
Bode, M., Falkenstein, T., Davidovic, M., Pitsch, H., Taniguchi, H., Murayama, K., Arima, T., Moon, S., Wang, J. & Arioka, A., Effects of Cavitation and Hydraulic Flip in 3Hole GDI Injectors. SAE International Journal of Fuels and Lubricants, 10(2), pp.380393. 2017.
The performance of Gasoline Direct Injection (GDI) engines is governed<br />
by multiple physical processes such as the internal nozzle flow and the<br />
mixing of the liquid stream with the gaseous ambient environment. A<br />
detailed knowledge of these processes even for complex injectors is very<br />
important for improving the design and performance of combustion engines<br />
all the way to pollutant formation and emissions. However, many<br />
processes are still not completely understood, which is partly caused by<br />
their restricted experimental accessibility. Thus, highfidelity<br />
simulations can be helpful to obtain further understanding of GDI<br />
injectors. In this work, advanced simulation and experimental methods<br />
are combined in order to study the spray characteristics of two<br />
different 3hole GDI injectors. First, a simulation approach for<br />
computing cavitation and hydraulic flip is presented, which<br />
appropriately combines simulations with different levels of abstraction<br />
allowing for predictive GDI injector simulations on currently available<br />
supercomputers. Next, general resolution requirements and various<br />
initial conditions are discussed. Especially the effect of initial<br />
bubbles inside the sac on the cavitation formation process is<br />
investigated by performing a transient simulation with moving needle.<br />
Finally, coupled compressible LargeEddy Simulations (LES) of the<br />
internal nozzle flow with and without models for cavitation and<br />
hydraulic flip, Direct Numerical Simulations (DNS) of the resulting<br />
primary breakup, and farfield Lagrangian Particlebased LES (LPLES)<br />
are performed in order to achieve a detailed data set of the injection<br />
process of two different 3hole GDI injectors. Xray measurements of<br />
innozzle cavitation as well as droplet size distribution obtained by<br />
Phase Doppler Particle Analyzer (PDPA) measurements in the farfield are<br />
used for validation and understanding the impact of the different<br />
effects on the underlying breakup processes. Furthermore, sensitivities<br />
between nozzle design features and hydraulic flip are investigated.

Iliksu, M., Khetan, A., Yang, S., Simon, U., Pitsch, H. & Sauer, D.U., Elucidation and Comparison of the Effect of LiTFSI and LiNO3 Salts on
Discharge Chemistry in Nonaqueous LiO2 Batteries. ACS Applied Materials & Interfaces, 9(22), pp.1931919325. 2017.
The role,of lithium salts in determining the discharge capacity of<br />
LiO2 batteries has been highlighted in several recent studies; however<br />
questions pertaining to their effect on the cathode surface and in the<br />
solution phase still remain unanswered. We conducted galvanostatic<br />
discharge experiments with different compositions of a binary mixture of<br />
1 M of LiNO3 and LiTFSI in tetraglyine (TEGDME) as the electrolyte and<br />
analyzed the discharge products using techniques such as FTIR, Raman<br />
spectroscopy, and SEM. It was observed that there:is a nonlinear<br />
correlation between the electrolyte composition and the first discharge<br />
capacity, With the highest discharge capacity achieved with the<br />
electrolyte composition as 0.75 M LiNO3 and 0.25 M LiTFSI. The ID/IG<br />
values;Obtained from Raman spectroscopy, which represent the degree of<br />
order in the carbon cathode surface,'' were found to be correlated to<br />
the measured capacity. Our results indicate that at concentrations of<br />
LiNO3 higher than 0.75 M in the electrolyte, nitrogen doping of the<br />
carbon surface reaches a critical limit, beyond which it becomes<br />
unfavorable for the discharge process. On. the other hand, decomposition<br />
of the electrolyte and formation of an amorphous layer on the cathode<br />
surface was found to intensify with increasing LiTFSI concentration, Our<br />
results show that the maximum discharge capacity of the cells is<br />
strongly dependent on the surface structure of the carbon cathode, which<br />
in turn is heavily influenced by the electrolyte composition. Classical<br />
molecular dynamics simulations of the same system indicated no such<br />
nonlinearity in the coordination of Li+ ions with respect to electrolyte<br />
composition, indicating that the ionic association strength of the anion<br />
May have Only a liinited effect.

Davidovic, M., Falkenstein, T., Bode, M., Cai, L., Kang, S., Hinrichs, J. & Pitsch, H., LES of nDodecane Spray Combustion Using a Multiple Representative
Interactive Flamelets Model. Oil & Gas Science and Technology  Revue d'IFP Energies Nouvelles, 72(5). 2017.
A singlehole ndodecane spray flame is studied in a LargeEddy<br />
Simulation (LES) framework under Dieselrelevant conditions using a<br />
Multiple Representative Interactive Flamelets (MRIF) combustion model.<br />
Diesel spray combustion is strongly affected by the mixture formation<br />
process, which is dominated by several physical processes such as the<br />
flow within the injector, breakup of the liquid fuel jet, evaporation<br />
and turbulent mixing with the surrounding gas. While the effects of<br />
nozzleinternal flow and primary breakup are captured within tuned model<br />
parameters in traditional Lagrangian spray models, an alternative<br />
approach is applied in this study, where the initial droplet conditions<br />
and primary fuel jet breakup are modeled based on results from highly<br />
resolved multiphase simulations with resolved interface. A highly<br />
reduced chemical mechanism consisting of 57 species and 217 reactions<br />
has been developed for ndodecane achiving a good computational<br />
performance at solving the chemical reactions. The MRIF model, which has<br />
demonstrated its capability of capturing combustion and pollutant<br />
formation under typical Diesel conditions in ReynoldsAveraged<br />
NavierStokes (RANS) simulations is extended for the application in LES.<br />
In the standard RIF combustion model, representative chemistry<br />
conditioned on mixture fraction is solved interactively with the flow.<br />
Subfilterscale mixing is modeled by the scalar dissipation rate. While<br />
the standard RIF model only includes temporal changes of the scalar<br />
dissipation rate, the spatial distribution can be accounted for by<br />
extending the model to multiple flamelets, which also enables the<br />
possibility of capturing different fuel residence times. Overall, the<br />
model shows good agreement with experimental data regarding both, low<br />
and high temperature combustion characteristics. It is shown that the<br />
ignition process and pollutant formation are affected by turbulent<br />
mixing. First, a cool flame is initiated at approximately stoichiometric<br />
mixture and propagates towards the rich side. Hence, heat and radicals<br />
are transported away from the most reactive mixture and thus the<br />
ignition is delayed. At the same time, the transported heat and radicals<br />
increase the reactivity of rich mixtures, which strongly affects the CO<br />
formation. NO was found to increase compared to the no transport case<br />
due to enhanced mixing, which is related to a broader hightemperature<br />
zone and the additional transport of oxygen from lean into<br />
hightemperature regions.

Boschung, J., Hennig, F., Denker, D., Pitsch, H. & Hill, R.J., Analysis of structure function equations up to the seventh
order. Journal of Turbulence, p.132. 2017.

Farazi, S., Sadr, M., Kang, S., Schiemann, M., Vorobiev, N., Scherer, V. & Pitsch, H., Resolved simulations of single char particle combustion in a laminar
flow field. FUEL, 201, pp.1528. 2017.
The aim of this work is to study spatially and chemically resolved<br />
particle combustion cases to understand chemical and laminar transport<br />
processes and to support model development. In the present study, the<br />
combustion process of a single char particle located in air or oxyfuel<br />
atmosphere composed of oxygen, carbon dioxide, and steam is<br />
investigated. Char burnout is represented in highly resolved numerical<br />
simulations including a detailed description of the surface and the gas<br />
phase chemistry. At the solidgas interface, heat and mass fluxes due to<br />
the surface reactions involving carbon oxidation and gasification are<br />
considered. The model is validated based on experimental results for<br />
char burnout phase in a flat flame burner. We perform a comprehensive<br />
set of fully resolved reactive 2D simulations by varying particle size,<br />
relative velocity, diluent, and oxygen composition in the surrounding<br />
gas. The simulation results are discussed regarding the CO2 and N2<br />
content of the atmosphere highlighting the effects of oxyfuel<br />
combustion. Furthermore, the impact of the particle flow motion on the<br />
flame that forms around the char particle is investigated by varying<br />
relative Reynolds number with particle size and relative slip velocity.<br />
(C) 2016 Elsevier Ltd. All rights reserved.

Ritter, D., Abel, D., Korkmaz, M., Pitsch, H. & Albin, T., Control of CNGDiesel DualFuel Engines. In AUTOREG 2017 – Automatisiertes Fahren und vernetzte Mobilität, 8. VDI/VDE Fachtagung, Berlin, Deutschland. 2017.

Korkmaz, M., Golc, D., Ritter, D., Jochim, B., Abel, D. & Pitsch, H., Experimental Investigation of Performance and Emissions Characteristics in a SingleCylinder Compression Ignition DualFuelEngine. In Proceedings of the European Combustion Meeting, April 18th21st, Dubrovnik, Croatia. 2017.

Jocher, A., Pitsch, H., Gomez, T., Bonnety, J. & Legros, G., Combustion instability mitigation by magnetic fields. PHYSICAL REVIEW E, 95(6). 2017.
The present interdisciplinary study combines electromagnetics and<br />
combustion to unveil an original and basic experiment displaying a<br />
spontaneous flame instability that is mitigated as the nonpremixed<br />
sooting flame experiences a magnetic perturbation. This magnetic<br />
instability mitigation is reproduced by direct numerical simulations to<br />
be further elucidated by a flow stability analysis. A key role in the<br />
stabilization process is attributed to the momentum and thermochemistry<br />
coupling that the magnetic force, acting mainly on paramagnetic oxygen,<br />
contributes to sustain. The spatial local stability analysis based on<br />
the numerical simulations shows that the magnetic field tends to reduce<br />
the growth rates of small flame perturbations.

Korkmaz, M., Ritter, D., Jochim, B., Beeckmann, J., Abel, D. & Pitsch, H., Experimental Investigation and Analysis of Performance and Emissions Characteristics of a SingleCylinder Compression Ignition DualFuelEngine for ModelBased Combustion Control. In Symposium for Combustion Control, June 28th29th, Aachen, Germany. 2017.

Cai, L., Kruse, S., Felsmann, D., Thies, C., Yalamanchi, K.K. & Pitsch, H., Experimental Design for Discrimination of Chemical Kinetic Models for OxyMethane Combustion. Energy & Fuels, 31(5), pp.55335542. 2017.
The concept of combustion under oxyfuel conditions has the potential to<br />
reduce greenhouse gas emissions. For the design of combustion devices<br />
operating under these conditions, a good understanding of fuel oxidation<br />
behavior in terms of chemical kinetic mechanisms is useful. For the<br />
oxidation of the main component of natural gas and coal devolatilization<br />
products, i.e., methane, various chemical mechanisms are available in<br />
the literature validated mostly with experiments using air and none of<br />
them is developed particularly or has been validated extensively for<br />
oxymethane combustion. An important prerequisite for model assessment<br />
is highquality data typically obtained from resource and<br />
timeconsuming measurements. The aim of this study is to identify the<br />
best methane mechanism for oxyfuel combustion from a set of models<br />
available in the literature with a minimum number of measurements. Five<br />
chemical models, which have been validated for the oxidation of<br />
methane/air mixtures, are compared in terms of their performance for<br />
extinction strain rates, ignition delay times, and laminar burning<br />
velocities of oxymethane mixtures. A modelbased experimental design<br />
method, i.e., Akaike weights design criterion, is applied to determine<br />
the optimal potential measurements. Ideally, at the conditions of<br />
designed experiments, model predictions are nicely separated and, thus,<br />
the best model can be identified by comparison to these measurements. It<br />
is shown that the employed experimental design strategy identifies<br />
informative experiments for model discrimination efficiently. While<br />
measurements of extinction strain rates are proposed to be carried out<br />
for flames with small methane mass fractions of the fuel stream and<br />
oxygen mass fractions of the oxidizer stream, shock tube experiments are<br />
evaluated as equally useful for model discrimination over the<br />
investigated range of conditions. Measurements of flame speeds are<br />
designed at very small and very large equivalence ratios particularly at<br />
relatively high pressures.

Bode, M., Goeb, D., Denker, D., Asuri Mukundan, A., Yellapantula, S. & Pitsch, H., Spray combustion and dissipation elements: a novel analysis approach. In Proceedings of the European Combustion Meeting, April 18th21st, Dubrovnik, Croatia. 2017.

Leitner, W., Klankermayer, J., Pischinger, S., Pitsch, H. & KohseHoeinghaus, K., Advanced Biofuels and Beyond: Chemistry Solutions for Propulsion and Production. ANGEWANDTE CHEMIEINTERNATIONAL EDITION, 56(20), pp.54125452. 2017.
Sustainably produced biofuels, especially when they are derived from<br />
lignocellulosic biomass, are being discussed intensively for future<br />
ground transportation. Traditionally, research activities focus on the<br />
synthesis process, while leaving their combustion properties to be<br />
evaluated by a different community. This Review adopts an integrative<br />
view of engine combustion and fuel synthesis, focusing on chemical<br />
aspects as the common denominator. It will be demonstrated that a<br />
fundamental understanding of the combustion process can be instrumental<br />
to derive design criteria for the molecular structure of fuel<br />
candidates, which can then be targets for the analysis of synthetic<br />
pathways and the development of catalytic production routes. With such<br />
an integrative approach to fuel design, it will be possible to improve<br />
systematically the entire system, spanning biomass feedstock, conversion<br />
process, fuel, engine, and pollutants with a view to improve the carbon<br />
footprint, increase efficiency, and reduce emissions.

Sudholt, A., Tripathi, R., Mayer, D., Glaude, P.A., BattinLeclerc, F. & Pitsch, H., The oxidation of the novel lignocellulosic biofuel γvalerolactone in a low pressure flame. Proceedings of the Combustion Institute, 36(1), pp.577–585. 2017.
A first oxidation study of the novel lignocellulosic biofuel γvalerolactone (GVL) has been performed in a low pressure premixed flat flame. A stoichiometric GVL/methane flame was investigated experimentally and numerically at a pressure of 50 Torr. The measurements include flame temperatures and species concentrations. Species profiles from online gas chromatography measurements are presented for about 40 species including oxygen, hydrogen, argon, carbon monoxide, carbon dioxide, water, hydrocarbon, and oxygenated species with separated isomers. The recent kinetic model for \GVL\ pyrolysis of De Bruycker et al. (2016) was extended for oxidation and the resulting model shows good agreement with the experimental flame data. A reaction pathway analysis was performed, which elucidates the fuel specific oxidation pathways of \GVL\ at the investigated conditions. Additionally, an enthalpy analysis of the flame was performed highlighting the consistency of the different measurement techniques.

Trisjono, P. & Pitsch, H., A direct numerical simulation study on NO formation in lean premixed flames. Proceedings of the Combustion Institute, 36(2), pp.2033–2043. 2017.
There is considerable interest in the development of ultralow emission fuelflexible combustion systems for power generation, which are typically operated under lean premixed conditions. In this work, NO formation in turbulent premixed combustion is investigated via a direct numerical simulation of a temporally evolving methane/air jet flame at a jet Reynolds number of 9000. A detailed chemical mechanism, which in addition to the methane chemistry contains comprehensive NO kinetics is employed in a threedimensional, shear driven, premixed turbulent configuration computed using 3 billion mesh points. After the fidelity of the DNS database and physical properties of the simulated flame are discussed, the results are used to investigate the interaction of NO chemistry and turbulence in lean premixed flames. To this end, the heat release and NO formation rates are analyzed. These indicate a decorrelation of the heat release and the NO production under highly turbulent conditions. In order to explain and understand this behavior, statistics of individual formation pathways and elementary reactions of the computed turbulent jet flame are compared to an unstretched premixed flame, which could be considered a simple approach used for modeling. Specifically, the formation via the NNH pathway is found to be important for flame generated NO and significantly affected by the turbulence, which is shown to be a consequence of differential diffusion.

Jocher, A., Foo, K.K., Sunc, Z., Bassam, D., Pitsch, H., Alwahabic, Z. & Nathanc, G., Impact of acoustic forcing on soot evolution and temperature in ethyleneair flames. Proceedings of the Combustion Institute, 36(1), pp.781–788. 2017.
This combined numerical and experimental study assesses the transient coupling of soot formation, flame chemistry and fluid transport in ethyleneair coflow flames at acoustic forcing frequencies of 20 and 40 Hz. The measurements report soot volume fraction and flow velocity. For the computational analysis, the numerical code’s capability in modeling soot formation is first demonstrated in a steady coflow flame. Soot volume fraction and temperature measurements from different laboratories and optical techniques are used for validation. Then, acoustic forcing is applied to investigate the transient behavior of this multidimensional combustion problem. Forcing at different frequencies and amplitudes provokes very distinct transient soot, temperature, and flow conditions. The discussed steady ethyleneair flame is excited with 20 and 40 Hz, corresponding to Strouhal numbers of 0.23 and 0.46. For both frequencies, forcing amplitudes of 20, 50, and 60% are studied numerically and validated against measurements at 50%. With a startup transient analysis, the computation time to reach a periodic state is evaluated and soot volume fraction predictions are then compared with the measurements. A reduction in maximum soot volume fraction for the increased forcing frequency is observed experimentally and numerically. The decrease in maximum soot volume fraction is explained by a residence time analysis revealing shorter maximum fluid parcel residence times for the 40 Hz than for the 20 Hz case. It is also found that at 40 Hz the transient evolution of maximum soot production and forced fuel velocity is almost synchronized, while for the 20 Hz case, a time lag of 32.5 ms is observed, corresponding to 65% of a full period.

Vierjahn, T., Schnorr, A., Weyers, B., Denker, D., Wald, I., Garth, C., Kuhlen, T.W. & Hentschel, B., Interactive Exploration of Dissipation Element Geometry. In Telea, A. & Bennett, J., eds. Eurographics Symposium on Parallel Graphics and Visualization. The Eurographics Association. 2017.

Wick, A., Nguyen, T.T., Laurent, F., O. Fox, R. & Pitsch, H., Modeling Soot Oxidation with the Extended Quadrature Method of Moments. Proceedings of the Combustion Institute, 36(1), pp.789–797. 2017.
Modeling the oxidation of soot particles in flames is a challenging topic both from a chemical point of view and regarding the statistical treatment of the evolution of the soot number density function (NDF). The method of moments is widelyused for the statistical modeling of aerosol dynamics in various applications, and a number of different moment methods have been established and successfully applied to the modeling of soot formation and growth. However, a shortcoming of existing moment methods is the lack of an accurate, numerically robust, and computationally efficient way to treat soot oxidation, especially regarding the prediction of the particle number density. In this work, the recently developed Extended Quadrature Method of Moments (EQMOM) is integrated with a physicochemical soot model and combined with a treatment for particle removal by oxidation. This leads to a modeling framework for the simulation of coupled inception, growth, coagulation, and oxidation of soot in flames. In EQMOM, the moment equations are closed by reconstructing the soot NDF with a superposition of continuous kernel functions. Various standard distribution functions can be used as kernel functions, and the algorithm has been implemented here using gamma and lognormal distributions. It is shown that and discussed why gamma distributions are more suitable as kernel functions than lognormal distributions in order to accurately predict soot oxidation. The integrated model is validated by comparisons with analytical solutions for the NDF, results from Monte Carlo simulations of soot formation and oxidation in flames, and experimental data.

Bode, M., vom Lehn, F., Satcunanathan, S., Le Chenadec, V. & Pitsch, H., Simulation of Hydraulic Flip in Cavitating Nozzles using OneFluid and TwoFluid Equilibrium Models. In The 3rd International Conference on Numerical Methods in Multiphase Flows, ICNMMFIII, June 26th29th, Tokyo, Japan. 2017.

Denker, D., Attili, A., Luca, S., Bisetti, F. & Pitsch, H., Dissipation Element Analysis of Premixed Spatial Evolving Jet Flames. In 16th SIAM International Conference on Numerical Combustion, April 3rd5th, Orlando, Florida, USA. 2017.

Jocher, A., Bonnety, J., Pitsch, H., Gomez, T. & Legros, G., Dual magnetic effects on soot production in partially premixed flames. Proceedings of the Combustion Institute, 36(1), pp.1377–1385. 2017.
The present paper further explores the potential of the gradient of the square of the magnetic flux density (∇(B2)) to control the soot production in flames. Experimental investigations assess for the first time the influence of ∇(B2) on the soot production in laminar axisymmetric partially premixed flames. The steady rich ethylene/oxygen flames are established in different coflowing mixtures, composed of oxygen and nitrogen, over a Santoro type burner, which is located in an electromagnet. The flame experiences different magnitudes of upward ∇(B2), ranging from 0 to 18.2 T2/m, as well as different oxygen contents of the coflow, ranging from 21% to 50% in volume, and different levels of the equivalence ratio, ranging from 5 to ∞. Soot temperature and volume fraction are mapped in the flame by a Modulated Absorption/Emission technique. Increasing the magnitude of ∇(B2) allows for the modification of soot production in the flame. A reversal of the magnetic effect compared to the nonpremixed case is observed. For nonpremixed flames, the upward ∇(B2) systematically leads to an increase in soot production, while this magnetic gradient can induce an overall reduction in soot production in some partially premixed flames. This original reversal is attributed to a switch of the magnetic force direction. As a result, this supplementary force can induce an increase or a decrease of the residence time in the rich region of a partially premixed flame, depending mainly on the fields of oxygen mole fraction and temperature in this region. In conjunction with more elaborated magnetic fields, these opposite trends may contribute to strategies for the control of soot production.

Farazi, S., Schiemann, M., Vorobiev, N., Scherer, V., Kang, S. & Pitsch, H., A new variable transport coefficient singlefilm model for char combustion. In Proceedings of the European Combustion Meeting, April 18th21st, Dubrovnik, Croatia. 2017.

Schmid, P.J. & Sayadi, T., Lowdimensional representation of nearwall dynamics in shear flows, with implications to wallmodels. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375(2089). 2017.
The dynamics of coherent structures near the wall of a turbulent<br />
boundary layer is investigated with the aim of a lowdimensional<br />
representation of its essential features. Based on a triple<br />
decomposition into mean, coherent and incoherent motion and a dynamic<br />
mode decomposition to recover statistical information about the<br />
incoherent part of the flow field, a driven linear system coupling<br />
firstand secondorder moments of the coherent structures is derived and<br />
analysed. The transfer function for this system, evaluated for a<br />
wallparallel plane, confirms a strong bias towards streamwise elongated<br />
structures, and is proposed as an `impedance' boundary condition which<br />
replaces the bulk of the transport between the coherent velocity field<br />
and the coherent Reynolds stresses, thus acting as a wall model for<br />
largeeddy simulations (LES). It is interesting to note that the<br />
boundary condition is nonlocal in space and time. The extracted model<br />
is capable of reproducing the principal Reynolds stress components for<br />
the pretransitional, transitional and fully turbulent boundary layer.<br />
This article is part of the themed issue `Toward the development of<br />
highfidelity models of wall turbulence at large Reynolds number'.

Tripathia, R., Lee, C., Fernandes, R.X., Oliviere, H., Curranc, H.J., Sarathyf, S.M. & Pitsch, H., Ignition characteristics of 2methyltetrahydrofuran: An experimental and kinetic study. Proceedings of the Combustion Institute, 36(1), pp.587–595. 2017.
The present paper elucidates oxidation behavior of 2methyltetrahydrofuran (2MTHF), a novel secondgeneration biofuel. New experimental data sets for 2MTHF including ignition delay time measurements in two different combustion reactors, i.e. rapid compression machine and highpressure shock tube, are presented. Measurements for 2MTHF/oxidizer/diluent mixtures were performed in the temperature range of 639−1413639−1413 K, at pressures of 10, 20, and 40 bar, and at three different equivalence ratios of 0.5, 1.0, and 2.0. A detailed chemical kinetic model describing both lowand hightemperature chemistry of 2MTHF was developed and validated against new ignition delay measurements and already existing flame species profiles and ignition delay measurements. The mechanism provides satisfactory agreement with the experimental data. For identifying key reactions at various combustion conditions and to attain a better understanding of the combustion behavior, reaction path and sensitivity analyses were performed.

Jochim, B., Korkmaz, M. & Pitsch, H., Scalar dissipation rate based multizone model for earlyinjected and conventional diesel engine combustion. Combustion and Flame, 175, pp.138154. 2017.

Cai, L., Minwegen, H., Beeckmann, J., Burke, U., Tripathi, R., Ramalingam, A., Kröger, L.C., Sudholt, A., Leonhard, K., Klankermayer, J., Heufer, K.A. & Pitsch, H., Experimental and numerical study of a novel biofuel: 2Butyltetrahydrofuran. Combustion and Flame, 178(1), pp.257–267. 2017.

Chen, P., Khetan, A., Yang, F., Migunov, V., Weide, P., Stürmer, S.P., Guo, P., Kähler, K., Xia, W., Mayer, J., Pitsch, H., Simon, U. & Muhler, M., Experimental and Theoretical Understanding of NitrogenDopingInduced Strong Metal–Support Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogenation. ACS Catalysis, 7(2), pp.11971206. 2017.
By doping the TiO2 support with nitrogen, strong metal–support interactions (SMSI) in Pd/TiO2 catalysts can be tailored to obtain highperformance supported Pd nanoparticles (NPs) in nitrobenzene (NB) hydrogenation catalysis. According to the comparative studies by Xray diffraction, Xray photoelectron spectroscopy (XPS), and diffuse reflectance CO FTIR (CO–DRIFTS), Ndoping induced a structural promoting effect, which is beneficial for the dispersion of Pd species on TiO2. Highangle annular darkfield scanning transmission electron microscopy study of Pd on Ndoped TiO2 confirmed a predominant presence of sub2 nm Pd NPs, which are stable under the applied hydrogenation conditions. XPS and CO–DRIFTS revealed the formation of strongly coupled Pd–N species in Pd/TiO2 with Ndoped TiO2 as support. Density functional theory (DFT) calculations over model systems with Pdn (n = 1, 5, or 10) clusters deposited on TiO2(101) surface were performed to verify and supplement the experimental observations. In hydrogenation catalysis using NB as a model molecule, Pd NPs on Ndoped TiO2 outperformed those on Nfree TiO2 in terms of both catalytic activity and stability, which can be attributed to the presence of highly dispersed Pd NPs providing more active sites, and to the formation of Pd–N species favoring the dissociative adsorption of the reactant NB and the easier desorption of the product aniline.

Rubbert, A., Hennig, F., Klaas, M., Pitsch, H., Schröder, W. & Peters, N., Streamline segment scaling behavior in a turbulent wavy channel flow. Experiments in Fluids, 58(2), p.10. 2017.
A turbulent flow in a wavy channel was investigated by tomographic particleimage velocimetry measurements and direct numerical simulations. To analyze the turbulent structures and their scaling behavior in a flow undergoing favorable and adverse pressure gradients, the streamline segmentation method proposed by Wang (J Fluid Mech 648:183203, 2010) was employed. This method yields joint statistical information about velocity fluctuations and length scale distributions of nonoverlapping structures within the flow. In particular, the joint statistical properties are notably influenced by the pressure distribution. Previous findings from flat channel flows and synthetic turbulence simulations concerning the normalized segment length distribution could be reproduced and therefore appear to be largely universal. However, the mean streamline segment length of accelerating and decelerating segments varies within one wavelength typically elongating segments of the type which corresponds to the local mean flow. Furthermore, the local pressure gradient was found to significantly impact local joint streamline segmentation statistics as a main influence on their inherent asymmetry.

Yellapantula, S., Bode, M., Mukundan, A.A. & Pitsch, H., Numerical Study of AutoIgnition in a Liquid nheptane Jet. In 10th U.S. National Combustion Meeting, April 23rd26th, College Park, Maryland, US. 2017.

Abdelgadir, A., Rakha, I.A., Steinmetz, S.A., Attili, A., Bisetti, F. & Roberts, W.L., Effects of hydrodynamics and mixing on soot formation and growth in laminar coflow diffusion flames at elevated pressures. Combustion and Flame, 181, pp.39  53. 2017.
The formation, growth, and oxidation of soot are studied in a set of laminar coflow diffusion flames at pressures ranging from 1 to 8 atm. The modeling approach combines detailed finite rate chemical kinetics mechanisms that model the formation of Polycyclic Aromatic Hydrocarbon (PAH) species up to pyrene, and a bivariate method of moments that describes soot particles and aggregates by their volume and surface area. The spatial distribution of soot observed experimentally and that predicted numerically are in good qualitative agreement with the peak soot volume fraction located at the flame tip and soot appearing on the flame wings and closer to the nozzle as pressure increases. A detailed analysis of the effect of hydrodynamics and mixing on soot formation is presented. We show that the scalar dissipation rate is lower for the higher pressure flames, promoting the formation of \PAH\ species and soot. Thus, the observed increase in soot volume fraction across flames with increasing pressure is not due solely to mixture density and kinetics effects, rather is affected by hydrodynamics and mixing processes also. Similarly, our results indicate that the decrease in the scalar dissipation rate contribute to changing the location where soot forms in the flame, with soot formation occurring closer to the nozzle and outward on the flame’s wings as pressure increases. Radiative heat losses are found to lower the flame temperature, inducing a reduction of the \PAH\ species and associated rates of soot formation. However, heat losses are responsible for a slightly longer flame, which allows for more soot. The overall effect is a modest variation of soot volume fraction if radiation is included.

Sayadi, T., Farazi, S., Kang, S. & Pitsch, H., Transient multiple particle simulations of char particle combustion. Fuel, 199, pp.289298. 2017.
Transient combustion behavior of char particle arrays in oxyfuel<br />
atmosphere is analyzed with a resolvedparticle twodimensional<br />
simulation framework, considering both heterogenous and homogeneous<br />
chemical reactions. A range of various parameters is considered, such as<br />
particle distance, oxygen levels in the incoming flow, particle Reynolds<br />
number, and the particle arrangement. The analysis shows a large<br />
dependence on the distance between the particles and the location of<br />
each particle within the array. While particles facing the incoming flow<br />
show the highest burning efficiency, this efficiency drops as<br />
consecutive rows are considered. This effect is stronger for arrays,<br />
where the distance between the consecutive rows is less than three<br />
particle diameters. Within the range of particle Reynolds numbers<br />
considered in this study (1 <= Re <= 8), higher Reynolds numbers result<br />
in more efficient burning. The arrangement of the particles inside the<br />
array has a mild influence on the combustion behavior. As arrays with<br />
lower particle number densities are considered, the influence of the<br />
arrangement of the particle array diminishes. Finally, considering the<br />
temperature and the oxygen mass fraction around all the simulated<br />
particles in the parameter space provides insight towards the global<br />
relation between these variables, highlighting possible modeling<br />
directions. (C) 2017 Elsevier Ltd. All rights reserved.

Kleinheinz, K., Kubis, T., Trisjono, P., Bode, M. & Pitsch, H., Computational study of flame characteristics of a turbulent piloted jet burner with inhomogeneous inlets. Proceedings of the Combustion Institute, 36(2), pp.1747  1757. 2017.
Recent experimental studies of a piloted turbulent jet burner at Sydney and Sandia with inhomogeneous mixture composition at the inlet have revealed that the flame stability of a partially premixed flame can be significantly increased with a tailored mixture fraction profile at the burner exit. In the present work, largeeddy simulations of this burner are performed with three different levels of mixture inhomogeneity. A multiregime flamelet model is employed, which accounts for the range of combustion modes found in these flames. The results of the simulations are validated against experimental data for the case with the highest blowoff velocity. Good agreement is observed for velocity, mixture fraction, and temperature fields. The simulations with the multiregime model provide detailed data of the prevalent combustion regime as well as the heat release. The location of heat release and differences in the combustion modes are analyzed for three cases with different mixture inhomogeneities. The progress variable source term is split up into the individual contributions of the combustion regimes. Substantial differences are found for the contributions of premixed and nonpremixed combustion in the different flames as well as the location of the heat release. These results are used to explain the respective flame stabilities. For the inhomogeneous case, which features substantially increased flame stability, hot pilot gases are in contact with reactive mixture directly at the nozzle. Therefore a predominantly premixed zone of strong heat release develops at the jet exit and stabilizes the flame. For both other cases, the heat release at the nozzle is lower leading to smaller blowoff velocities. For the nonpremixed case, this is due to air shielding the pilot from the reactive mixture. For the premixed case, the homogeneous mixture issuing from the jet is above the flammability limit and consequently the heat release is diffusion dominated.

Lucchesi, M., Abdelgadir, A., Attili, A. & Bisetti, F., Simulation and analysis of the soot particle size distribution in a turbulent nonpremixed flame. Combustion and Flame, 178, pp.3545. 2017.

Korkmaz, M., Zweigel, R., Niemetz, K., Jochim, B.K., Abel, D. & Pitsch, H., Assessment of Different Included Spray Cone Angles and Injection Strategies for PCCI Diesel Engine Combustion. In WCX 17: SAE World Congress Experience, April 4th6th, Detroit, Michigan, USA. SAE Technical Paper Series. SAE International. 2017.

Beeckmann, J., Hesse, R., Kruse, S., Berens, A., Peters, N., Pitsch, H. & Matalon, M., Propagation speed and stability of spherically expanding hydrogen/air flames: Experimental study and asymptotics. Proceedings of the Combustion Institute, 36(1), pp.1531–1538. 2017.
Here, outwardly propagating spherical hydrogen/air flames are examined theoretically and experimentally with respect to flame propagation speed and the onset of instabilities which develop due to thermal expansion and nonequal diffusivities. Instabilities increase the surface area of the spherical flame, and hence the flame propagation speed. The theory applied here accounts for both hydrodynamic and diffusivethermal effects, incorporating temperature dependent transport coefficients. Experiments are performed in a spherical combustion chamber over a wide range of equivalence ratios (0.6–2.0), initial temperatures (298–423 K), and initial pressures (1 atm to 15 bar). The evolution of the flame propagation speed as a function of flame radius is compared to predictions from theory showing excellent agreement. Also the wrinkling of hydrogen/air flames is examined under increased pressure and temperature for various equivalence ratios. Critical flame radii, defined as the point of transition to cellular flames, are extracted from highspeed Schlieren flame imaging. Overall, the critical radius is found to decrease with increasing pressure. The predictions yield the growth rate of small disturbances and the critical flame radius. Experimental flame radii, as expected, are underpredicted by the theoretical findings. Experimental data are provided in the form of an approximation formula.

Dahms, R.N., Paczko, G., Skeen, S.A. & Pickett, L.M., Understanding the ignition mechanism of highpressure spray flames. Proceedings of the Combustion Institute, 36(2), pp.26152623. 2017.

Xu, K., Zhang, H., Wu, Y., Baroncelli, M. & Pitsch, H., Transient model for soot formation during the combustion of single coal particles. Proceedings of the Combustion Institute, 36(2), pp.2131–2138. 2017.
A transient mathematical model was developed to describe soot formation during the combustion of single coal particles based on the static semiempirical model presented by Fletcher and coworkers. Sensitivity analyses of the model parameters show that soot emissivity and mass diffusivity of tar play an important role in predicting soot volume fraction (fv) and flame temperature (Tf). The model was applied to simulate the combustion of single bituminous coal particles with initial diameter (2r0) of 83 µm in a drop tube furnace and air atmosphere. It was found that soot is only formed within the first ∼ 5 ms after the appearance of the volatile flame. Although most of the soot is oxidized during the volatile flame phase, a small portion of soot still remains during the char combustion. Due to the soot presence, the volatile flame duration is extended by 2.6 ms. Compared with the sootfree flame, the sooting flame has remarkable lower Tf and its peak Tf value is ∼ 410 K lower. As a consequence, char combustion starts at a temperature that is ∼ 125 K lower than that of the sootfree case. Spatially, the peak fv at 16.6 ms appears at 4.5 r0 and soot oxidation zone spans to ∼ 10 r0. The model was validated by comparing the predicted Tf and fv under different O2 mole fractions (xO2) with recent experimental results reported by Khatami and coworkers. The predicted trends are consistent with those of the experimental results. With increasing xO2, Tf increases, but the increase rate becomes more gradual at a large xO2. While for fv, a nonmonotonic variation is observed, where soot first increases and then decreases with a peak value occurring at xO2 ≈ 40%.

Bode, M., Davidovic, M. & Pitsch, H., Multiscale Coupling for Predictive Injector Simulations. In Di Napoli, E., Hermanns, M. A., Iliev, H., Lintermann, A., & Peyser, A., eds. HighPerformance Scientific Computing: First JARAHPC Symposium, JHPCS 2016, October 4th5th, Aachen, Germany, Revised Selected Papers. Springer International Publishing, pp. 96108, . 2017.

Pitsch, H. & Williams, F.A., Preface. Combustion and Flame, 175, p.1. 2017.

Hennig, F., Boschung, J. & Peters, N., Statistical Description of Streamline Segments in a Turbulent Channel Flow with a Wavy Wall. New Results in Numerical and Experimental Fluid Mechanics X, pp.135143. 2016.
In this study we investigate the statistical properties of so called streamline segments in a turbulent channel flow with one plane and one wavy wall. We give a short overview of the concept of streamline segments and recent results in description and modeling in this field. We find that streamline segments in the vicinity of the wavy wall are significantly smaller on average than in the core region. However, normalizing the length distribution with the mean segment length leads to an almost perfect collapse of the pdfs. This quasiuniversal behavior is further highlighted by the comparison to statistics of streamline segments in homogeneous isotropic turbulence. Finally, we investigate the kinematic behavior of streamline segments by means of conditional moments and show differences in scaling behavior compared to the classical structure function analysis.

Bode, M., Deshmukh, A., Göbbert, J.H. & Pitsch, H., CIAO: Multiphysics, multiscale NavierStokes solver for turbulent reacting flows in complex geometries Brömmel, D., Frings, W., & Wylie, B. J. N., eds. , 2016.

Attili, A., Bisetti, F., Mueller, M.E. & Pitsch, H., Effects of nonunity Lewis number of gasphase species in turbulent nonpremixed sooting flames. Combustion and Flame, 166, pp.192202. 2016.
Turbulence statistics from two threedimensional direct numerical simulations of planar nheptane/air turbulent jets are compared to assess the effect of the gasphase species diffusion model on flame dynamics and soot formation. The Reynolds number based on the initial jet width and velocity is around 15, 000, corresponding to a Taylor scale Reynolds number in the range 100 ≤ Reλ ≤ 150. In one simulation, multicomponent transport based on a mixtureaveraged approach is employed, while in the other the gasphase species Lewis numbers are set equal to unity. The statistics of temperature and major species obtained with the mixtureaveraged formulation are very similar to those in the unity Lewis number case. In both cases, the statistics of temperature are captured with remarkable accuracy by a laminar flamelet model with unity Lewis numbers. On the contrary, a flamelet with a mixtureaveraged diffusion model, which corresponds to the model used in the multicomponent diffusion threedimensional DNS, produces significant differences with respect to the \DNS\ results. The total mass of soot precursors decreases by 20–30% with the unity Lewis number approximation, and their distribution is more homogeneous in space and time. Due to the nonlinearity of the soot growth rate with respect to the precursors’ concentration, the soot mass yield decreases by a factor of two. Being strongly affected by coagulation, soot number density is not altered significantly if the unity Lewis number model is used rather than the mixtureaveraged diffusion. The dominant role of turbulent transport over differential diffusion effects is expected to become more pronounced for higher Reynolds numbers.

Boschung, J., Hennig, F., Gauding, M., Pitsch, H. & Peters, N., Generalised higherorder Kolmogorov scales. Journal of Fluid Mechanics, 794, pp.233251. 2016.
Kolmogorov introduced dissipative scales based on the mean dissipation ⟨ε⟩ and the viscosity ν, namely the Kolmogorov length η=(ν3/⟨ε⟩)1/4 and the velocity uη=(ν⟨ε⟩)1/4. However, the existence of smaller scales has been discussed in the literature based on phenomenological intermittency models. Here, we introduce exact dissipative scales for the evenorder longitudinal structure functions. The derivation is based on exact relations between evenorder moments of the longitudinal velocity gradient (∂u1/∂x1)2m and the dissipation ⟨εm⟩. We then find a new length scale ηC,m=(ν3/⟨εm/2⟩2/m)1/4 and uC,m=(ν⟨εm/2⟩2/m)1/4, i.e. the dissipative scales depend rather on the moments of the dissipation ⟨εm/2⟩ and thus the full probability density function (p.d.f.) P(ε) instead of powers of the mean ⟨ε⟩m/2. The results presented here are exact for longitudinal evenordered structure functions under the assumptions of (local) isotropy, (local) homogeneity and incompressibility, and we find them to hold empirically also for the mixed and transverse as well as odd orders. We use direct numerical simulations (DNS) with Reynolds numbers from Reλ=88 up to Reλ=754 to compare the different scalings. We find that indeed P(ε) or, more precisely, the scaling of ⟨εm/2⟩/⟨ε⟩m/2 as a function of the Reynolds number is a key parameter, as it determines the ratio ηC,m/η as well as the scaling of the moments of the velocity gradient p.d.f. As ηC,m is smaller than η, this leads to a modification of the estimate of grid points required for DNS.

Fiscaletti, D., Elsinga, G.E., Attili, A., Bisetti, F. & Buxton, O.R.H., Scale dependence of the alignment between strain rate and rotation in turbulent shear flow. Phys. Rev. Fluids, 1(6), p.064405. 2016.

Göbbert, J.H., Bode, M. & Wylie, B.J.N., ExtremeScale In Situ Visualization of Turbulent Flows on IBM Blue Gene/Q JUQUEEN. In Taufer, M., Mohr, B., & Kunkel, J. M., eds. High Performance Computing: ISC High Performance 2016 International Workshops, ExaComm, EMuCoCoS, HPCIODC, IXPUG, IWOPH, P3MA, VHPC, WOPSSS, 19th23rd June, Frankfurt, Germany. pp. 4555. 2016.

Fiscaletti, D., Attili, A., Bisetti, F. & Elsinga, G.E., Scale interactions in a mixing layer – the role of the largescale gradients. Journal of Fluid Mechanics, 791, pp.154–173. 2016.

Narayanaswamy, K., Pitsch, H. & Pepiot, P., A component library framework for deriving kinetic mechanisms for multicomponent fuel surrogates: Application for jet fuel surrogates. Combustion and Flame, 165, pp.288  309. 2016.
Surrogate fuels are often used in place of real fuels in computational combustion studies. However, many different choices of hydrocarbons to make up surrogate mixtures have been reported in the literature, particularly for jet fuels. To identify the best choice of surrogate components, the capabilities of different surrogate mixtures in emulating the combustion kinetic behavior of the real fuel must be examined. To allow extensive assessment of the combustion behavior of these surrogate mixtures against detailed experimental measurements for real fuels, accurate and compact kinetic models are most essential. To realize this goal, a flexible and evolutive component library framework is proposed here, which allows mixing and matching between surrogate components to obtain short chemical mechanisms with only the necessary kinetics for the desired surrogate mixtures. The idea is demonstrated using an extensively validated multicomponent reaction mechanism developed in stages (Blanquart et al., 2009; Narayanaswamy et al., 2010, 2014, 2015), thanks to its compact size and modular assembly. To display the applicability of the component library framework, (i) a jet fuel surrogate consisting of ndodecane, methylcyclohexane, and mxylene, whose kinetics are described in the multicomponent chemical mechanism is defined, (ii) a chemical model for this surrogate mixture is derived from the multicomponent chemical mechanism using the component library framework, and (iii) the predictive capabilities of this jet fuel surrogate and the associated chemical model are assessed extensively from low to high temperatures in well studied experimental configurations, such as shock tubes, premixed flames, and flow reactors.

Pitsch, H. & Keylock, C., Obituary for Norbert Peters In Memoriam Professor Norbert Peters (19422015) Obituary. Fluid Dynamics Research, 48(2). 2016.

Bode, M. & Pitsch, H., Novel simulations of fuel injection systems investigating spray characteristics by use of highfidelity multiphase methods and supercomputers. PRACEdays16, May 10th12th, Prague, Czech Republic. 2016.

Bode, M., Göbbert, J.H. & Pitsch, H., HighFidelity Multiphase Simulations and InSitu Visualization Using CIAO. In NIC Symposium, 11th12th February, Jülich, Germany. 2016.

Boschung, J., Peters, N., Laizet, S. & Vassilicos, J.C., Streamlines in stationary homogeneous isotropic turbulence and fractalgenerated turbulence. Fluid Dynamics Research, 48(2), p.021403. 2016.

Mohamed, S.Y., Cai, L., Khaled, F., Banyon, C., Wang, Z., Rashidi, M.J.A., Pitsch, H., Curran, H.J., Farooq, A. & Sarathy, S.M., Modeling Ignition of a Heptane Isomer: Improved Thermodynamics, Reaction Pathways, Kinetics, and Rate Rule Optimizations for 2Methylhexane. The Journal of Physical Chemistry A, 120(14), pp.22012217. 2016.
Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2methylhexane, based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxyalkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new highpressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate the ability to optimize the kinetic model using both manual and automated techniques for rate parameter tunings to improve agreement with the measured ignition delay time data. Finally, additional low temperature chain branching reaction pathways are shown to improve the model’s performance. The present approach to model development provides better performance across extended operating conditions while also strengthening the fundamental basis of the model.

Denker, D., Boschung, J., Hennig, F. & Pitsch, H., Dissipation Element Analysis of Reactingand NonReacting Flows. In APS Meeting Abstracts. 2016.

Korkmaz, M., Zweigel, R., Jochim, B., Beeckmann, J., Abel, D. & Pitsch, H., Tripleinjection Strategy for Modelbased Control of PCCI Diesel Engine Combustion. In SCC Symposium for Combustion Control 2016  Thinking the Future, June 15th16th, Aachen, Germany. pp. 201211. 2016.

Bates, L., Bradley, D., Paczko, G. & Peters, N., Engine hot spots: Modes of autoignition and reaction propagation. Combustion and Flame, 166, pp.80  85. 2016.
Many direct numerical simulations of spherical hot spot autoignitions, with different fuels, have identified different autoignitive regimes. These range from benign autoignition, with pressure waves of small amplitude, to superknock with the generation of damaging overpressures. Results of such simulations are generalised diagrammatically, by plotting boundary values of ξ, the ratio of acoustic to autoignition velocity, against ɛ. This latter parameter is the residence time of the developing acoustic wave in the hot spot of radius ro, namely ro/a, normalised by the excitation time for the chemical heat release, τe. This ratio controls the energy transfer into the developing acoustic front. A third relevant parameter involves the product of the activation temperature, E/R, for the autoignition delay time, τi, normalised by the mixture temperature. T, the ratio, τi/τe, and the dimensionless hot spot temperature gradient, ( ∂ ln T / ∂ r ¯ ) , where r ¯ is a dimensionless radius. These parameters define the boundaries of regimes of thermal explosion, subsonic autoignition, developing detonations, and nonautoignitive deflagrations, in plots of ξ against ɛ.The regime of developing detonation forms a peninsula and contours, throughout the field. The product parameter ( E / R T ) ( τ i / τ e ) / ∂ ln T / ∂ r ¯ expresses the influences of hot spot temperature gradient and fuel characteristics, and a unique value of it might serve as a boundary between autoignitive and deflagrative regimes. Other combustion regimes can also be identified, including a mixed regime of both autoignitive and “normal” deflagrative burning. The paper explores the performances of a number of different engines in the regimes of controlled autoignition, normal combustion, combustion with mild knock and, ultimately, superknock. The possible origins of hot spots are discussed and it is shown that the dissipation of turbulent energy alone is unlikely to lead directly to sufficiently energetic hot pots. The knocking characterisation of fuels also is discussed.

Kerschgens, B., Cai, L., Pitsch, H., Heuser, B. & Pischinger, S., Dinbuthylether, noctanol, and noctane as fuel candidates for diesel engine combustion. Combustion and Flame, 163, pp.6678. 2016.
In this study, three different \C8\ fuels, namely noctanol, dinbuthylether (DnBE), and noctane, are investigated with regard to diesel engine combustion to assess and analyze the effects of fuel structure. DnBE and noctanol are isomers, i.e. they have the same elementary composition (C8H18O), but a different structure and very different chemical and physical properties. The ignition behavior of these fuels in enginelike configurations is studied in terms of chemical modeling using recently published detailed kinetic mechanisms. The reduced and enhanced homogeneous ignition propensity of the alcohol and ether functional groups, respectively, are explained by reaction pathway analysis. However, the difference in low temperature reactivity of noctanol and noctane is relatively small compared to the difference in their cetane ratings. To study the engine behavior of these fuels, experiments and simulations were performed. A single cylinder diesel engine was operated with DnBE, noctanol, and noctane as single component fuels. For all three fuels, very low soot emissions at nitrogen oxide emissions within the Euro 6 regulation limits can be reached. Combustion and pollutant formation in the diesel engine are computed using the representative interactive flamelet approach. Results from the numerical simulations show good agreement with the experimental diesel engine tests. A subsequent analysis of the diesel engine simulation results shows that the differences in engine operation and carbon monoxide emissions can be attributed to the different ignitability, reflected by the different cetane ratings of the fuels. The substantially lower cetane rating of noctanol compared to noctane is explained by analysis of engine simulations and homogeneous reactor calculations. The effect of the different physical fuel properties does not play a significant role in the operation range studied. However, it is found that one of the main reasons for the large differences in cetane numbers between noctane and noctanol is the different stoichiometric mixture fraction, which leads in diesel engines to lower temperatures at the ignition location and hence to longer ignition delay times.

Farazi, S., Sayadi, T. & Pitsch, H., Numerical analysis of the drag force acting on the reactive single char particle under oxyfuel condition. In Proceedings of the China National Symposium on Combustion. 2016.

Kim, Y.J., Khetan, A., Wu, W., Chun, S.E., Viswanathan, V., Whitacre, J.F. & Bettinger, C.J., Evidence of PorphyrinLike Structures in Natural Melanin Pigments Using Electrochemical Fingerprinting. Advanced Materials, 28(16), pp.31733180. 2016.
Eumelanins are extended heterogeneous biopolymers composed of molecular<br />
subunits with ambiguous macromolecular topology. Here, an<br />
electrochemical fingerprinting technique is described, which suggests<br />
that natural eumelanin pigments contain indolebased tetramers that are<br />
arranged into porphyrinlike domains. Spectroscopy and density<br />
functional theory calculations suggest that sodium ions undergo<br />
occupancydependent stepwise insertion into the core of porphyrinlike<br />
tetramers in natural eumelanins at discrete potentials.

Korkmaz, M., Jochim, B., Zweigel, R., Beekmann, J., Abel, D. & Pitsch, H., Assessment of a tripleinjection strategy for PCCI diesel engine combustion. In THIESEL 2016  Conference on thermo and fluid dynamic processes in direct injection engines, September 13th16th, Valencia, Spain. 2016.

Gauding, M., Wick, A., Goebbert, J.H., Hempel, M., Peters, N. & Hasse, C., Generalized Energy Budget Equations for LargeEddy Simulations of Scalar Turbulence. In New Results in Numerical and Experimental Fluid Mechanics X. pp. 123133. 2016.
The energy transfer between different scales of a passive scalar<br />
advected by homogeneous isotropic turbulence is studied by an exact<br />
generalized transport equation for the second moment of the scalar<br />
increment. This equation can be interpreted as a scalebyscale energy<br />
budget equation, as it relates at a certain scale r terms representing<br />
the production, turbulent transport, diffusive transport and dissipation<br />
of scalar energy. These effects are analyzed by means of direct<br />
numerical simulation where each term is directly accessible. To this<br />
end, a variation of the Taylor microscale based Reynolds number between<br />
88 and 754 is performed. Understanding the energy transport between<br />
scales is crucial for LargeEddy Simulation (LES). For an analysis of<br />
the energy transfer in LES, a transport equation for the second moment<br />
of the filtered scalar increment is introduced. In this equation new<br />
terms appear due to the interaction between resolved and unresolved<br />
scales, which are analyzed in the context of an a priori and an a<br />
posteriori test. It is further shown that LES using an eddy viscosity<br />
approach is able to fulfill the correct interscale energy transport for<br />
the present configuration.

Boschung, J., Gauding, M., Hennig, F., Denker, D. & Pitsch, H., Corrections to the 4/5law for decaying turbulence. In APS Meeting Abstracts. 2016.

Farazi, S. & Pitsch, H., Fully resolved simulation of single and group particle combustion in an oxyfuel atmosphere. In Oxyflame International Workshop, February 10th11th, Montabaur, Germany. 2016.

Trisjono, P., Kang, S. & Pitsch, H., On a consistent highorder finite difference scheme with kinetic energy conservation for simulating turbulent reacting flows. J. Comput. Physics, 327, pp.612628. 2016.

Göbbert, J.H. & Bode, M., Case Study: Multiphase Flow Simulation Analysis with VisIt/LibSim. In Scalable HPC Visualization and Data Analysis using VisIt, Supercomputing 16, 13th18th November, Salt Lake City, Utah, United States. 2016.

Baroncelli, M., Schenk, M. & Pitsch, H., Time of Flight mass spectrometry for species measurements in a counterflow flame: methodology and test rig set up. Oxyflame International Workshop, February 10th11th, Montabaur, Germany. 2016.

Trisjono, P., Kleinheinz, K., Hawkes, E.R. & Pitsch, H., Modeling turbulence–chemistry interaction in lean premixed hydrogen flames with a strained flamelet model. Combustion and Flame, 174, pp.194207. 2016.
Premixed flames in the thin and broken reaction zones regime exhibit strong nonlinear turbulence–chemistry interactions and are hence challenging to model. In the present study, a direct numerical simulation (DNS) database for a hydrogen–air flame is used to understand the effect of turbulence–chemistry interactions on the flame speed. To this end, statistics of the local flame front displacement speed are investigated and the roles of flame stretch due to curvature and strain are analyzed. The local flame speed is found to be on average around 30% lower than the laminar burning velocity, which is shown to be a consequence of strain. Strain effects are then modeled by a recently proposed strained flamelet model, which is validated a priori against the DNS in terms of the flame speed and the reaction source term. The strained flamelet model leads to a moderate improvement of the flame speed prediction and is able to reproduce the decrease of the local flame speed. Moreover, it significantly improves the source term closure, which is demonstrated on the basis of the optimal estimator approach and, overall, describes the investigated DNS flame with very good accuracy.

Boschung, J., Gauding, M., Hennig, F., Denker, D. & Pitsch, H., Finite Reynolds number corrections of the 4/5 law for decaying turbulence. Phys. Rev. Fluids, 1(6), p.064403. 2016.

Cai, L., Pitsch, H., Mohamed, S.Y., Raman, V., Bugler, J., Curran, H. & Sarathy, S.M., Optimized reaction mechanism rate rules for ignition of normal alkanes. Combustion and Flame, 173, pp.468–482. 2016.

Trisjono, P., Kang, S. & Pitsch, H., On a consistent highorder finite difference scheme with kinetic energy conservation for simulating turbulent reacting flows. J. Comput. Phys., 327, pp.612628. 2016.

Keylock, C., Kida, S. & Peters, N., JSPS Supported Symposium on Interscale Transfers and Flow Topology in Equilibrium and Nonequilibrium Turbulence (Sheffield, UK, September 2014) Preface. Fluid Dynamics Research, 48(020001). 2016.

Davidovic, M., Bode, M., Falkenstein, T., Cai, L. & Pitsch, H., LES of ndodecane spray combustion and pollutant formation using a Multiple Representative Interactive Flamelet model. In LES for Internal Combustion Engine Flows LES4ICE, November 30thDecember 1st, France. 2016.

Qu, M., Kruse, S., Pitsch, H., Pallua, N. & Nourbakhsh, M., Viability of Human Composite Tissue Model for Experimental Study of Burns. Discovery Medicine, 22(119), pp.1928. 2016.
Experimental studies of burns are primarily performed with animal models that have important anatomical and physiological differences relative to human systems. The aim of this study was to develop a human experimental burn model using composite tissue obtained from bariatric surgery. We established a new protocol to maintain viable sections of human cutaneous and subcutaneous (sub/cutaneous) tissue in vitro. Under the conditions selected, multiparametric flow cytometry and histological analysis confirmed the viability and integrity of the human sub/cutaneous tissue for at least 5 days. Furthermore, we utilized a precision McKenna burner to inflict burns on the human tissue model under welldefined thermal conditions in vitro. Our data showed a localized, temporally restricted polarization of the resident macrophages in the subcutaneous human tissue in response to specific thermal forces. Therefore, our model provides a useful alternative to animal studies for further detailed investigations of human responses to injuries and treatments.

Deshmukh, A.Y., Mayer, D., Bode, M., Falkenstein, T., Pitsch, H., Khosravi, M., van Overbrüggen, T. & Schröder, W., LES of Direct Gas Injection in Internal Combustion Engines. In LES for Internal Combustion Engine Flows LES4ICE, November 30thDecember 1st, France. 2016.

Cai, L., Baroncelli, M., Kruse, S., Felsmann, D., Thies, C. & Pitsch, H., Extinction Characteristics of Methane Counterflow Flames under OxyFuel and Air Conditions. In Proceedings of the China National Symposium on Combustion. 2016.

Sudholt, A., Lee, C., Klankermayer, J., Fernandes, R.X. & Pitsch, H., Ignition characteristics of saturated and unsaturated furans. Combustion and Flame, 171, pp.133  136. 2016.

Baroncelli, M., Felsmann, D., Kruse, S., Beeckmann, J. & Pitsch, H., Extinction strain rates for coal light volatiles under oxyfuel conditions. Proceedings of the China National Symposium on Combustion. 2016.

Falkenstein, T., Kang, S., Davidovic, M., Bode, M. & Pitsch, H., LES of Internal Combustion Engine Flows Using Cartesian Overset Grids. In LES for Internal Combustion Engine Flows LES4ICE, November 30thDecember 1st, France. 2016.

Bode, M., Davidovic, M. & Pitsch, H., MultiScale Coupling for Predictive Injector Simulations. In JARAHPC Symposium JHPCS'16, October 4th5th, Aachen, Germany. 2016.

Burke, U., Beeckmann, J., Kopp, W.A., Uygun, Y., Olivier, H., Leonhard, K., Pitsch, H. & Heufer, K.A., A comprehensive experimental and kinetic modeling study of butanone. Combustion and Flame, 168, pp.296  309. 2016.
Through a large interdisciplinary approach the “Tailor Made Fuels from Biomass” (TMFB) cluster of excellence has been pursuing the identification of next generation biofuels. By first using chemical synthesis to procure suitable chemical components from biomass followed by initial screening experiments a large information database is compiled and can be used to guide fuel design. As a result of this method butanone has been identified as a particularly interesting target owing to its potential usage as a sparkignition fuel, thanks to its impressive knock resistant properties. This has motivated this study to consider the fundamental combustion chemistry controlling its reactivity under engine relevant conditions. A detailed chemical kinetic model which includes both high and lowtemperature oxidation reaction pathways for butanone is developed. This model (PCFCbutanone_v1) is validated using the available experimental data from the literature (included as supplemental material) and newly measured data collected during the course of this study. Ignition delay times are measured using both a shock tube and a rapid compression machine. They are measured for φ = 1.0 butanone/air mixtures covering a range of temperatures (850–1280 K) and pressures (20 and 40 bar) that incorporates engine relevant conditions. In addition, laminar burning velocities for butanone are measured for a range of equivalence ratios (0.7–1.3) and pressures (1 and 5 bar), an important parameter considering the potential use of butanone within spark ignition engines. The new model also incorporates new quantum chemical calculations of the thermodynamic parameters for butanone and the lowtemperature species associated with its oxidation. The thermodynamic parameters used in the model are necessary to calculate the reverse rate constants in the model making them important parameters, in particular in predicting the lowtemperature ignition delay times presented here. Butanone shows no evidence of negative temperature coefficient behavior. However, inclusion of the lowtemperature oxidation pathways is shown to be important to accurately predict the lowtemperature ignition delay times of butanone. Of particular importance are the radical βscission and HȮ2 elimination reactions which are essential in accurately predicting the ignition delay times.

Göbbert, J.H., Bode, M. & Wylie, B.J.N., ExtremeScale InSitu Visualization of Turbulent Flows on IBM Blue Gene/Q JUQUEEN. In ISC High Performance, June 19th23rd, Frankfurt, Germany. 2016.

Sudholt, A., Lee, C., Klankermayer, J., Fernandes, R.X. & Pitsch, H., Ignition characteristics of saturated and unsaturated furans. Combustion and Flame, 171, pp.133136. 2016.

Göbbert, J.H., Bode, M., Lintermann, A. & Zilken, H., Lowering the Barriers to InSitu Visualization. In ISC Workshop on InSitu Visualization WOIV, June 23rd, Frankfurt, Germany. 2016.

Peters, N., Boschung, J., Gauding, M., Goebbert, J.H., Hill, R.J. & Pitsch, H., Higherorder dissipation in the theory of homogeneous isotropic turbulence. Journal of Fluid Mechanics, 803, pp.250274. 2016.
The twopoint theory of homogeneous isotropic turbulence is extended to source terms appearing in the equations for higherorder structure functions. For this, transport equations for these source terms are derived. We focus on the trace of the resulting equations, which is of particular interest because it is invariant and therefore independent of the coordinate system. In the trace of the evenorder source term equation, we discover the higherorder moments of the dissipation distribution, and the individual evenorder source term equations contain the higherorder moments of the longitudinal, transverse and mixed dissipation distribution functions. This shows for the first time that dissipation fluctuations, on which most of the phenomenological intermittency models are based, are contained in the Navier–Stokes equations. Noticeably, we also find the volumeaveraged dissipation εr used by Kolmogorov (J. Fluid Mech., vol. 13, 1962, pp. 82–85) in the resulting system of equations, because it is related to dissipation correlations.

Beeckmann, J., Cai, L., Schaback, D., Hesse, R. & Pitsch, H., Reduced Chemical Mechanism for the Calculation of Ethanol / Air Flame Speeds. In SAE Technical Paper Series. p. 6. 2015.
Ethanol currently remains the leading biofuel in the transportation sector, with special focus on spark ignition engines, as a pure as well as a blend component. In order to provide reliable numerical simulations of gasoline combustion processes under the influence of ethanol for modern engine research, it is mandatory to develop well validated detailed kinetic combustion models. One key parameter for the numerical simulation is the laminar burning velocity. Under the aspect of minimizing the general simulation effort for burning velocities, wellvalidated models have to be reduced. As a base kinetic mechanism for the reduction and optimisation process with respect to burning velocity calculations, a detailed model presented by Zhao et al. (Int. J. Chem. Kin. 40 (1) (2007) 118) is chosen. The model has been extensively validated against shock tube, rapid compression machine and burning velocity data. The detailed model consists of 55 species and 290 reactions. A stochastic model calibration approach is undertaken for the optimisation of the base mechanism against data found in the literature. New experimental data at 5 bar and 373 K are used for validation. The optimised mechanism is significantly reduced within this work applying a multistage reduction strategy using the directed relation graph with error propagation (DRGEP) technique. The reduced mechanism is again validated with the experimental data used before. Overall, the reduced mechanism consists of 36 species and 215 reactions. It predicts experimental flame speeds very well.

Jocher, A., Pitsch, H., Gomez, T., Bonnety, J. & Legros, G., Impact of magnetic fields on the stability of nonpremixed flames. In Proceedings of the 7th European Combustion Meeting, March 30thApril 2nd, Budapest, Hungary. 2015.

Beeckmann, J., Hesse, R., Cai, L., Pitsch, H., Heufer, A. & Yang, Y., 2Butanone Laminar Burning Velocities  Experimental and Kinetic Modelling Study. In SAE Technical Paper Series. p. 10. 2015.

Bode, M., Falkenstein, T., Chenadec, V.L., Kang, S., Pitsch, H., Arima, T. & Taniguchi, H., A New Euler/Lagrange Approach for Multiphase Simulations of a MultiHole GDI Injector. In SAE Technical Paper. SAE International. 2015.
Compared to conventional injection techniques, Gasoline Direct Injection (GDI) has a lot of advantages such as increased fuel efficiency, high power output and low emission levels, which can be more accurately controlled. Therefore, this technique is an important topic of today's injection system research.Although the operating conditions of GDI injectors are simpler from a numerical point of view because of smaller Reynolds and Weber numbers compared to Diesel injection systems, accurate simulations of the breakup in the vicinity of the nozzle are very challenging. Combined with the complications of experimental techniques that could be applied inside the nozzle and at the nozzle exit, this is the reason for the lack of understanding the primary breakup behavior of current GDI injectors.In this work, this issue is addressed by combining highfidelity primary breakup simulations in the vicinity of the nozzle exit, which use the velocity profile at the nozzle exit as boundary condition, and common Lagrange spray simulations. In detail, these enhanced simulations of a 6hole stateoftheart GDI injector are compared with the results of fullyLagrange spray (FLS) simulations and experiments.It is shown that the accuracy of common Lagrange spray simulations can be increased by the combination with highfidelity primary breakup simulations. This approach avoids the necessity of assuming a droplet size distribution at the nozzle exit, which requires usually tuning by using experimental data. Thus, its potential for apriori spray predictions, which are for example desired within the injector design process, is much higher.

Sudholt, A., Glaude, P.A., BattinLeclerc, F. & Pitsch, H., The oxidation of γvalerolactone in a low pressure ﬂame. In Proceedings of the 7th European Combustion Symposium, March 30thApril 2nd, Budapest, Hungary. 2015.

Zweigel, R., Korkmaz, M., Pitsch, H. & Abel, D., Modellbasierte Prädiktive Regelung der dieselmotorischen Niedertemperaturverbrennung unter Berücksichtigung von Emissionen. AUTOREG 2015  Auf dem Weg zum automatisierten Fahren, 7. Fachtagung BadenBaden, June 9th10th, VDIBerichte 2233, pp.317328. 2015.

Foo, K.K., Jocher, A., Thong, C.X., Dally, B., Nathan, G., Medwell, P.R., Alwahabi, Z.T. & Pitsch, H., Preliminary Experimental and Computational Study of Time varying Laminar NonPremixed Ethylene/Nitrogen Flames. Australian Combustion Symposium, December 7th9th, Melbourne, Australia. 2015.

Jochim, B., Zweigel, R., Korkmaz, M., Abel, D. & Pitsch, H., Reduced Order Combustion Model For A Virtual Control Testbed Of A PCCI Diesel Engine. In Symposium for Combustion Control, June 17th18th, Aachen, Germany. pp. 3540. 2015.

Gauding, M., Boschung, J., Hasse, C. & Peters, N., Dissipative Range Scaling of Higher Order Structure Functions
for Velocity and Passive Scalars. In 15th European Turbulence Conference (ETC15), August 25th28th, Delft, Netherlands. 2015.

Falkenstein, T., Bode, M., Kang, S., Pitsch, H., Arima, T. & Taniguchi, H., LargeEddy Simulation Study on Unsteady Effects in a Statistically Stationary SI Engine Port Flow. In SAE Technical Paper. SAE International. 2015.
Although sparkignited engines have a considerable development history, the relevant flow physics and geometry design implications are still not fully understood. One reason is the lack of experimental and numerical methods with sufficiently high resolution or capabilities of capturing stochastic phenomena which could be used as part of the development cycle. More recently, LargeEddy simulation (LES) has been identified as a promising technique to establish a better understanding of incylinder flow variations. However, simulations of engine configurations are challenging due to resolution as well as modeling requirements and computational cost for these unsteady multiphysics problems. LES on full engine geometries can even be prohibitively expensive. For this reason, the size of the computational LES domain is here reduced to the region of physical interest and boundary conditions are obtained from a RANS simulation of the whole experimental flow domain. This approach required modifications to the compressible in and outflow boundary conditions of the highly accurate structured LES framework. The extended method allows for oblique in and outflows with an assumed velocity profile. Results for canonical test cases are presented to verify the approach and show its limitations. The extended numerical framework is used to study port flow under steady boundary conditions in a stateoftheart gasoline engine geometry using boundary conditions from a RANS simulation for the whole flow domain. Time averaged results are validated against PIV measurement data and port flow performance parameters are evaluated. Additionally, comparison to the classical RANS approach is made. Effects of stochastic flow phenomena are analyzed in detail, which make port flow performance parameters like, e.g. tumble ratio, hard to predict by conventional ensembleaveraged methods. Fluctuations in pressure and velocity fields mostly originate from the complex incylinder flow, but propagate upstream to the chosen inflow boundary location.

Sapunkov, O., Pande, V., Khetan, A., Choomwattana, C. & Viswanathan, V., Quantifying the promise of 'beyond' Liion batteries. Translational Material Research, 2(4). 2015.
There is a growing consensus that future specific energy improvements in<br />
Liion batteries may not ever be sufficient to allow mass market<br />
adoption of electric vehicles, as we approach the physical limits of<br />
storage capacity of current Liion batteries. Several `beyond liion'<br />
(BLI) chemistries are being explored as possible highenergydensity<br />
alternatives to Liion batteries. In this article, we focus on analyzing<br />
three BLI battery systems: Liair, Lisulphur and Naair. We present a<br />
comprehensive discussion of the fundamental material challenges<br />
associated with these chemistries and document the progress being made<br />
in translating nextgeneration battery systems from the lab to the<br />
market. We also carry out a critical examination of the hype surrounding<br />
emerging battery technologies. We report, for the first time, a hype<br />
chart for batteries akin to those popularized by Gartner, Inc. for<br />
emerging technologies. We expect this hype chart to give us better<br />
insights on the respective standings of the current BLI technologies.

Sakai, Y., Ando, H., Chakravarty, H.K., Pitsch, H. & Fernandes, R.X., A computational study on the kinetics of unimolecular reactions of ethoxyethylperoxy radicals employing CTST and VTST. Proceedings of the Combustion Institute, 35(1), pp.161169. 2015.
Diethyl ether (DEE) has favorable properties as a diesel fuel component, including its outstanding cetane number. To utilize this promising fuel, more and more knowledge on the chemical kinetics of \DEE\ oxidation will be required. For the present article, the rate constants of unimolecular reactions of ethoxyethylperoxy radicals, which are main intermediates in the oxidation of \DEE\ under the engine relevant conditions, have been computationally investigated and compared with those of alkanes. Geometries, vibrational frequencies, and energies of reactants, products, and transition states with pronounced barrier were calculated according to the procedure of the CBSQB3 method. The highpressure limiting rate constants were calculated in the temperature range of 500−2000 K by using a conventional transition state theory with hindered rotor approximation for low frequency torsional mode. The oxygen dissociation reactions have been investigated by using a variational transition state theory based on the CASPT2(7,5)/augccpvdz single point calculations at UB3LYP/CBSB7 geometries and vibrational frequencies. It was found that the oxidation pathways are equal to those of alkane oxidations, however, the rate constants are significantly different from those of alkanes due to the oxygen vicinity. The rate constants of intramolecular hydrogen shift reactions are from 3 to 8 times larger at 700 K than those of alkylic peroxy radical when the abstracted hydrogen is in the βposition of the ether. The rate constant of βscission reactions for 1,5intramolecular hydrogen shift products of 1ethoxyethylperoxy radial is 163 times larger at 700 K than that of alkylic hydroperoxy radical, and this reaction becomes a main reaction pathway, whereas cyclicether is a main product in alkane oxidation. These characteristic rate constants are given in threeparameter modified Arrhenius form for the refinement of predictive chemical kinetic models being developed.

Baroncelli, M., Kruse, S. & Pitsch, H., Experimental and numerical investigation of extinction strain rates of coal devolatilization products under oxyfuel conditions. In Proceedings of the European Combustion Meeting. 2015.

Boschung, J., Exact relations between the moments of dissipation and longitudinal velocity derivatives in turbulent flows. Physical Review E, 92(4), p.043013. 2015.
Following an approach by Siggia, we present coefficients C(n) relating the moments of the dissipation of kinetic energy 〈ɛ〉 and the longitudinal velocity gradient 〈∂u(1)/∂x(1)〉 under the assumption of isotropy and continuity. Particularly, we find that the moment 〈ɛ(n)〉 of order n is completely determined by 〈(∂u(1)/∂x(1))(2n)〉 and an order (and viscosity) dependent coefficient for all n under the assumption of (local) isotropy. This implies that all theories which specify 〈ɛ(n)〉 also implicitly determine 〈(∂u(1)/∂x(1))(2n)〉 and vice versa. As a corollary to the direct connection between the moments of the dissipation and the longitudinal velocity gradient, the even standardized moments of order 2n of ∂u(1)/∂x(1) (flatness, hyperflatness, and so on) are directly related to the ratio of the moments 〈ɛ(n)〉/〈ɛ〉(n). We compare the theoretical values of the coefficients C(n) up to n=6 with homogeneous isotropic DNS data ranging from Re(λ)=88 to Re(λ)=529.

Fiorina, B., Mercier, R., Kuenne, G., Ketelheun, A., Avdić, A., Janicka, J., Geyer, D., Dreizler, A., Alenius, E., Duwig, C., Trisjono, P., Kleinheinz, K., Kang, S., Pitsch, H., Proch, F., Marincola, F.C. & Kempf, A., Challenging modeling strategies for LES of nonadiabatic turbulent stratified combustion. Combustion and Flame, 162(11), pp.4264  4282. 2015.
Five different lowMach large eddy simulations are compared to the turbulent stratified flame experiments conducted at the Technical University of Darmstadt (TUD). The simulations were contributed by TUD, the Institute for Combustion Technology (ITV) at Aachen, Lund University (LUND), the \EM2C\ laboratory at Ecole Centrale Paris, and the University of DuisburgEssen (UDE). Combustion is modeled by a premixed flamelet tabulation with local flame thickening (TUD), a premixed flamelet progress variable approach coupled to a level set method (ITV), a 4steps mechanism combined with implicit ŁES\ (LUND), the FTACLES model that is based on filtered premixed flamelet tabulation (EM2C), and a flame surface density approach (UDE). An extensive comparison of simulation and experimental data is presented for the first two moments of velocity, temperature, mixture fraction, and major species mass fractions. The importance of heatlosses was assessed by comparing simulations for adiabatic and isothermal boundary conditions at the burner walls. The adiabatic computations predict a flame anchored on the burner lip, while the nonadiabatic simulations show a flame liftoff of one half pilot diameter and a better agreement with experimental evidence for temperature and species concentrations. Most simulations agree on the mean flame brush position, but it is evident that subgrid turbulence must be considered to achieve the correct turbulent flame speed. Qualitative comparisons of instantaneous snapshots of the flame show differences in the size of the resolved flame wrinkling patterns. These differences are (a) caused by the influence of the ŁES\ combustion model on the flame dynamics and (b) by the different simulation strategies in terms of grid, inlet condition and numerics. The simulations were conducted with approaches optimized for different objectives, for example low computational cost, or in another case, short turn around.

Beeckmann, J., Cai, L., Berens, A., Peters, N. & Pitsch, H., An analytical approximation for low and hightemperature autoignition for dimethyl ether–air mixtures. Proceedings of the Combustion Institute, 35(1), pp.275281. 2015.
Dimethyl ether has proven to be one of the most attractive alternatives to traditional fossil oil derived fuels for compression ignition engines. In this study, a skeletal mechanism consisting of 32 species and 49 elementary reactions, based on the detailed mechanism proposed by Fischer et al. Int. J. Chem. Kinet. 32 (12) (2000) 713–740, is further reduced to a short 36step mechanism describing first and second stage ignition as well as the rapid transition to final products. A global 4step mechanism is derived by introducing steady state assumptions of intermediate species. An analytical solution for the ignition delay times of the first stage of ignition in the low temperature regime and the beginning of its transition to the intermediate temperature regime is presented. The important competition of the β scission and the reaction with molecular oxygen of the hydroperoxymethoxymethyl radical (C2OCH2O2H) is quantified by the introduction of a parameter β , related to the competition of chainbranching and chainpropagation. The calculated values agree very well with those of the 36step mechanism. Also for the high temperature regime, an analytical solution is presented, which agrees well with the experiments and the values calculated with the 36step mechanism.

Schmitt, C. & Pitsch, H., Reactive linearized equations of perturbed compressible variables for lowMach number variabledensity flows. Journal of Computational Physics, 281, pp.127. 2015.
A hydrodynamic/acoustic splitting approach is proposed to study noise emitted from reactive variabledensity flows. A simulation using the variabledensity lowMach number equations provides a solution of the hydrodynamic motions of the flow (baseflow), and a set of equations for perturbed variables is additionally solved to capture the acoustic motions. A rigorous derivation of these equations for the assumed baseflow is given, which compared to its nonreacting counterpart includes additional terms related to variabledensity flows. Two different test cases are presented. First, the Kirchhoff vortex is simulated to highlight instability issues related to constantdensity flows. Second, an academic test case for variabledensity flows is proposed in the form of a reacting dipole, which is used to underline the stability of the proposed perturbed equations in such a scenario. Various intermediate forms of the derived perturbation equations are juxtaposed and analyzed with respect to their stability for these two test cases, and assumptions made in their derivation are numerically justified.

Attili, A., Bisetti, F., Mueller, M.E. & Pitsch, H., Damköhler number effects on soot formation and growth in turbulent nonpremixed flames. Proceedings of the Combustion Institute, 35(2), pp.12151223. 2015.
The effect of Damköhler number on turbulent nonpremixed sooting flames is investigated via large scale direct numerical simulation in threedimensional nheptane/air jet flames at a jet Reynolds number of 15,000 and at three different Damköhler numbers. A reduced chemical mechanism, which includes the soot precursor naphthalene, and a highorder method of moments are employed. At the highest Damköhler number, local extinction is negligible, while flames holes are observed in the two lowest Damköhler number cases. Compared to temperature and other species controlled by fuel oxidation chemistry, naphthalene is found to be affected more significantly by the Damköhler number. Consequently, the overall soot mass fraction decreases by more than one order of magnitude for a fourfold decrease of the Damköhler number. On the contrary, the overall number density of soot particles is approximately the same, but its distribution in mixture fraction space is different in the three cases. The total soot mass growth rate is found to be proportional to the Damköhler number. In the two lowest Da number cases, soot leakage across the flame is observed. Leveraging Lagrangian statistics, it is concluded that soot leakage is due to patches of soot that cross the stoichiometric surface through flame holes. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames.

Knudsen, E., Shashank, & Pitsch, H., Modeling partially premixed combustion behavior in multiphase LES. Combustion and Flame, 162(1), pp.159180. 2015.
A partially premixed flamelet model is extended to account for liquid fuel, and its accuracy and dependence on spray modeling inputs are investigated. The spray and combustion model coupling is considered by simulating single drop evaporation experiments. These simulations confirmed previous findings showing that small changes in spray model assumptions significantly influence evaporation rates. The analysis is then extended to Large Eddy Simulation (LES) using a \NASA\ model aircraft combustor burning JetA fuel. Results indicate the partially premixed flamelet approach can describe the range of combustion physics found in the burner. However, they also reveal that the liquid model sensitivities hold in LES, where a tight coupling exists between combustion model predictions and spray model assumptions. Details of the liquid modeling approach, including thermodynamic specifications and descriptions of liquid spray at the nozzle outlet, influence the burning regime, mean gas temperatures, and emissions levels predicted by LES. The results suggest that while the combustion model is capable, spray model influences must be firmly understood before validation can be accomplished in liquid fueled contexts.

Cai, L., Uygun, Y., Togbé, C., Pitsch, H., Olivier, H., Dagaut, P. & Sarathy, S.M., An experimental and modeling study of noctanol combustion. Proceedings of the Combustion Institute, 35(1), pp.419427. 2015.
This study presents the first investigation on the combustion chemistry of noctanol, a long chain alcohol. Ignition delay times were determined experimentally in a highpressure shock tube, and stable species concentration profiles were obtained in a jet stirred reactor for a range of initial conditions. A detailed kinetic model was developed to describe the oxidation of noctanol at both low and high temperatures, and the model shows good agreement with the present dataset. The fuel’s combustion characteristics are compared to those of nalkanes and to short chain alcohols to illustrate the effects of the hydroxyl moiety and the carbon chain length on important combustion properties. Finally, the results are discussed in detail.

Pitsch, H. & Williams, F.A., Professor Norbert Peters (19422015). Combustion and Flame, 162(10), pp.3447  3448. 2015.

Pitsch, H. & Williams, F.A., Professor Norbert Peters 10 July 1942–4 July 2015 Obituary. Journal of Fluid Mechanics, 781, pp.12. 2015.

Khetan, A., Luntz, A. & Viswanathan, V., TradeOffs in Capacity and Rechargeability in Nonaqueous LiO2 Batteries: SolutionDriven Growth versus Nucleophilic Stability. The Journal of Physical Chemistry Letters, 6(7), pp.12541259. 2015.
The development of highcapacity rechargeable Li–O2 batteries requires the identification of stable solvents that can promote a solutionbased discharge mechanism, which has been shown to result in higher discharge capacities. Solutiondriven discharge product growth requires dissolution of the adsorbed intermediate LiO2*, thus generating solvated Li+ and O2– ions. Such a mechanism is possible in solvents with high Gutmann donor or acceptor numbers. However, O2– is a strong nucleophile and is known to attack solvents via proton/hydrogen abstraction or substitution. This kind of a parasitic process is extremely detrimental to the battery’s rechargeability. In this work, we develop a thermodynamic model to describe these two effects and demonstrate an anticorrelation between solvents’ stability and their ability to enhance capacity via solutionmediated discharge product growth. We analyze the commonly used solvents in the same framework and describe why solvents that can promote higher discharge capacity are also prone to degradation. Solvating additives for practical Li–O2 batteries will have to be outliers to this observed anticorrelation.

Deshmukh, A., Bode, M., Pitsch, H., Kirsch, V., Reddemann, M., Palmer, J., Kneer, R., van Overbrüggen, T., Schröder, W., Liebergesell, B., Bardow, A., Ottenwälder, T. & Pischinger, S., Spray breakup and mixture formation of novel biofuels. In 3rd TMFB International Conference, June 23rd25th, Aachen, Germany. 2015.

Kim, S.H. & Pitsch, H., On the lattice Boltzmann method for multiphase flows with large density ratios. Journal of Computational Physics, 303, pp.19  27. 2015.
An analysis of the lattice Boltzmann (LB) method for multiphase flows with large density ratios is presented. It is shown that for incompressible, multiphase LB methods, the divergencefree condition is not satisfied within the formal accuracy of the LB method, when the density ratio between the two phases is large enough. The discrete differentiationbyparts rule is responsible for this error. A new multiphase LB method to resolve this issue is proposed.

Stoehr, K.D., Peters, N. & Beeckmann, J., Low temperature oscillations of DME combustion in a jetstirred reactor. Proceedings of the Combustion Institute, 35(3), pp.36013607. 2015.
The low temperature oxidation behavior of \DME\ in a jetstirred reactor is investigated. There have been numerous investigations on the low temperature reaction kinetics of DME. In this work, the focus is set on the instabilities that arise at very low temperatures (in the order of 580 K) in a jetstirred reactor and a numerical analysis of the source of these oscillations is performed. A stability diagram was created where the parameters at which the oscillations were observed are displayed. The temperature oscillations at higher temperatures (around 1000 K) have been shown to be caused by ignition–extinction phenomena, whereas the oscillations observed at low temperatures are believed to be of thermokinetical origin, with the buildup of a semistable species (hydroperoxidemethylformate HPMF), its dissociation at increasing temperatures with the subsequent formation of large quantities of formaldehyde that increase the temperature giving rise to the firststage ignition observed and at the same time act as inhibitor to the reaction. The continuous flow through the reactor brings the system back to its original state, and the process begins anew.

Hammer, N., Satzger, H., Jamitzky, F., Allalen, M., Block, A., Karmakar, A., Brehm, M., Bader, R., Iapichino, L., Ragagnin, A., Karakasis, V., Kranzlmüller, D., Bode, A., Huber, H., Kühn, M., Machado, R., Grünewald, D., Edelmann, P.V.F., Röpke, F.K., Wittmann, M., Zeiser, T., Wellein, G., Mathias, G., Schwörer, M., Lorenzen, K., Federrath, C., Klessen, R., Bamberg, K., Ruhl, H., Schornbaum, F., Bauer, M., Nikhil, A., Qi, J., Klimach, H., Stüben, H., Deshmukh, A., Falkenstein, T., Dolag, K. & Petkova, M., Extreme ScaleOut SuperMUC Phase 2, lessons learned. ParCo. 2015.

Bode, M., Göbbert, J.H. & Pitsch, H., Novel multiphase simulations investigating cavitation by use of insitu visualization and Euler/Lagrange coupling. In PRACEdays15, May 26th28th, Dublin, Ireland. 2015.

Göbbert, J.H., Gauding, M., Ansorge, C., Hentschel, B., Kuhlen, T. & Pitsch, H., Direct Numerical Simulation of Fluid Turbulence at Extreme Scale with psOpen. ParCo. 2015.

Bode, M., Deshmukh, A., Kirsch, V., Reddemann, M., Kneer, R. & Pitsch, H., Direct numerical simulations of novel biofuels for predicting spray characteristics. In ICLASS 2015, 13th International Conference on Liquid Atomization and Spray Systems, August 23rd27th, Tainan, Taiwan. 2015.

Trisjono, P. & Pitsch, H., Systematic Analysis Strategies for the Development of Combustion Models from DNS: A Review. Flow, Turbulence and Combustion, 95(23), pp.231259. 2015.
Direct numerical simulation (DNS) of turbulent combustion is a research area becoming ever more important and has been established as a powerful tool in combustion science to complement theory and experiment. The rapid advancement of supercomputing has lately enabled a series of interesting DNS studies with some practical relevance. This advent along with the severe lack of experimental data sets with a high degree of data completeness motivates the use of DNS for combustion modeling. Simultaneously, enormous progress is made in the utilization of DNS data as a valuable resource to develop and validate combustion models from DNS data. In this paper, several promising and useful analysis techniques are identified and reviewed. Their usefulness is demonstrated on the basis of selected modeling studies, which comprise examples of various commonly employed modeling frameworks, and which integrate DNS in a systematic way into the process of model development and validation. It is expected that the modeling routes outlined here can be used to address currently open modeling questions and to advance the fidelity of existing models.

Kerschgens, B., Cai, L., Pitsch, H., Janssen, A., Jakob, M. & Pischinger, S., Surrogate fuels for the simulation of diesel engine combustion of novel biofuels. International Journal of Engine Research, 16(4), pp.531546. 2015.
Recently, several promising biomassderived fuels for diesel engines have been identified, produced, and tested. Diesel engine experiments confirmed very low soot and low nitrogen oxide emissions. With regard to further improvements of the combustion system, it is desirable to complement the diesel engine experiments with numerical simulations. To date, this is hindered by the lack of suitable chemical reaction mechanisms for these novel fuels. Therefore, a surrogate approach is presented here and applied in computational fluid dynamics simulations. Combustion and pollutant formation is simulated using the representative interactive flamelet model. Ignition, combustion, and pollutant formation are described in a consistent manner by inclusion of detailed reaction chemistry. Different mixtures of nheptane, toluene, ethanol, dimethylether, ethane, and phenol are employed to describe the combustion chemistry of the biofuels. The compositions of the surrogate fuels are compiled according to hydrogen/carbon ratio, oxygen content, cetane rating, and molecular properties of the experimental fuels. Spray, injection, and evaporation properties of the experimental fuels, as obtained from spray vessel experiments, are included in the computational fluid dynamics simulations. By systematic comparison of experimental and numerical results, the surrogate methodology is validated and an improved understanding of the limitations of the current surrogates is achieved. Thus, a methodology for the fast adoption of novel fuels for simulations is proposed that can be used regardless of the availability of specific chemical reaction mechanisms.

Gauding, M., Goebbert, J.H., Hasse, C. & Peters, N., Line segments in homogeneous scalar turbulence. Physics of Fluids, 27(9). 2015.
The local structure of a turbulent scalar field in homogeneous isotropic turbulence is analyzed by direct numerical simulations (DNS) with different Taylor microscale based Reynolds numbers between 119 and 529. A novel signal decomposition approach is introduced where the signal of the scalar along a straight line is partitioned into segments based on the local extremal points of the scalar field. These segments are then parameterized by the distance ℓ between adjacent extremal points and the scalar difference Δϕ at the extrema. Both variables are statistical quantities and a joint distribution function of these quantities contains most information to statistically describe the scalar field. It is highlighted that the marginal distribution function of the length becomes independent of Reynolds number when normalized by the mean length ℓm. From a statistical approach, it is further shown that the mean length scales with the Kolmogorov length, which is also confirmed by DNS. For turbulent mixing, the scalar gradient plays a paramount role. Turbulent scalar fields are characterized by clifframplike structures manifesting the occurrence of localized large scalar gradients. To study turbulent mixing, a segmentbased gradient is defined as Δϕ/ℓ. Joint statistics of the length and the segmentbased gradient provide novel understanding of clifframplike structures. Ramplike structures are unveiled by the asymmetry of the joint distribution function of the segmentbased gradient and the length. Clifflike structures are further analyzed by conditional statistics and it is shown from DNS that the width of cliffs scales with the Kolmogorov length scale.

Mayer, D., Moshammer, K., Cai, L., Pitsch, H. & KohseHöinghaus, K., A numerical study of highlydiluted, burnerstabilised dimethyl ether flames. Combustion Theory and Modelling, 19(2), pp.238259. 2015.

Cai, L. & Pitsch, H., Optimized chemical mechanism for combustion of gasoline surrogate fuels. Combustion and Flame, 162(5), pp.1623  1637. 2015.
Since real petroleum fuels are composed of a huge variety of hydrocarbon components, surrogate mixtures of various hydrocarbon fuels are typically employed in computational research and in engine development to represent transportation fuels. In this study, a reduced combustion mechanism of Primary Reference Fuel (PRF) mixtures (nheptane and isooctane) is integrated into the published kinetic model (Narayanaswamy et al., 2010), allowing for the formulation of multicomponent surrogate fuels (e.g. PRF/toluene) and for the prediction of Polycyclic Aromatic Hydrocarbon (PAH) formation in gasoline engines. In order to optimize the model performance, a recently developed optimization technique based on rate rules (Cai and Pitsch, 2014) is extended in this study. The goal is to calibrate automatically the multicomponent kinetic mechanism, which also leads to a chemically consistent \PRF\ mechanism and a computational advantage for the calibration process. In addition, this work contributes to the development of general rate rules for various hydrocarbon fuels. An ethanol model is also incorporated into the proposed mechanism. This facilitates the prediction of gasoline/ethanol blend combustion. The resulting mechanism retains a compact size and is successfully validated against experimental measurements.

Sudholt, A., Cai, L., Heyne, J., Haas, F.M., Pitsch, H. & Dryer, F.L., Ignition characteristics of a bioderived class of saturated and unsaturated furans for engine applications. Proceedings of the Combustion Institute, 35(3), pp.29572965. 2015.
Ignition characteristics in the form of derived cetane numbers (DCN) of (hydro) furanic species are investigated experimentally in an Ignition Quality Tester. Further, quantum chemistry calculations at CBSQB3 level of theory are applied to determine bond dissociation energies (BDEs) and thereby suggest the initial reactions of the ignition process for all of these fuels. Using the calculated BDEs, it is found that the ignition characteristics are similar among furans and among tetrahydrofurans, but strongly differ between these molecular classes. It is shown that the ignition behavior of aromatic furans is determined by the ring structure, which correlates with a negligible side chain influence. Hence, furan fuel structures can be chosen with respect to feasibility of the production pathways and engine compatibility. On the contrary, the side chain length for tetrahydrofurans defines the potential application paradigm. Tetrahydrofurans with short side chains are candidates for \SI\ application, whereas 2butyltetrahydrofuran may be a candidate for diesel application. The influence of the number and location of double bonds in the ring is illustrated with the additional study of dihydrofurans, and the influence of other functional groups is evaluated for (tetrahydro) furfuryl alcohols. To investigate possible fuel application scenarios, a second part of this study investigates \DCNs\ of furanic fuel blends in nheptane and diesel fuel. A \DCN\ mixing rule is found to be approximately linear and for blends with up to 20 mol% furans, the side chain structure (alkyl, alcohol) has no distinct influence on the blend DCN.

Boschung, J., Gauding, M., Hennig, F., Peters, N. & Pitsch, H., An alternative definition of order dependent dissipation scales, 15th European Turbulence Conference, August 25th28th, Delft, the Netherlands. 2015.

Bode, M., Falkenstein, T., Pitsch, H., Kimijima, T., Taniguchi, H. & Arima, T., Numerical study on the impact of cavitation on the spray development
processes for GDI Injection. In ICLASS 2015, 13th International Conference on Liquid Atomization and Spray Systems, August 23rd27th, Tainan, Taiwan. 2015.

Varea, E., Beeckmann, J., Pitsch, H., Chen, Z. & Renou, B., Determination of burning velocities from spherically expanding H2/air flames. Proceedings of the Combustion Institute, 35(1), pp.711719. 2015.
Laminar burning velocities of hydrogen/air mixtures can show discrepancies up to 30%, making chemical mechanism validation and improvement difficult. The source of uncertainties may come from different factors influencing at each processing and postprocessing steps the final value. Considering a spherically expanding flame configuration, reflection on the accuracy of the formulations, used to derive the desired quantity, is proposed. Starting from the exact definition of the laminar burning velocity, two formulations – direct and indirect flame speeds formulations – are derived for spherical flames. Each single source of uncertainty involved in the formulations is pointed out. The emphasis is focused on a specific mixture at an equivalence ratio of 0.50, atmospheric pressure, and an initial temperature of 300 K. This point represents the best tradeoff between low ratio of flame velocity and recording sampling rate and the occurrence of cellular flames (Le < 1). An extensive experimental and numerical study (1D spherically expanding flames) of this mixture is carried out. As a result, the experimental laminar burning velocities determined by using the direct flame speed or the indirect flame speed formulae depict different values. However, when numerically determined, both formulae yield the same value. This paves the way to understand and identify the experimental error sources. Stretch and Lewis numbers effects (superadiabatic temperatures) as well as radiation processes (burned gas motion) are studied. Nonetheless, they do not show to be the main source of uncertainty. The extrapolation procedure (linear or nonlinear) according to the limited number of experimental points (rapid apparition of cellular structure) appears as the main factor influencing the discrepancy in laminar burning velocities.

Ye, J., Medwell, P.R., Varea, E., Kruse, S., Dally, B.B. & Pitsch, H.G., An experimental study on MILD combustion of prevaporised liquid fuels. Applied Energy, 151, pp.93  101. 2015.
This paper presents an experimental study on moderate or intense low oxygen dilution (MILD) combustion of prevaporised liquid fuels burning in a reverseflow \MILD\ combustor under elevated pressures. The influence of fuel type, equivalence ratio, carrier gas, operating pressure and air jet velocity on the combustion stability and emissions are investigated. Ethanol, acetone and nheptane are vaporised and carried to the combustor using either nitrogen or air. It is found that the combustion stability is highly dependent on fuel type, with nheptane being the most unstable due to its fast ignition under all highpressure conditions studied. Measured \CO\ emissions emitted from all fuels are very low except when the equivalence ratio approaches the lean extinction limit, and this effect is not dependent on the pressure. The joint regime of low \CO\ and \NOx\ emission becomes narrower under elevated pressure as \NOx\ emissions emitted from all fuels increased with pressure. The enhanced \NOx\ formation rate via the nitrous oxide mechanism, the slower mixing, the increased flame temperature and residence time are believed to cause higher \NOx\ emissions as pressure increases. The \NOx\ emissions are reduced by increasing the air jet velocity, which is attributed to a lower peak temperature. The \NOx\ emissions are also reduced when the fuel is carried by nitrogen instead of air. Further research is required to understand this trend which will help in reducing \NOx\ emissions under these conditions, especially at elevated pressures.

Jocher, A., Pitsch, H., Gomez, T. & Legros, G., Modification of sooting tendency by magnetic effects. Proceedings of the Combustion Institute, 35(1), pp.889895. 2015.
Experimental investigations assess for the first time the influence of the gradient of the square of the magnetic flux density ( ∇ ( B 2 ) ) on the soot production in a laminar axisymmetric nonpremixed flame. The steady nonsmoking ethylene flame is established in a coflowing mixture, composed of oxygen and nitrogen, over a Santoro type burner. This burner is located in an electromagnet. The flame experiences different magnitudes of upward ∇ ( B 2 ) , ranging from 0 to 18.2 T2/m, as well as different oxygen contents of the coflow, ranging from 21% to 50% in volume. Soot volume fraction is mapped in the flame by Laser Extinction Measurement technique. Increasing the magnitude of the ∇ ( B 2 ) allows for the modification of soot production in the flame. This modification is enhanced by increasing oxygen content, as oxygen exhibits a relatively high paramagnetic susceptibility. Furthermore, the aforementioned modification is shown to enable the shift among similar soot concentration profiles in the flame, just as the variation of oxygen content can do. Consequently, modification of the inner flame structure by magnetic effects could contribute to the control of oxyfuel combustion.

Kruse, S., Kerschgens, B., Berger, L., Varea, E. & Pitsch, H., Experimental and numerical study of MILD combustion for gas turbine applications. Applied Energy, 148, pp.456  465. 2015.
In this paper, the pressure influence on the \MILD\ combustion process and on emissions is examined. Experiments were performed in a combustion chamber based on the reverse flow configuration to enhance mixing of fresh and burnt gases. First, investigations under atmospheric pressure were performed to determine the regime of jointly low \NOx\ and \CO\ emissions. Afterwards, the effects of burnt gas recirculation and mixing under ambient pressure were evaluated by decreasing the inlet nozzle diameter without changing the residence time. In the second step, measurements were conducted under higher pressure while keeping the mass flow rates constant. Thereby, the residence time is extended, the \NOx\ formation chemistry is changed. These effects result in a strong rise in \NOx\ emissions and simulations indicate that at higher pressure a flame is established at the nozzle exit for higher equivalence ratios. In order to decrease the Damköhler number and shift the combustion process to the wellstirred reactor regime, the inlet nozzle diameter was decreased without changing the residence time. Thereby, burnt gas recirculation and gas mixing is enhanced, while simultaneously extending the chemical time scales. A significant decrease in \NOx\ emissions was detected.

Saghafian, A., Terrapon, V.E. & Pitsch, H., An efficient flameletbased combustion model for compressible flows. Combustion and Flame, 162(3), pp.652  667. 2015.
A combustion model based on a flamelet/progress variable approach for highspeed flows is introduced. In the proposed formulation, the temperature is computed from the transported total energy and tabulated species mass fractions. Only three additional scalar equations need to be solved for the combustion model. Additionally, a flamelet library is used that is computed in a preprocessing step. This approach is very efficient and allows for the use of complex chemical mechanisms. An approximation is also introduced to eliminate costly iterative steps during the temperature calculation. To better account for compressibility effects, the chemical source term of the progress variable is rescaled with the density and temperature. The compressibility corrections are analyzed in an a priori study. The model is also tested in both Reynoldsaveraged Navier–Stokes (RANS) and largeeddy simulation (LES) computations of a hydrogen jet in a supersonic transverse flow. Comparison with experimental measurements shows good agreement, particularly for the ŁES\ case. It is found that the disagreement between \RANS\ results and experimental data is mostly due to the mixing model deficiencies and the presumed probability density functions used in the \RANS\ formulation. A sensitivity study of the proposed model shows the importance of the compressibility corrections especially for the source term of the progress variable.

Narayanaswamy, K., Pitsch, H. & Pepiot, P., A chemical mechanism for low to high temperature oxidation of methylcyclohexane as a component of transportation fuel surrogates. Combustion and Flame, 162(4), pp.1193  1213. 2015.
Surrogate fuels consisting of a mixture of wellstudied hydrocarbons are often used to model real fuels in typical combustion studies. A major challenge, however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multicomponent surrogates. This task is further complicated by the diverse nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A wellcharacterized mechanism appropriate for the oxidation of smaller hydrocarbon species (Blanquart et al., 2009), as well as several substituted aromatic species and ndodecane (Narayanaswamy et al., 2010, 2014), well suited as a base to model surrogates, has now been extended to describe the oxidation of methylcyclohexane, a representative of the cyclic alkane class, which is often used in jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of methylcyclohexane is extracted from the detailed scheme of Pitz et al. (2007) and integrated in a systematic way into the previous model. Rate coefficient changes based on recent recommendations from literature, and an additional concerted elimination pathway important at moderate to low temperatures are introduced to the resulting chemical mechanism, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.

Zhou, K., Attili, A., Alshaarawi, A. & Bisetti, F., Simulation of aerosol nucleation and growth in a turbulent mixing layer. Physics of Fluids, 26(6), p.065106. 2014.

Schaefer, P., Gampert, M., Hennig, F. & Peters, N., Geometrical Features of Streamlines and Streamline Segments in Turbulent Flows. In Dillmann, A., Heller, G., Krämer, E., Kreplin, H. P., Nitsche, W., & Rist, U., eds. New Results in Numerical and Experimental Fluid Mechanics IX. Notes on Numerical Fluid Mechanics and Multidisciplinary Design. pp. 8592. 2014.
Streamlines constitute natural geometries in turbulent flows. In this work streamlines are segmented based on local extrema of the field of the absolute value of the velocity along the streamline coordinate. Streamline segments are parameterized based on their arclength and a theoretical scaling of the mean length with the geometric mean of the Kolmogorov length and the Taylor microscale is derived and found to be in good agreement with direct numerical simulations.

Paczko, G., Peters, N., Seshadri, K. & Williams, F.A., The role of coolflame chemistry in quasisteady combustion and extinction of nheptane droplets. Combustion Theory and Modelling, 18(45), pp.515531. 2014.
Experiments on the combustion of large nheptane droplets, performed by the National Aeronautics and Space Administration in the International Space Station, revealed a second stage of continued quasisteady burning, supported by lowtemperature chemistry, that follows radiative extinction of the first stage of burning, which is supported by normal hotflame chemistry. The second stage of combustion experienced diffusive extinction, after which a large vapour cloud was observed to form around the droplet. In the present work, a 770step reduced chemicalkinetic mechanism and a new 62step skeletal chemicalkinetic mechanism, developed as an extension of an earlier 56step mechanism, are employed to calculate the droplet burning rates, flame structures, and extinction diameters for this coolflame regime. The calculations are performed for quasisteady burning with the mixture fraction as the independent variable, which is then related to the physical variables of droplet combustion. The predictions with the new mechanism, which agree well with measured autoignition times, reveal that, in decreasing order of abundance, H2O, CO, H2O2, CH2O, and C2H4 are the principal reaction products during the lowtemperature stage and that, during this stage, there is substantial leakage of nheptane and O2 through the flame, and very little production of CO2 with no soot in the mechanism. The fuel leakage has been suggested to be the source of the observed vapour cloud that forms after flame extinction. While the new skeletal chemicalkinetic mechanism facilitates understanding of the chemical kinetics and predicts ignition times well, its predicted droplet diameters at extinction are appreciably larger than observed experimentally, but predictions with the 770step reduced chemicalkinetic mechanism are in reasonably good agreement with experiment. The computations show how the key ketohydroperoxide compounds control the diffusionflame structure and its extinction.

Hennig, F., Boschung, J. & Peters, N., Properties of Streamline Segments in Turbulent Channel Flows
with Wavy Walls. In APS Meeting Abstracts. 2014.

Boschung, J., Hennig, F. & Peters, N., Local behavior of streamlines in turbulent flows. In APS Meeting Abstracts. 2014.

Bode, M., Diewald, F., Broll, D.O., Heyse, J.F., Chenadec, V.L. & Pitsch, H., Influence of the Injector Geometry on Primary Breakup in Diesel Injector Systems. In SAE Technical Paper. SAE International. 2014.
Diesel injection systems have a significant impact on the performance as well as emission and pollutant formation of modern diesel engines. Even though the geometry of atomizers became more and more complex over the last years, injection systems still have a large potential for improving the overall diesel engine combustion process. Due to the complexity of the atomization process, reliable models are not available, yet these are highly desired for supporting the design process. They have to be developed using detailed numerical simulations.In this work, the “Spray A” reference case defined by the Engine Combustion Network is simulated under realistic operation conditions using a recently developed numerical framework for multiphase flows. A LargeEddy Simulation of the nozzle internal flow is coupled with a Direct Numerical Simulation of the interfacial outside flow and the resulting primary breakup is analyzed. Additionally, the impact of the injector geometry on primary breakup is studied. For this purpose, the effect of the taper ratio of the injector nozzle and the turbulent kinetic energy at the injector exit on the primary breakup is discussed and a parametric study is performed.It is shown that the taper ratio of the nozzle of modern injection systems has a large impact on the primary breakup because it directly influences the turbulent kinetic energy as well as the radial velocity component at the nozzle exit, which in turn affect the primary breakup.

Zweigel, R., Jochim, B., Korkmaz, M., Albin, T., Pitsch, H. & Abel, D., Virtual Test Bed of a Cascaded PCCI Combustion Control Loop with Underlying Air Path Controller and MultiZone Combustion Model. FISITA 2014 World Automotive Congress, June 2nd6th, Maastricht, the Netherlands, F2014CET072, pp.110. 2014.

Kwak, D., Khetan, A., Noh, S., Pitsch, H. & Han, B., First Principles Study of Morphology, Doping Level, and Water Solvation Effects on the Catalytic Mechanism of NitrogenDoped Graphene in the Oxygen Reduction Reaction. ChemCatChem, 6(9), pp.26622670. 2014.
By using first principles DFT calculations, we reveal oxygen reduction reaction mechanisms in Ndoped graphene (NGr). Considering both the morphology and the concentration of dopant N atoms in bulk and edge NGr forms, we calculate the energies of a large number of NGr model systems to cover a wide range of possible NGr structures and determine the most stable NGr forms. In agreement with experiments, our DFT calculations suggest that doping levels in stable NGr forms are limited to less than approximately 30 at. % N, above which the hexagonal graphene framework is broken. The ground state structures of bulk and edge NGr forms are found to differ depending on the doping level and poisoning of the edge bonds. Oxygen reduction reaction mechanisms are evaluated by using Gibbs freeenergy diagrams with and without water solvation. Our results indicate that N doping significantly alters the catalytic properties of pure graphene and that dilutely doped bulk NGr forms are the most active.

Gampert, M., Kleinheinz, K., Peters, N. & Pitsch, H., Experimental and Numerical Study of the Scalar Turbulent/NonTurbulent Interface Layer in a Jet Flow. Flow, Turbulence and Combustion, 92(12), pp.429449. 2014.
Based on two largeeddy simulations (LES) of a nonreacting turbulent round jet with a nozzle based Reynolds number of 8,610 with the same configuration as the one that has recently been investigated experimentally (Gampert et al., 2012; J Fluid Mech, 2012; J Fluid Mech 724:337, 2013), we examine the scalar turbulent/nonturbulent (T/NT) interface layer in the mixture fraction field of the jet flow between ten and thirty nozzle diameters downstream. To this end, the LES—one with a coarse grid and one with a fine grid—are in a first step validated against the experimental data using the axial decay of the mean velocity and the mean mixture fraction as well as based on radial selfsimilar profiles of mean and root mean square values of these two quantities. Then, probability density functions (pdf) of the mixture fraction at various axial and radial positions are compared and the quality of the LES is discussed. In general, the LES results are consistent with the experimental data. However, in the flow region where the imprint of the T/NT interface layer is dominant in the mixture fraction pdf, discrepancies are observed. In a next step, statistics of the T/NT interface layer are studied, where a satisfactory agreement for the pdf of the location of the interface layer from the higher resolved LES with the experimental data is observed, while the one with the coarse grid exhibits considerable deviations. Finally, the mixture fraction profile across the interface is investigated where the same trend as for the pdf of the location is present. In particular, it is found that the sharp interface that is present in experimental studies (Gampert et al., J Fluid Mech, 2013; Westerweel et al., J Fluid Mech 631:199, 2009) is less distinct in the LES results and rather diffused in radial direction outside of the T/NT interface layer.

Schenk, M., Lieb, S., Vieker, H., Beyer, A., Gölzhäuser, A., Wang, H. & KohseHöinghaus, K., Morphology of nascent soot in ethylene flames. Proceedings of the Combustion Institute, 35. 2014.
Because of their significant impact on climate, environment and health, reducing the emission of soot from combustion processes remains a problem that requires detailed understanding of its formation as well as of the principles that govern how the result in terms of size, morphology, and chemical reactivity of soot particles depends upon the formation process. Especially very small, below ten nanometersized, particles in the early stages of the soot nucleation process are interesting targets for more detailed inspection, to reveal useful insight and to guide further model development. In this study, Heliumion microscopy (HIM) is applied as an imaging technique new to combustion studies to analyze the morphology of soot particles >2 nm and to determine their geometrical characteristics. For this analysis, a series of premixed ethylene flames are investigated. Mobility size measurements from an earlier investigation have been compared with the particle sizes determined by HIM. Observed particle shapes and geometrical statistics suggest that in all flames under investigation, nascent soot possesses no welldefined morphologies. Additionally, investigations have been made using Xray photoelectron spectroscopy (XPS) to obtain more information on the chemical characteristics of these particles.

Schenk, M., Hansen, N., Vieker, H., Beyer, A., Gölzhäuser, A. & KohseHöinghaus, K., PAH formation and soot morphology in flames of C4 fuels. Proceedings of the Combustion Institute, 35. 2014.
In this work, we describe experimental studies on the formation of polycyclic aromatic hydrocarbons (PAH’s) in opposedflow atmosphericpressure flames of nbutane, ibutane, ibutene, and ibutanol and on the morphology of nascent soot particles sampled from premixed atmosphericpressure flames of the same fuels. To identify the major contributors to the molecular growth mechanism in opposedflow flames, we employed flamesampling molecularbeam mass spectrometry with electron ionization (EI) and in situ gaschromatography (GC) with mass spectrometric detection. The EI and GCEI mass spectra indicate that several pathways with different building blocks can contribute to molecular growth. Besides the commonly accepted hydrogenabstractionC2H2addition steps, we found reactions of the methyl radical to be important steps. This observation is also supported by the complexity of the mass spectra which indicates that at least one of the building blocks is rather small. The importance of phenyl radicals as building blocks seems to be limited. We also sampled nascent soot particles from premixed atmosphericpressure flames of the above mentioned fuels and used heliumion microscopy to unravel the influence of the fuel structure on the morphology of the sampled particles. While differences in flame temperatures and residence times are known to influence the particle sizes, the observed different morphologies are likely due to slightly different C/O ratios and potentially the chemical nature of the fuel.

Trisjono, P., Kleinheinz, K., Pitsch, H. & Kang, S., Large Eddy Simulation of Stratified and Sheared Flames of a Premixed Turbulent Stratified Flame Burner Using a Flamelet Model with Heat Loss. Flow, Turbulence and Combustion, 92(12), pp.201235. 2014.
This paper presents large eddy simulations (LES) of the Darmstadt turbulent stratified flame burner (TSF) at different operating conditions including detailed heat loss modeling. The target cases are a nonreacting and two reacting cases. Both reacting cases are characterized by stratification, while one flame additionally features shear. In the regime diagram for premixed combustion, the studied flames are found at the border separating the thin reaction zones regime and the broken reaction zones regime. A coupled level set/progress variable model is utilized to describe the combustion process. To account for heat loss, an enthalpy defect approach is adopted and reformulated to include differential diffusion effects. A novel powerlaw rescaling methodology is proposed to integrate the enthalpy defect approach into the level set/progress variable model which is extensively validated in two validation scenarios. It is demonstrated that the LES with the newly developed model captures the influence of heat loss well and that the incorporation of heat loss effects improves the predictions of the TSFburner over adiabatic simulations, while reproducing the experimentally observed flame liftoff from the pilot nozzle.

Herrmann, F., Jochim, B., Oßwald, P., Cai, L., Pitsch, H. & KohseHöinghaus, K., Experimental and numerical lowtemperature oxidation study of ethanol and dimethyl ether. Combustion and Flame, 161(2), pp.384397. 2014.
Lowtemperature combustion (LTC) receives increasing attention because of its potential to reduce NOx and soot emissions. For the application of this strategy in practical systems such as internal combustion engines and gas turbines, the fundamental chemical reactions involved must be understood in detail. To this end, reliable experimental data are needed including quantitative speciation to assist further development of reaction mechanisms and their reduction for practical applications.<br />
<br />
The present study focuses on the investigation of lowtemperature oxidation of ethanol and dimethyl ether (DME) under identical conditions in an atmosphericpressure laminar flow reactor. The gas composition was analyzed by timeofflight (TOF) mass spectrometry. This technique allows detection of all species simultaneously within the investigated temperature regime. Three different equivalence ratios of ϕ = 0.8, 1.0, and 1.2 were studied in a wide, highlyresolved temperature range from 400 to 1200 K, and quantitative species mole fraction profiles have been determined.<br />
<br />
The experiments were accompanied by numerical simulations. Their results clearly show the expected different lowtemperature oxidation behavior of both fuels, with a distinct negative temperature coefficient (NTC) region only observable for DME. With detailed species information including intermediates, differences of the kinetics for both fuels are discussed. Small modifications of the mechanisms served to identify sensitivities in the model. The experimental results may assist in the improvement of kinetic schemes and their reduction.

Bisetti, F., Attili, A. & Pitsch, H., Advancing predictive models for particulate formation in turbulent flames via massively parallel direct numerical simulations. Philos Trans A Math Phys Eng Sci., 372(2022). 2014.
Combustion of fossil fuels is likely to continue for the near future due to the growing trends in energy consumption worldwide. The increase in efficiency and the reduction of pollutant emissions from combustion devices are pivotal to achieving meaningful levels of carbon abatement as part of the ongoing climate change efforts. Computational fluid dynamics featuring adequate combustion models will play an increasingly important role in the design of more efficient and cleaner industrial burners, internal combustion engines, and combustors for stationary power generation and aircraft propulsion. Today, turbulent combustion modelling is hindered severely by the lack of data that are accurate and sufficiently complete to assess and remedy model deficiencies effectively. In particular, the formation of pollutants is a complex, nonlinear and multiscale process characterized by the interaction of molecular and turbulent mixing with a multitude of chemical reactions with disparate time scales. The use of direct numerical simulation (DNS) featuring a state of the art description of the underlying chemistry and physical processes has contributed greatly to combustion model development in recent years. In this paper, the analysis of the intricate evolution of soot formation in turbulent flames demonstrates how DNS databases are used to illuminate relevant physicochemical mechanisms and to identify modelling needs.

Mittal, V., Kang, S., Doran, E., Cook, D. & Pitsch, H., LES of Gas Exchange in IC Engines. Oil & Gas Science and Technology, 69(1), pp.2940. 2014.

Attili, A., Bisetti, F., Mueller, M.E. & Pitsch, H., Formation, growth, and transport of soot in a threedimensional turbulent nonpremixed jet flame. Combustion and Flame, 161, pp.18491865. 2014.

Cai, L. & Pitsch, H., Mechanism optimization based on reaction rate rules. Combustion and Flame, 161(2), pp.405415. 2014.
Abstract Accurate chemistry models form the backbone of detailed computational fluid dynamics (CFD) tools used for simulating complex combustion devices. Combustion chemistry is often very complex and chemical mechanisms generally involve more than one hundred species and one thousand reactions. In the derivation of these large chemical mechanisms, typically a large number of reactions appears, for which rate data are not available from experiment or theory. Rate data for these reactions are then often assigned using socalled reaction classes. This method categorizes all possible fuelspecific reactions as classes of reactions with prescribed rules for the rate constants. This ensures consistency in the chemical mechanism. In rate parameter optimizations found in the published literature, rate constants of single elementary reactions are usually systematically optimized to achieve good agreement between model performance and experimental measurements. However, it is not kinetically reasonable to modify the rate parameters of single reactions, because this will violate consistency of rate parameters of kinetically similar reactions. In this work, the rate rules, that determine the rates for reaction classes are calibrated instead of the rates of single elementary reactions leading to a chemically more consistent model optimization. This is demonstrated by optimizing an npentane combustion mechanism. The rate rules are studied with respect to reaction classes, abstracting species, broken C–H bonds, and ring strain energy barriers. Furthermore, the uncertainties of the rate rules and model predictions are minimized and the pressure dependence of reaction classes dominating low temperature oxidation is optimized.

Bode, M., Le Chenadec, V. & Pitsch, H., High Fidelity multiphase simulations studying primary breakup. In PRACEdays14, 20th22nd May, Barcelona, Spain. 2014.

Cai, L., Sudholt, A., Lee, D.J., Egolfopoulos, F., Pitsch, H.G., Westbrook, C.K. & Sarathy, S.M., Chemical kinetic study of a novel lignocellulosic biofuel: Dinbutyl ether oxidation in a laminar flow reactor and flames. Combustion and Flame, 161(3), pp.798–809. 2014.
The combustion characteristics of promising alternative fuels have been studied extensively in the recent years. Nevertheless, the pyrolysis and oxidation kinetics for many oxygenated fuels are not well characterized compared to those of hydrocarbons. In the present investigation, the first chemical kinetic study of a longchain linear symmetric ether, dinbutyl ether (DBE), is presented and a detailed reaction model is developed. DBE has been identified recently as a candidate biofuel produced from lignocellulosic biomass. The model includes both high temperature and low temperature reaction pathways with reaction rates generated using appropriate rate rules. In addition, experimental studies on fundamental combustion characteristics, such as ignition delay times and laminar flame speeds have been performed. A laminar flow reactor was used to determine the ignition delay times of lean and stoichiometric DBE/air mixtures. The laminar flame speeds of DBE/air mixtures were measured in the stagnation flame configuration for a wide rage of equivalence ratios at atmospheric pressure and an unburned reactant temperature of 373 K. All experimental data were modeled using the present kinetic model. The agreement between measured and computed results is satisfactory, and the model was used to elucidate the oxidation pathways of DBE. The dissociation of ketohydroperoxides, leading to radical chain branching was found to dominate the ignition of DBE in the low temperature regime. The results of the present numerical and experimental study of the oxidation of dinbutyl ether provide a good basis for further investigation of long chain linear and branched ethers.

Khetan, A., Pitsch, H. & Viswanathan, V., Solvent Degradation in Nonaqueous LiO2 Batteries: Oxidative Stability versus HAbstraction. Journal of Physical Chemistry Letters, 5(14), pp.2419–2424. 2014.

Narayanaswamy, K., Pepiot, P. & Pitsch, H.G., A chemical mechanism for low to high temperature oxidation of ndodecane as a component of transportation fuel surrogates. Combustion and Flame, 161(4), pp.866–884. 2014.
Using surrogate fuels in lieu of real fuels is an appealing concept for combustion studies. A major limitation however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multicomponent surrogates. This task is further complicated by the fairly large nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A wellcharacterized mechanism appropriate for the oxidation of smaller hydrocarbon species G. Blanquart, P. PepiotDesjardins, H. Pitsch, Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors, Combust. Flame 156 (2009) 588–607, and several substituted aromatic species K. Narayanaswamy, G. Blanquart, H. Pitsch, A consistent chemical mechanism for the oxidation of substituted aromatic species, Combust. Flame 157 (10) (2010) 1879–1898, ideally suited as a base to model surrogates, has now been extended to describe the oxidation of ndodecane, a representative of the paraffin class, which is often used in diesel and jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of ndodecane is extracted from the detailed scheme of Sarathy et al. S. M. Sarathy, C. K.Westbrook, M. Mehl, W. J. Pitz, C. Togbe, P. Dagaut, H. Wang, M. A. Oehlschlaeger, U. Niemann, K. Seshadri, Comprehensive chemical kinetic modeling of the oxidation of 2methylalkanes from C7 to C20, Combust. Flame 158 (12) (2011) 2338–2357 and integrated in a systematic way into the base model. Rate changes based on recent rate recommendations from literature are introduced to the resulting chemical mechanism in a consistent manner, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets

Gauding, M., Wick, A., Pitsch, H. & Peters, N., Generalised scalebyscale energybudget equations and largeeddy simulations of anisotropic scalar turbulence at various Schmidt numbers. Journal of Turbulence, 15(12), pp.857882. 2014.
A necessary condition for the accurate prediction of turbulent flows using largeeddy simulation (LES) is the correct representation of energy transfer between the different scales of turbulence in the LES. For scalar turbulence, transfer of energy between turbulent length scales is described by a transport equation for the second moment of the scalar increment. For homogeneous isotropic turbulence, the underlying equation is the wellknown Yaglom equation. In the present work, we study the turbulent mixing of a passive scalar with an imposed mean gradient by homogeneous isotropic turbulence. Both direct numerical simulations (DNS) and LES are performed for this configuration at various Schmidt numbers, ranging from 0.11 to 5.56. As the assumptions made in the derivation of the Yaglom equation are violated for the case considered here, a generalised Yaglom equation accounting for anisotropic effects, induced by the mean gradient, is derived in this work. This equation can be interpreted as a scalebyscale energybudget equation, as it relates at a certain scale r terms representing the production, turbulent transport, diffusive transport and dissipation of scalar energy. The equation is evaluated for the conducted DNS, followed by a discussion of physical effects present at different scales for various Schmidt numbers. For an analysis of the energy transfer in LES, a generalised Yaglom equation for the second moment of the filtered scalar increment is derived. In this equation, new terms appear due to the interaction between resolved and unresolved scales. In an apriori test, this filtered energybudget equation is evaluated by means of explicitly filtered DNS data. In addition, LES calculations of the same configuration are performed, and the energy budget as well as the different terms are thereby analysed in an aposteriori test. It is shown that LES using an eddy viscosity model is able to fulfil the generalised filtered Yaglom equation for the present configuration. Further, the dependence of the terms appearing in the filtered energybudget equation on varying Schmidt numbers is discussed.

Jochim, B., Zweigel, R., Korkmaz, M., Abel, D. & Pitsch, H., Comparing accuracy and efficiency between a multizone model and artificial neural networks in dynamic simulations of PCCI Diesel engine combustion for engine control. THIESEL 2014  Conference on thermo and fluid dynamic processes in direct injection engines, September 9th12th, Valencia, Spain, 385. 2014.

Boschung, J., Schaefer, P., Peters, N. & Meneveau, C., The local topology of streamand vortex lines in turbulent flows. Physics of Fluids (1994present), 26(4), p.045107. 2014.

Gampert, M., Boschung, J., Hennig, F., Gauding, M. & Peters, N., The vorticity versus the scalar criterion for the detection of the turbulent/nonturbulent interface. Journal of Fluid Mechanics, 750, pp.578596. 2014.
Based on a direct numerical simulation (DNS) of a temporally evolving mixing layer, we present a detailed study of the turbulent/nonturbulent (T/NT) interface that is defined using the two most common procedures in the literature, namely either a vorticity or a scalar criterion. The different detection approaches are examined qualitatively and quantitatively in terms of the interface position, conditional statistics and orientation of streamlines and vortex lines at the interface. Computing the probability density function (p.d.f.) of the mean location of the T/NT interface from vorticity and scalar allows a detailed comparison of the two methods, where we observe a very good agreement. Furthermore, conditional mean profiles of various quantities are evaluated. In particular, the position p.d.f.s for both criteria coincide and are found to follow a Gaussian distribution. The terms of the governing equations for vorticity and passive scalar are conditioned on the distance to the interface and analysed. At the interface, vortex stretching is negligible and the displacement of the vorticity interface is found to be determined by diffusion, analogous to the scalar interface. In addition, the orientation of vortex lines at the vorticity and the scalar based T/NT interface are analyzed. For both interfaces, vorticity lines are perpendicular to the normal vector of the interface, i.e. parallel to the interface isosurface.

Beeckmann, J., Cai, L. & Pitsch, H., Experimental investigation of the laminar burning velocities of methanol, ethanol, npropanol, and nbutanol at high pressure. Fuel, 117, Part A, pp.340350. 2014.
Due to their increasing share, the combustion of alternative fuels and in particular oxygenated, bioderived fuel components need to be characterised. The laminar burning velocity is one key parameter for the characterisation of fuels, and it also serves as an important quantity to validate chemical kinetic models. Methanol, ethanol, npropanol, and nbutanol laminar burning velocities experiments were conducted in a spherical combustion vessel at an unburnt temperature of 373 K and a pressure of 10 bar. Measured burning velocities from this study and from the published literature are compared with numerical simulation data from published chemical mechanisms. The models tend to underpredict the experimentally measured values. A sensitivity analysis suggests further investigation of the pressure dependence for the fuel specific reactions with hydrogen and hydroxyl radicals.

Lange, L., Korkmaz, M., Magens, E. & Heinze, J., Assessment of 2photon PLIF for COContentration measurement in a reseach combustor. In MOTAR Meeting, April 10th11th, Stuttgart, Germany. 2013.

Varea, E., Modica, V., Renou, B., Halter, F., MounaïmRousselle, C. & Chen, Z., On the accurate determination of unstretched laminar burning velocity from spherically expanding flames. In COST Topical Workshop: Kinetic studies using laminar flame, Lund, Sweden. 2013.

Schaefer, P., Gampert, M. & Peters, N., A model equation for the joint distribution of the length and velocity difference of streamline segments in turbulent flows. Physics of Fluids, 25(11), p.115107. 2013.
Streamlines recently received attention as natural geometries of turbulent flow fields. Similar to dissipation elements in scalar fields, streamlines are segmented into smaller subunits based on local extreme points of the absolute value of the velocity field u along the streamline coordinate s, i.e., points where the projected gradient in streamline direction u s = 0. Then, streamline segments are parameterized using their arclength l between two neighboring extrema and the velocity difference Δ at the extrema. Both parameters are statistical variables and streamline segments are characterized by the joint probability density function (jpdf) P(l, Δ). Based on a previously formulated model for the marginal pdf of the arclength, P(l), which contains terms that account for slow changes as well as fast changes of streamline segments, a model for the jpdf is formulated. The jpdf's, when normalized with the mean length, l m , and the standard deviation of the velocity difference σ, obtained from two different direct numerical simulations (DNS) cases of homogeneous isotropic decaying and forced turbulence at Taylor based Reynolds number of Re λ = 116 and Re λ = 206, respectively, turn out to be almost Reynolds number independent. The steady model solution is compared with the normalized jpdf's obtained from DNS and it is found to be in good agreement. Special attention is paid to the intrinsic asymmetry of the jpdf with respect to the mean length of positive and negative streamline segments, where due to the kinematic stretching of positive segments and compression of negative ones, the mean length of positive segments turns out to be larger than the mean length of negative ones. This feature is reproduced by the model and the ratio of the two length scales, which turns out to be an almost Reynolds number independent, dimensionless quantity, is well reproduced. Finally, a relation between the kinetic asymmetry of streamline segments and the dynamic asymmetry of the pdf of longitudinal velocity gradients in turbulent flows, which manifests itself in a negative velocity gradient skewness, is established and it is theoretically shown that negative streamline segments are only smaller than positive ones, if the gradient is negatively skewed.

Gampert, M., Narayanaswamy, V. & Peters, N., Scalar gradient trajectory measurements using highfrequency cinematographic planar Rayleigh scattering. Experiments in Fluids, 54(12), pp.115. 2013.
In this work, we perform an experimental investigation into statistics based on scalar gradient trajectories in a turbulent jet flow, which have been suggested as an alternative means to analyze turbulent flow fields by Wang and Peters (J Fluid Mech 554:457–475, 2006, 608:113–138, 2008). Although there are several numerical simulations and theoretical works that investigate the statistics along gradient trajectories, only few experiments in this area have been reported. To this end, highfrequency cinematographic planar Rayleigh scattering imaging is performed at different axial locations of a turbulent propane jet issuing into a CO2 coflow at nozzlebased Reynolds numbers Re 0 = 3,000–8,600. Taylor’s hypothesis is invoked to obtain a threedimensional reconstruction of the scalar field in which then the corresponding scalar gradient trajectories can be computed. These are then used to examine the local structure of the mixture fraction with a focus on the scalar turbulent/nonturbulent interface. The latter is a layer that is located between the fully turbulent part of the jet and the outer flow. Using scalar gradient trajectories, we partition the turbulent scalar field into these three regions according to an approach developed by Mellado et al. (J Fluid Mech 626:333–365, 2009). Based on the latter, we investigate the probability to find the respective regions as a function of the radial distance to the centerline, which turns out to reveal the meandering nature of the scalar T/NT interface layer as well as its impact on the local structure of the turbulent scalar field.

Farisco, F., Rochhausen, S., Korkmaz, M. & Schroll, M., Validation of Flow Field and Heat Transfer in a TwoPass Internal Cooling Channel Using Different Turbulence Models. In ASME Turbo Expo: Power for Land, Sea, and Air. 2013.

Lefebvre, A., Varea, E., Modica, V., Renou, B. & Boukhalfa, M., Experimental Study of Radiation Absorption on Laminar Flame Speed of CO2 Diluted Methane Flames at Elevated Pressure. In Proceedings of the European Combustion Meeting 2013, 25th  28th June 2013, Lund, Sweden. 2013.

Gampert, M., Narayanaswamy, V., Schaefer, P. & Peters, N., Conditional statistics of the turbulent/nonturbulent interface in a jet flow. Journal of Fluid Mechanics, 731, pp.615638. 2013.
Using twodimensional highspeed measurements of the mixture fraction Z in a turbulent round jet with nozzlebased Reynolds numbers Re0 between 3000 and 18 440, we investigate the scalar turbulent/nonturbulent (T/NT) interface of the flow. The mixture fraction steeply changes from Z=0 to a final value which is typically larger than 0.1. Since combustion occurs in the vicinity of the stoichiometric mixture fraction, which is around Z=0.06 for typical fuel/air mixtures, it is expected to take place largely within the turbulent/nonturbulent interface. Therefore, deep understanding of this part of the flow is essential for an accurate modelling of turbulent nonpremixed combustion. To this end, we use a composite model developed by Effelsberg & Peters (Combust. Flame, vol. 50, 1983, pp. 351–360) for the probability density function (p.d.f.) P(Z) which takes into account the different contributions from the fully turbulent as well as the turbulent/nonturbulent interface part of the flow. A very good agreement between the measurements and the model is observed over a wide range of axial and radial locations as well as at varying intermittency factor γ and shear. Furthermore, we observe a constant mean mixture fraction value in the fully turbulent region. The p.d.f. of this region is thus of nonmarching character, which is attributed physically to the meandering nature of the fully turbulent core of the jet flow. Finally, the location and in particular the scaling of the thickness δ of the scalar turbulent/nonturbulent interface are investigated. We provide the first experimental results for the thickness of the interface over the abovementioned Reynolds number range and observe δ/L∼Re−1λ, where L is an integral length scale and Reλ the local Reynolds number based on the Taylor scale λ, meaning that δ∼λ. This result also supports the assumption often made in modelling of the stoichiometric scalar dissipation rate χst being a Reynoldsnumberindependent quantity.

Gampert, M., Schaefer, P., Boschung, J. & Peters, N., Gradient trajectory analysis of the scalar superlayer in a jet flow. Proceedings of the 8th Symposium on Turbulence and Shear Flow Phenomena, August 28th30th, Poitiers, France. 2013.
Based on planar highspeed Rayleigh scattering measurements of the mass fraction of propane discharging from a turbulent round jet into coflowing carbon dioxide at nozzle based Reynolds numbers Re0=3,0008,600, we investigate the scalar superlayer. The latter is located between the fully turbulent part of the jet and the outer flow and has the so called turbulent/nonturbulent interface embedded within it. It is termed in analogy to the laminar superlayer introduced by Corrsin and Kistler (NACA Report 1244, 1955). Using scalar gradient trajectories, we partition the turbulent scalar field into the afore mentioned three regions according to an approach developed by Mellado et al. (J. Fluid Mech. 626:333365, 2009) based on which we in a next step investigate conditioned zonal statistics of the scalar pdf as well as the scalar difference along the trajectory and its mean scalar value. Finally, we relate our results for the scalar superlayer on the one hand to the findings made in other experimental and numerical studies of the turbulent/nonturbulent interface and discuss them on the other hand in the context of the flamelet approach in turbulent nonpremixed combustion.

Boschung, J., Hennig, F. & Peters, N., A Comparison of the Scalar and Vorticity Criterion defining the T/NT Interface. 66th Annual Meeting of the APS Division of Fluid Dynamics, November 24th26th, Pittsburgh, USA. 2013.

Attili, A. & Bisetti, F., Fluctuations of a passive scalar in a turbulent mixing layer. Phys. Rev. E, 88(3), p.033013. 2013.

Gauding, M. & Peters, N., Statistical Investigation of Turbulent Mixing by Means of Turbulent Line Segments. 66th Annual Meeting of the APS Division of Fluid Dynamics, November 24th26th, Pittsburgh, USA. 2013.
We examine the turbulent mixing of a passive scalar with imposed mean gradient. The Taylor microscale based Reynolds number varies between 85 and 530. A straight line through the turbulent field of a passive scalar ϕ is decomposed into piecewise monotonously increasing or decreasing segments. These so called turbulent line segments (TLS) start at a local minimum point and end at a local maximum point or vice versa and are parameterized by the distance ℓ between the extreme points and by the corresponding scalar difference Δϕ. The implication is that TLS, whose mean length is about ten times the Kolmogorov length, characterize the dynamic process of scalarenergy dissipation. Firstly, we examine the joint distribution function of Δϕ and ℓ and define the gradient Δϕ/ℓ of TLS. This helps to understand clifframp structures as we can show at which length scale large gradients arise. Based on a statistical approach we can further relate the mean gradient to the local gradient and can examine the scaling of the kurtosis of the local gradient with the Reynolds number. Secondly, we define a structure function based on TLS, that relates the extreme points and calculate the scaling exponents. The result is compared with the KOCtheory.

Attili, A. & Bisetti, F., Application of a robust and efficient Lagrangian particle scheme to soot transport in turbulent flames. Computers & Fluids, 84, pp.164  175. 2013.
A Lagrangian particle scheme is applied to the solution of soot dynamics in turbulent nonpremixed flames. Soot particulate is described using a method of moments and the resulting set of continuum advectionreaction equations is solved using the Lagrangian particle scheme. The key property of the approach is the independence between advection, described by the movement of Lagrangian notional particles along pathlines, and internal aerosol processes, evolving on each notional particle via source terms. Consequently, the method overcomes the issues in Eulerian gridbased schemes for the advection of moments: errors in the advective fluxes pollute the moments compromising their realizability and the stiffness of source terms weakens the stability of the method. The proposed scheme exhibits superior properties with respect to conventional Eulerian schemes in terms of stability, accuracy, and grid convergence. Taking into account the quality of the solution, the Lagrangian approach can be computationally more economical than commonly used Eulerian schemes as it allows the resolution requirements dictated by the different physical phenomena to be independently optimized. Finally, the scheme posseses excellent scalability on massively parallel computers.

Moshammer, K., Vranckx, S., Chakravarty, H.K., Parab, P., Fernandes, R.X. & KohseHöinghaus, K. eds., An experimental and kinetic modeling study of 2methyltetrahydrofuran flames. Combustion and Flame, 160(12), pp.2729–2743. 2013.
Clean combustion processes are of paramount importance in the transition of the energy system towards increased sustainability. In an attempt to partially replace conventional fossil fuels, bioderived oxygenates attract rising attention as alternative transportation fuels. Among this class of fuels, cyclic structures that can be derived from cellulosic biomass are particularly interesting. Here we present a study of premixed, laminar lowpressure flames of 2methyltetrahydrofuran (2MTHF) with an equivalence ratio of ϕ = 1.7 at 40 mbar. Timeofflight molecularbeam mass spectrometry (MBMS) with electron ionization (EI) was used to analyze and quantify mole fraction profiles of reactants, products, and most intermediate species including radicals involved in the combustion process. As a valuable complement, MBMS using singlephoton ionization (PI) by vacuum ultraviolet radiation permitted isomer identification as well as independent concentration information under similar flame conditions. A detailed combustion model for 2MTHF was developed, and the flame structure and species information were examined in conjunction with these experiments.

Chakravarty, H.K. & Fernandes, R.X., Reaction Kinetics of Hydrogen Abstraction Reactions by Hydroperoxyl Radical from 2Methyltetrahydrofuran and 2,5Dimethyltetrahydrofuran. Journal of Physical Chemistry A, 117(24), pp.5028–5041. 2013.
Highly accurate rate parameters for Habstraction reactions by HO2 radicals are needed for development of predictive chemical kinetic models for ignition. In this article, we report the rate coefficients for reaction of hydroperoxyl radical (HO2) with 2methyltetrahydrofuran (MTHF) and 2,5dimethyltetrahydrofuran (DMTHF) computed employing CBSQB3 and CCSD(T)/ccpVTZ//B3LYP/ccpVTZ level of theory in the temperature range of 500–2000 K. Conventional transition state theory (CTST) with hindered rotor approximation for low frequency torsional modes and RRHO (rigidrotor harmonic oscillator) approximation for all other vibrational modes is employed to evaluate the high pressure rate constants as a function of temperature. Rate constant of each individual hydrogen abstraction channel is taken into account to calculate the overall rate constant. Threeparameter Arrhenius expressions have been obtained by fitting to the computed rate constants of all abstraction channels between 500 and 2000 K. Eight transition states have been identified for MTHF and four for slightly more stable transDMTHF. Intrinsic reaction coordinates (IRC) calculations were performed to verify the connectivity of all the transition states (TSs) with reactants and products. One dimensional Eckart’s asymmetrical method has been used to calculate quantum mechanical tunneling effect. Results of the theoretically calculated rate coefficients indicate that the hydrogen abstraction by HO2 from the C2 carbon of both MTHF and DMTHF is the most dominant path among all reaction pathways attributed to its lowest barrier height. The total rate coefficients of the MTHF and DMTHF with HO2 at CCSD(T)/ccpVTZ//B3LYP/ccpVTZ level of theory are k(T) = 8.60T3.54 exp(−8.92/RT) and k(T)= 3.17T3.63 exp(−6.59/RT) cm3 mol–1 s–1, respectively. At both the level of theories, the predicted total abstraction rate constant for DMTHF is found to be higher as compared to that of MTHF over an entire temperature range of investigation. The overall rate constant calculated at CCSD(T)/ccpVTZ//B3LYP/ccpVTZ level of theory is lower by 1.43 and 3.44 times at 2000 K than the CBSQB3 level for MTHF and DMTHF, respectively.

Stoehr, K.D., Peters, N. & Pitsch, H., Niedertemperaturoszillationen in einem DME betriebenen Jetstirredflow Reaktor. 26. Deutscher Flammentag, VDIBericht. 2013.

Vranckx, S., Lee, C., Chakravarty, H.K. & Fernandes, R.X., A rapid compression machine study of the low temperature combustion of cyclohexane at elevated pressures. Proceedings of the Combustion Institute, 34(1), pp.377–384. 2013.
Ignition delay times have been measured in a rapid compression machine for cyclohexane/O2/N2/Ar mixtures with equivalence ratios of 0.5, 1.0 and 2.0 at elevated pressures of up to 40 bar and temperatures between 680 and 910 K. These data clearly show the negativetemperaturecoefficient behavior for cyclohexane in the temperature range investigated. The predictions of several detailed kinetic models are compared to these new experimental validation data and these mechanisms have been analyzed to explain the obtained differences with a focus on the crucial peroxychemistry of the primary radicals. The presented data are the first set of ignition delay times at elevated pressures for the lowtemperature range.

Jocher, A., Pitsch, H., Gomez, T. & Legros, G., Modification of Soot Production inside Laminar Diffusion Flames by Static NonUniform Magnetic Fields. In Proceedings of the European Combustion Meeting 2013, June 25th28th, Lund, Sweden. 2013.

Varea, E., Modica, V., Renou, B. & Boukhalfa, A.M., Pressure effects on laminar burning velocities and Markstein lengths for IsooctaneEthanolAir mixtures. Proceedings of the Combustion Institute, 34(1), pp.735–744. 2013.
A new method for extracting the laminar burning velocity from the difference between the flame speed and the local fresh gas velocity at the entrance of the flame front is used for ethanol/isooctane/air mixtures. This approach gives additional information in terms of flame sensitivity to flame stretch represented by the Markstein length relative to the fresh gases L u and a direct measurement of the unstretched laminar burning velocity without any assumptions on adiabaticity and on the fuel mixture properties. The overall accuracy of the measurements obtained from the repeatability of the experiments is less than ±1.5 cm/s at P = 0.1 MPa and increases with pressure to reach ±3 cm/s at P = 1 MPa. Measurements of the unstretched laminar burning velocities and Markstein lengths of ethanol/isooctane/air flames in the range of initial pressure from 0.1 to 1 MPa and for various ethanol mole fractions, ∊ , are presented. A globally good agreement is obtained with literature data for pure fuels, and with both mechanisms of Marinov and of Saxena for pure ethanol/air mixtures (except for rich ethanol/air flames in the case of Marinov mechanism) and with Jerzembeck mechanism for pure isooctane/air flames. The effect of pressure on the burning velocity of pure fuel is interpreted using the correlation View the MathML sourceun0(P)=un0(P0)(P/P0)β. Particular attention has been paid to the effect of equivalence ratio and fuel blending on the power exponent β. Both experimental and numerical data show the existence of a minimum in β for stoichiometric mixtures and a globally quadratic approximation with ϕ. From our results, we demonstrate also the quasi insensitivity of β with fuel blending. A general correlation is then proposed to express the effect of pressure, equivalence ratio and ethanol mole fraction in isooctane at a constant temperature of 373 K. The accuracy of this correlation and the ranges of validity are also provided.

Chenadec, V.L. & Pitsch, H., A monotonicity preserving conservative sharp interface flow solver for high density ratio twophase flows. Journal of Computational Physics, 249, pp.185203. 2013.
This paper presents a novel approach for solving the conservative form of the incompressible twophase NavierStokes equations. In order to overcome the numerical instability induced by the potentially large density ratio encountered across the interface, the proposed method includes a VolumeofFluid type integration of the convective momentum transport, a monotonicity preserving momentum rescaling, and a consistent and conservative Ghost Fluid projection that includes surface tension effects. The numerical dissipation inherent in the VolumeofFluid treatment of the convective transport is localized in the interface vicinity, enabling the use of a kinetic energy conserving discretization away from the singularity. Two and threedimensional tests are presented, and the solutions shown to remain accurate at arbitrary density ratios. The proposed method is then successfully used to perform the detailed simulation of a round water jet emerging in quiescent air, therefore suggesting the applicability of the proposed algorithm to the computation of realistic turbulent atomization.

Mittal, V. & Pitsch, H., A flamelet model for premixed combustion under variable pressure conditions. Proceedings of the Combustion Institute, 34(2), pp.29953003. 2013.
Studying premixed combustion under variable pressure conditions is important for modeling devices such as internal combustion engines. Various models have been developed for isobaric premixed combustion, but few models exist for variable pressure conditions. In this study, a new model is proposed for studying combustion at these conditions. The model is based on the flamelet progress variable approach that has been used extensively to study constant pressure premixed combustion. The quantities required by the combustion model are tabulated only at a reference pressure. At other pressure values, they are obtained using a quadratic logarithmic expansion in pressure. Expensive tabulation due to an additional pressure dimension is avoided, and only the coefficients for the expansion need to be tabulated. Additionally, under constant volume conditions, the burnt gas temperature may become larger than the unstretched premixed postflame temperature due to compression of the gases. This effect is modeled by considering the mixture enthalpy. A polynomial expansion method is developed to obtain the temperature from the mixture enthalpy accurately at minimal computational cost. The model is validated against direct numerical simulation data with multistep finite rate chemistry under isochoric conditions. Different turbulence intensities and length scales are studied to better assess the model performance. The model is shown to capture the effects on temperature due to the variable pressure.

Karwasara, S., Sharma, M.K., Tripathi, R. & Nagendran, S., Synthesis and Reactivity of NAminotroponiminatogermylenepyrrole and Its Derivatives. Organometallics, 32(14), pp.3830–3836. 2013.
Through the reaction of the aminotroponiminatogermylene monochloride complex (Bui2ATI)GeCl (1) with sodium pyrrolide, the stable Ngermylene pyrrole complex (Bui2ATI)GeNC4H4 (2) has been isolated. The reaction of compound 2 with thiophenol and selenophenol afforded the first germylene thio and selenophenoxide complexes (Bui2ATI)GeSPh (3) and (Bui2ATI)GeSePh (4) through the substitution of the pyrrole moiety (NC4H4) with an EPh moiety (E = S (3), Se (4)), respectively. Interestingly, the chalcogenide derivatives of compound 2, such as the Ngermathioacylpyrrole complex (Bui2ATI)Ge(S)NC4H4 (5) and Ngermaselenoacylpyrrole complex (Bui2ATI)Ge(Se)NC4H4 (6), also underwent the aforementioned substitution reaction with thiophenol and selenophenol, resulting in the first examples of germa thioester complexes ((Bui2ATI)Ge(S)SPh (7) and (Bui2ATI)Ge(Se)SPh (8)) and germa selenoester complexes ((Bui2ATI)Ge(S)SePh (9) and (Bui2ATI)Ge(Se)SePh (10)), respectively. All the novel germanium compounds 3–10 have been unequivocally characterized through multinuclear NMR spectroscopy along with the germylene complex 2. Further, compounds 3–5 and 7–10 were characterized through singlecrystal Xray diffraction studies. The GeII–S and GeII–Se bond lengths in compounds 3 and 4 are 2.367(1) and 2.511(1) Å, respectively. The average GeIV–S and GeIV–Se bond lengths in germa thioester (7 and 8) and germa selenoester (9 and 10) complexes are 2.241(1) and 2.362(1) Å, respectively.

Mueller, M.E., Chan, Q.N., Qamar, N.H., Dally, B.B., Pitsch, H., Alwahabi, Z.T. & Nathan, G.J., Experimental and computational study of soot evolution in a turbulent nonpremixed bluff body ethylene flame. Combustion and Flame, 160(7), pp.12981309. 2013.
A turbulent nonpremixed bluff body ethylene flame is studied both experimentally and computationally. Experimentally, the soot volume fraction is measured using laserinduced incandescence (LII). Three distinct regions are observed in the flame: a lowstrain recirculation zone, a downstream jetlike region, and a highstrain neck region connecting these two regions. The maximum soot volume fraction is found in the recirculation zone, but most of the soot volume is contained in the larger jetlike region further downstream. In the neck region between these two zones, soot cannot form due to large strain rates, and the small amounts of soot in this region indicate that soot rarely escapes the recirculation zone before being oxidized. The recirculation zone is characterized by a low soot intermittency, in contrast to the downstream jetlike region and previously investigated jet flames in which the soot intermittency is high. Large Eddy Simulation (LES) is used to further investigate this distinctly different evolution of soot in the recirculation zone. The LES model is found to predict the soot volume fraction profiles quite accurately, albeit with significant sensitivity to the inflow profiles of the fuel jet and air coflow. Soot is formed near the inner shear layer between the fuel jet and recirculation zone where the mixture fraction is sufficiently large to support Polycyclic Aromatic Hydrocarbon (PAH) formation. A portion of this soot is entrained into the interior of the recirculation zone where the soot growth rates are relatively low, despite the rich mixture fraction in this region. The circulation vortex then transports the soot from the interior of the recirculation zone toward less rich mixture fractions near the flame, which is situated in the outer shear layer between the air coflow and the recirculation zone. Here, the majority of soot growth occurs due to surface growth, that is, mass addition due to surface reactions with acetylene. The dominance of acetylenebased surface growth in the recirculation zone contrasts findings in previous simulations of turbulent jet flames that do not exhibit a recirculation zone, in which nucleation and PAH condensation were found to overwhelm acetylenebased surface growth. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Boschung, J., Meneveau, C. & Peters, N., Properties of the curvature tensor of streamtubes in turbulent flows. European Turbulence Conference 14, September 1st4th, Lyon, France. 2013.
Important features of the shape of streamtubes can be described by the first and second invariant of the curvature tensor ∂ti/∂xj where t is the unit vector tangent to the streamlines forming the tube. In threedimensional turbulence, three different generic behaviors can be found, as well as four degenerate cases. For instance, the first invariant of t represents the relative change of the tube’s crosssection. The joint PDF of the first and second invariant is evaluated from DNS, and the results stemming from the presented classification scheme are discussed. An attempt to reproduce the observed PDF from a restrictedEuler like model is described.

Schenk, M., Lieb, S., Vieker, H., Beyer, A., Gölzhäuser, A., Wang, H. & KohseHöinghaus, K., Imaging Nanocarbon Materials: Soot Particles in Flames are Not Structurally Homogeneous. ChemPhysChem, 14(14), pp.3248–3254. 2013.
For the first time, nascent soot particles are probed by using heliumion microscopy (HIM). HIM is a technique that is similar to scanning electron microscopy (SEM) but it can achieve higher contrast and improved surface sensitivity, especially for carbonaceous materials. The HIM microscope yields images with a high contrast, which allows for the unambiguous recognition of smaller nascent soot particles than those observed in previous transmission electron microscopy studies. The results indicate that HIM is ideal for rapid and reliable probing of the morphology of nascent soot, with surface details visible down to approximately 5 nm, and particles as small as 2 nm are detectable. The results also show that nascent soot is structurally and chemically inhomogeneous, and even the smallest particles can have shapes that deviate from a perfect sphere.

Schenk, M., Leonb, L., Moshammera, K., Oßwalda, P., Zeuch, T., Seider, L., Mauss, F. & KohseHöinghaus, K., Detailed mass spectrometric and modeling study of isomeric butene flames. Combustion and Flame, 160(3), pp.487–503. 2013.
Understanding the combustion chemistry of the butene isomers is a prerequisite for a comprehensive description of the chemistry of C1 to C4 hydrocarbon and oxygenated fuels such as butanol. For the development and validation of combustion models, it is thus crucial to improve the knowledge about the C4 combustion chemistry in detail.<br />
<br />
Premixed lowpressure (40 mbar) flat argondiluted (25%) flames of the three butene isomers (1butene, trans2butene and ibutene) were studied under fuelrich (ϕ = 1.7) conditions using a newly developed analytical combination of highresolution in situ molecularbeam mass spectrometry (MBMS) and in situ gas chromatography (GC). The timeofflight MBMS with its high mass resolution enables the detection of both stable and reactive species, while the gas chromatograph permits the separation of isomers from the same sampling volume. The isomerspecific species information and the quantitative mole fraction profiles of more than 30 stable and radical species measured for each fuel were used to extend and validate the C4 subset of a comprehensive flame simulation model. The experimental data shows different destruction pathways for the butene isomers, as expected, and the model is well capable to predict the different combustion behavior of the isomeric flames. The detailed analysis of the reaction pathways in the flame and the respective model predictions are discussed.

Aryanpour, M., Khetan, A. & Pitsch, H., Activity Descriptor for Catalytic Reactions on Doped Cerium Oxide. ACS Catalysis, 3(6), pp.12531262. 2013.
It is wellknown that ceria enhances chemical activity and catalyst durability in several important catalytic reactions, including CO oxidation and NO reduction. Of great practical value is then having a theoretical model to predict the effect of doping on the ceria activity before the actual. synthesis of its compounds. Such a model is developed in the present work on the basis of experimentally observed data, where we verify our hypothesis that the energy for oxygen vacancy formation is a simple yet powerful activity descriptor for this class of materials. We further benchmark and use our DFT + U computations to estimate this descriptor and to suggest a few transition metals that would increase the activity of ceria toward, redox reactions. This new activity descriptor might be an important factor in similar systems because it does not require any knowledge about the exact chemistry or mechanism of catalysis.

Schaefer, P., Gampert, M. & Peters, N., Joint statistics and conditional mean strain rates of streamline segments. Physica Scripta, T155(014004). 2013.
Based on four different direct numerical simulations of turbulent flows with Taylorbased Reynolds numbers ranging from Re λ = 50 to 300 among which are two homogeneous isotropic decaying, one forced and one homogeneous shear flow, streamlines are identified and the obtained space curves are parameterized with the pseudotime as well as the arclength. Based on local extrema of the absolute value of the velocity along the streamlines, the latter are partitioned into segments following Wang (2010 J. Fluid Mech. 648 183–203). Streamline segments are then statistically analyzed based on both parameterizations using the joint probability density function of the pseudotime lag τ (arclength l , respectively) between and the velocity difference Δ u at the extrema: P ( τ ,Δ u ), ( P ( l ,Δ u )). We distinguish positive and negative streamline segments depending on the sign of the velocity difference Δ u . Differences as well as similarities in the statistical description for both parameterizations are discussed. In particular, it turns out that the normalized probability distribution functions (pdfs) (of both parameterizations) of the length of positive, negative and all segments assume a universal shape for all Reynolds numbers and flow types and are well described by a model derived in Schaefer P et al (2012 Phys. Fluids 24 045104). Particular attention is given to the conditional mean velocity difference at the ending points of the segments, which can be understood as a firstorder structure function in the context of streamline segment analysis. It determines to a large extent the stretching (compression) of positive (negative) streamline segments and corresponds to the convective velocity in phase space in the transport model equation for the pdf. While based on the random sweeping hypothesis a scaling <vertical bar Delta u parallel to tau > proportional to (u(rms)epsilon tau)(1/3) is found for the parameterization based on the pseudotime, the parameterization with the arclength l yields a much larger than expected l 1/3 scaling. A theoretical indication for this finding is given and a scaling of ⟨Δ u ∥ l ⟩∝ l 2/3 is found from the DNS.

Chenadec, V.L. & Pitsch, H., A 3D Unsplit Forward/Backward VolumeofFluid Approach and Coupling to the Level Set Method. Journal of Computational Physics, 233, pp.1033. 2013.
This paper presents a novel methodology for interface capturing in twophase flows by combining a LagrangianEulerian VolumeofFluid approach with a Level Set method. While the VolumeofFluid transport relies on a robust and accurate polyhedral library, any highorder Level Set transport may be used. The method is shown to be less restrictive in terms of CFL conditions than split VolumeofFluid methods. Various geometric integration schemes are proposed and tested. For linear velocity fields, mass error is shown to vanish, and to be third order otherwise. The method is validated on 2D and 3D test cases. Conservation properties are shown to be excellent, while geometrical accuracy remains satisfactory even for complex flows such as the primary breakup of a liquid jet.

Flouros, M., Cottier, F., Hirschmann, M., Kutz, J. & Jocher, A., Ejector Scavenging of Bearing Chambers: A Numerical and Experimental Investigation. Journal Engineering for Gas Turbines and Power, 135(8). 2013.
Oil system architecture in aero engines has remained almost the same for the last 30 years. At least one oil feed pump is responsible for distributing pressurized oil into the bearing chambers, and several scavenge pumps are responsible for evacuating the bearing chambers from the oil and the air mixture. Air is used as the sealing medium in bearing chambers and is the dominant medium in terms of volume occupation and expansion phenomena. In order to simplify the oil system architecture and thus improve the system's reliability with less mechanical parts and also decrease weight, an ejector system has been designed for scavenging bearing chambers. The idea behind the ejector is to use highpressure oil from the feed pump and use it for feeding the ejector's primary jet. Through the momentum transfer between the pressurized oil at the jet's tip and the twophase mixture of air and oil from the bearing chamber, the mixture will be discharged into the oil tank. In order to design the ejector for aero engine applications, enginerelevant performance conditions had to be considered. The design was performed using a onedimensional analysis tool and then considerably refined by using the numerical tool ansys cfx. In a further step, the ejector was manufactured out of pure quartz glass and was tested in a lube rig with a bearing chamber, which has evolved from a real engine application. In the bearing chamber, enginerelevant performance conditions were simulated. Through the provided instrumentation for pressures, temperatures, and air/oil flows, the performance characteristics of the ejector were assessed and were compared to the analytic and numerical results. A highspeed camera was used to record the twophase flow downstream of the bearing chamber in the scavenge pipe. This work is part of the European Unionfunded research program Engine LUBrication System TechnologieS (ELUBSYS) within the 7th EU Frame Programme for Aeronautics and Transport (AAT.2008.4.2.3).

Beeckmann, J., Cai, L. & Pitsch, H., Reduced Chemical Mechanism for the Calculation of Ethanol/Air Flame Speeds. Proceedings of the 9th International Colloquium Fuels  Conventional and Future Energy for Automobiles, January 15th17th, Stuttgart, Germany. 2013.

Vranckx, S., Beeckmann, J., Kopp, W.A., Lee, C., Cai, L., Chakravarty, H.K., Olivier, H., Leonhard, K., Pitsch, H. & Fernandes, R.X., An experimental and kinetic modelling study of nbutyl formate combustion. Combustion and Flame, 160(12), pp.26802692. 2013.
The oxidation of nbutyl formate, a potential biofuel candidate, is studied using three different experimental approaches. Ignition delay times have been measured for stoichiometric mixtures of fuel and air for pressures of about 20 and 90 bar at temperatures from 846 up to 1205 K in a highpressure shock tube. A rapid compression machine has been used to determine the lowtemperature ignition delay times for stoichiometric mixtures at pressures close to 20 bar over the temperature range from 646 K up to 861 K. Laminar burning velocities have been determined for stoichiometric ratios ranging from 0.8 to 1.2 using the highpressure chamber method combined with an optical Schlieren cinematography setup in order to acquire experimental data at elevated pressures of about 10 bar and a temperature of 373 K. A detailed kinetic model has been constructed including hightemperature and lowtemperature reaction pathways. The enthalpies of formation, entropies, and specific heats at constant pressure for the fuel, its primary radicals, and several combustion intermediates have been computed with the CBSQB3 methods and included in the mechanism. This model was validated successfully against the presented data and used to elucidate the combustion of this interesting ester. The importance of accurate inclusion of the lowtemperature peroxy chemistry has been highlighted through sensitivity and reaction path analysis. This study presents the first combustion study of nbutyl formate and leads to an improved understanding of the chemical kinetics of alkyl ester oxidation.

Regele, J.D., Knudsen, E., Pitsch, H. & Blanquart, G., A twoequation model for nonunity Lewis number differential diffusion in lean premixed laminar flames. Combustion and Flame, 160(2), pp.240  250. 2013.
Premixed flames are prone to develop thermodiffusive instabilities when the diffusivity of the fuel is different from the rest of the mixture. Even when a uniform premixed composition is used, the local equivalence ratio across a flame front will not be constant. Therefore, a single quantity such as the progress variable is incapable of modeling accurately the combustion of nonunity Lewis number flames. In this work, a twoequation model is presented for the simulation of premixed laminar flames with nonunity Lewis number fuels. This model relies on the progress variable approach, which is suited for modeling premixed flames in which the fuel's Lewis number is near unity. An additional transport equation for a mixture fraction is derived for nonunity Lewis numbers. The model is verified to be consistent with simple laminar unstretched premixed flames. Hydrogen and propaneair mixtures are used to demonstrate the model's ability to capture the respectively unstable and stable properties of each lean mixture. One dimensional spherical simulations reproduce the effects of flame stretch due to flow strain rate and flame curvature. Finally in two dimensions, the model captures the creation of cellular structures for negative Markstein length flames and the stable propagation of positive Markstein length flames.

Schaefer, L., Göbbert, J.H. & Schröder, W., Dissipation element analysis in experimental and numerical shear flow. European Journal of Mechanics  B/Fluids, 38, pp.85–92. 2013.
Tomographic particleimage velocimetry (TPIV) measurements and direct numerical simulations (DNS) are performed to analyze the small and large scale structure of channel flow data, i.e., the fields of the fluctuations of the three velocity components and the turbulent kinetic energy, respectively, are investigated by dissipation element analysis which is a new method to gain statistical information from turbulent scalar fields based on distinct, spacefilling elements.<br />
<br />
In the inertial range, the conditional mean of the experimental turbulent kinetic energy difference reveals a scaling in the range of the expected characteristic 2/3Kolmogorov scaling. This is in excellent agreement with the numerical results from the channel flow DNS and comparative data from the DNS of a homogeneous shear flow. Furthermore, the shape of the marginal pdfs of the normalized element length and the scalar difference, i.e., the linear rise and the exponential decay, is shown to agree with the findings from the associated DNS. On average, the dissipation elements possess an orientation over a certain bandwidth in the mean flow direction.

Gampert, M., Schaefer, P. & Peters, N., Experimental investigation of dissipationelement statistics in scalar fields of a jet flow. Journal of Fluid Mechanics, 724, pp.337366. 2013.
We present a detailed experimental investigation of conditional statistics obtained from dissipation elements based on the passive scalar field theta and its instantaneous scalar dissipation rate chi. Using highfrequency planar Rayleigh scattering measurements of propane discharging as a round turbulent jet into coflowing carbon dioxide, we acquire with Taylor's hypothesis a highly resolved threedimensional field of the propane mass fraction theta. The Reynolds number (based on nozzle diameter and jet exit velocity) varies between 3000 and 8600. The experimental results for the joint probability density of the scalar difference Delta theta and the length l of dissipation elements resembles those previously obtained from direct numerical simulations of Wang & Peters (J. Fluid Mech., vol. 554, 2006, pp. 457475). In addition, the normalized marginal probability density function (P) over tilde((l) over tilde) of the length of dissipation elements follows closely the theoretical model derived by Wang & Peters (J. Fluid Mech., vol. 608, 2008, pp. 113138). We also find that the mean linear distance l(m) between two extreme points of an element is of the order of the scalar Taylor microscale lambda(u). Furthermore, the conditional mean <Delta theta vertical bar l > scales with Kolmogorov's 1/3 power law. The investigation of the orientation of long dissipation elements in the jet flow reveals a preferential alignment, perpendicular to the streamwise direction for long elements, while the orientation of short elements is close to isotropic. Following an approach proposed by Kholmyansky & Tsinober (Phys. Lett. A, vol. 373, 2009, pp. 23642367), we finally investigate the probability density function of the scalar increment delta theta in the streamwise direction, when strong dissipative events are either retained in or excluded from the measurement volume. In the present study, however, these events are related to maximum points of the scalar dissipation rate chi together with their local extent. When these regions are excluded from the scalar field, we observe a tendency of the probability density function P(delta theta(r)) towards a Gaussian bellshaped curve.

Chenadec, V.L. & Pitsch, H., A CONSERVATIVE FRAMEWORK FOR PRIMARY ATOMIZATION COMPUTATION AND APPLICATION TO THE STUDY OF NOZZLE AND DENSITY RATIO EFFECTS. Atomization and Sprays, 23(12), pp.11391165. 2013.
The present work focuses on the application of two recent developments of the volumeoffluid method to the computational study of primary atomization: a secondorder unsplit volumeoffluid algorithm and a conservative monotonicity preserving discretization of the twophase NavierStokes equations. This novel combination is motivated in the context of primary atomization at arbitrary density ratios. A validation of this framework is first carried in a controlled yet complex numerical experiment, a turbulent twophase temporal jet. An analysis of the conservation errors and the convergence of the dropsize distribution under grid refinement is provided, and was found to substantiate the ability of the method to resolve small droplets. Finally, a comparison of the performances of a simplified yet realistic injector with the breakup resulting from a fully turbulent cylindrical pipe in similar conditions is provided for varying density ratios (40 and 800). The framework is shown to be able to yield quantitative trends regarding the atomizer performances.

Gampert, M., Schaefer, P., Goebbert, J.H. & Peters, N., Decomposition of the turbulent kinetic energy field into regions of compressive and extensive strain. Physica Scripta, 2013(T155), p.014002. 2013.
Based on direct numerical simulations of homogeneous shear turbulence, homogeneous isotropic decaying turbulence and a turbulent channel flow, the scaling of the twopoint velocity difference along gradient trajectories ⟨Δ u n  s<br />
⟩ as well as between the extreme points of the instantaneous turbulent kinetic energy field k is studied. In the first stt<br />
ep, we examine the linear ⟨Δ u n  s ⟩∝ s · a ∞ scaling, where s denotes the separation arclength along a gradient trajee<br />
ctory and a ∞ is the asymptotic value of the conditional mean strain rate of large dissipation elements. Then, we investigate the scaling of the velocity difference between scalar extreme points ⟨Δ u n  l ⟩ as well as the probability densityy<br />
function of the Euclidean distance P (l) between them, conditioned on compressive and extensive strain regions. We observe that while the overall velocity difference along gradient trajectories and between diffusively connected scalar extreme points exhibits linear scaling behaviour ⟨Δ u n  l ⟩∝ l , the conditional velocity differences of extensive ⟨Δ u n  ll<br />
+ ⟩ and compressive regions ⟨Δ u n  l − ⟩ are in contrast to the K41 theory proportional to l 2/3 . The scaling exponn<br />
ent of the overall comes to one part from the purely extensive (compressive) ⟨Δ u n  l + ⟩ (⟨Δ u n  l − ⟩), while thh) >e second contribution is due to the difference in weighting the different regions, thus involving the conditioned pdfs P ( l + ) and P ( l − ). We find that the latter relation scales with l 1/3 . The decomposition of ⟨Δ u n  l ⟩ into two coo ntributions scaling with l 2/3 and l 1/3 , respectively, hence yields an alternative explanation for the observed linear regime.

Knudsen, E., Kolla, H., Hawkes, E.R. & Pitsch, H., LES of a premixed jet flame DNS using a strained flamelet model. Combustion and Flame, 160(12), pp.29112927. 2013.
Turbulent premixed flames in the thin and broken reaction zones regimes are difficult to model with Large Eddy Simulation (LES) because turbulence strongly perturbs subfilter scale flame structures. This study addresses the difficulty by proposing a strained flamelet model for LES of high Karlovitz number flames. The proposed model extends a previously developed premixed flamelet approach to account for turbulence's perturbation of subfilter premixed flame structures. The model describes combustion processes by solving strained premixed fiamelets, tabulating the results in terms of a progress variable and a hydrogen radical, and invoking a presumed PDF framework to account for subfilter physics. The model is validated using two dimensional laminar flame studies, and is then tested by performing an LES of a premixed slotjet direct numerical simulation (DNS). In the premixed regime diagram this slotjet is found at the edge of the broken reaction zones regime. Comparisons of the DNS, the strained flamelet model LES, and an unstrained flamelet model LES confirm that turbulence perturbs flame structure to leading order effect, and that the use of an unstrained flamelet LES model underpredicts flame height. It is shown that the strained flamelet model captures the physics characterizing interactions of mixing and chemistry in highly turbulent regimes. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Kang, S., Pitsch, H. & Hur, N., On a robust ALE method with discrete primary and secondary conservation. Journal of Computational Physics, 254, pp.17. 2013.
In this paper, the objective of the present study is to construct a robust, implicit discretization scheme with the ALE method for deforming grids. In order to minimize the need for an artificial stabilization technique, we aim to enforce the property of discrete secondary conservation as well as primary conservation. The requirements for the spatial and temporal discretization methods were discussed based on consistency between the continuous and discrete differential operators. It was shown that the existing methods combined with the ALE method do not satisfy the secondary conservation property discretely and produce secondorder error terms in time. Therefore, the integrated squared scalar value can be affected spuriously by the advection term. Then, a revised method based on a massweighted interpolation was derived to recover secondary conservation. It was found that the proposed method is an extension of the existing approaches for the secondary conservation law to the cases with deforming grids. When applied to laminar and turbulent flow cases with zero diffusivity or viscosity, the proposed method was found to recover discrete secondary conservation and thus can improve the stability of simulations.

Donde, P., Raman, V., Mueller, M.E. & Pitsch, H., LES/PDF based modeling of soot–turbulence interactions in turbulent flames. Proceedings of the Combustion Institute, 34(1), pp.11831192. 2013.
A large eddy simulation (LES)/probability density function (PDF) approach is used to describe the smallscale soot–turbulence–chemistry interactions. The PDF approach directly evolves the joint statistics of the gasphase scalars and a set of moments of the soot number density function. This LES/PDF approach is then used to simulate a turbulent natural gas jet diffusion flame. Since the PDF equation is high dimensional, a Lagrangian method formulated in cylindrical coordinates is coupled to the Eulerian solution technique for the LES flow equations. The LES/PDF simulations show that soot formation is highly intermittent and is always restricted to the fuelrich region of the flow. The PDF of soot moments has a wide spread leading to a large subfilter variance. Further, the conditional statistics of soot moments conditioned on mixture fraction and reaction progress variable show strong correlation between the gas phase composition and soot moments.

Lange, L., Korkmaz, M., Heinze, J., Magens, E. & Willert, C., Planar concentration measurements of
carbon monoxide (CO) using 2photon planar
LIF in a generic model combustor. In Gordon Research Seminar and Conference 2013 "Laser Diagnostics in Combustion". 2013.

Cai, L., Beeckmann, J. & Pitsch, H., A Consistent Chemical Mechanism for DME Oxidation. In Proceedings of the European Combustion Meeting, June 25th28th, Lund, Sweden. 2013.

Hennig, F., Boschung, J., Gauding, M. & Peters, N., Direct Numerical Simulation of a Temporal Mixing Layer and Detection of the Turbulent/NonTurbulent Interface. 66th Annual Meeting of the APS Division of Fluid Dynamics, November 24th26th, Pittsburgh, USA. 2013.
The direct numerical simulation of a temporally evolving mixing layer is presented. Using the DNS data we compare two different approaches of detecting the so called turbulent/nonturbulent interface that is found between the fully turbulent and the irrotational outer flow. Standard and conditional statistics are evaluated and compared with literature results.

Gampert, M., Schaefer, P., Narayanaswamy, V. & Peters, N., Gradient trajectory analysis in a Jet flow for turbulent combustion modelling. Journal of Turbulence, 14(1), pp.147164. 2013.
Based on planar highspeed Rayleigh scattering measurements of the mixture fraction Z of propane discharging from a turbulent round jet into coflowing carbon dioxide at nozzlebased Reynolds numbers Re0 = 30008600, we use scalar gradient trajectories to investigate the local structure of the turbulent scalar field with a focus on the scalar turbulent/nonturbulent interface. The latter is located between the fully turbulent part of the jet and the outer flow. Using scalar gradient trajectories, we partition the turbulent scalar field into these three regions according to an approach developed by Mellado et al. (J.P. Mellado, L. Wang, and N. Peters, Gradient trajectory analysis of a scalar field with external intermittency, J. Fluid Mech. 626 (2009), pp. 333365.). Based on these different regions, we investigate in a next step zonal statistics of the scalar probability density function (pdf) P(Z) as well as the scalar difference along the trajectory Delta Z and its mean scalar value Z(m), where the latter two quantities are used to parameterize the scalar profile along gradient trajectories. We show that the scalar pdf P(Z) can be reconstructed from zonal gradient trajectory statistics of the joint pdf P(Z(m), Delta Z). Furthermore, on the one hand we relate our results for the scalar turbulent/nonturbulent interface to the findings made in other experimental and numerical studies of the turbulent/nonturbulent interface, and on the other hand discuss them in the context of the flamelet approach and the modelling of pdfs in turbulent nonpremixed combustion. Finally, we compare the zonal statistics for P(Z) with the composite model of Effelsberg and Peters (E. Effelsberg and N. Peters, A composite model for the conserved scalar pdf, Combust. Flame 50 (1983), pp. 351360) and observe a very good qualitative and quantitative agreement.

Bode, M., Bisetti, F., Collier, N. & Pitsch, H., Multidimensional Flamelet Lookup Tables Using BSpline Interpolation. In Proceedings of the European Combustion Meeting 2013, June 25th28th, Lund, Sweden. 2013.

Beeckmann, J., Pitsch, H., Chaumeix, N., Dagaut, P., Dayma, G., Egolfopoulos, F., Foucher, F., Halter, F., MounaïmRousselle, C., Renou, B., Varea, E., de Goey, P. & Volkov, E., Collaborative Study for Accurate Measurements of Laminar Burning Velocity. Proceedings of the European Combustion Meeting 2013, June 25th28th, Lund, Sweden. 2013.

Jochim, B., Vanegas, A. & Pitsch, H., Development of an efficient and accurate multizone model for modelbased PCCI Diesel engine control. 26. Deutscher Flammentag, 2161, pp.377386. 2013.

Pitsch, H., Why You Need LES for Predicting Soot Emissions in Turbulent Combustion. ERCOFTAC WORKSHOP Direct and LargeEddy Simulation, April 3rd5th, Dresden, Germany. 2013.

Mueller, M.E., Iaccarino, G. & Pitsch, H., Chemical kinetic uncertainty quantification for Large Eddy Simulation of turbulent nonpremixed combustion. Proceedings of the Combustion Institute, 34(1), pp.12991306. 2013.
While the accuracy of chemical kinetic mechanisms continues to improve, these mechanisms are still models with, sometimes considerable, uncertainty. In order to rigorously validate turbulent combustion simulations against experimental data, this uncertainty must be separated from deficiencies in the turbulent combustion model itself. In this work, a method is developed for quantifying the uncertainty in turbulent flame simulations due to input uncertainty in the chemical mechanism. Here the method is developed for Large Eddy Simulation (LES) combined with a steady flamelet model. Rather than a brute force probabilistic approach in which hundreds or thousands of LES runs are required to compute statistics of outputs of interest, the method takes advantage of the actual algorithm employed with the steady flamelet model. First, the highdimensional uncertainty in the chemical kinetics is propagated through the flamelet equations, and the resulting lowerdimensional joint distribution of the temperature, species mass fractions, and other derived quantities is used as a stochastic equation of state in the LES. Since only a few äctive" quantities are needed to evolve the LES governing equations, efficient nonintrusive stochastic collocation is used to propagate the uncertainty in the density, requiring only a few LES runs. This process captures the uncertainty in the flow field induced by the uncertainty in the chemical kinetic rates. The remaining uncertainty in "passive" quantities, that is, quantities needed only for postprocessing such as the temperature and species mass fractions, is computed with random sampling during the LES runs. The uncertainty quantification algorithm is demonstrated with Sandia flame D, and it is shown that the uncertainty in the simulation results caused by uncertainties in the kinetic rates is sufficiently large to account for the discrepancies with the experimental measurements. The implication is that the turbulent combustion model cannot be fairly assessed with such a large uncertainty.

Kleinheinz, K., Gampert, M., Pitsch, H. & Peters, N., Experimental and numerical study of the turbulent/nonturbulent interface in a turbulent round jet flow. In European Turbulence Conference 14, September 1st4th, Lyon, France. 2013.
Based on two largeeddy simulations (LES) of a nonreacting turbulent round jet with a nozzle based Reynolds number of 8,610 with the same configuration as the one that has recently been investigated experimentally (Gampert et al., 2012; J Fluid Mech, 2012; J Fluid Mech 724:337, 2013), we examine the scalar turbulent/nonturbulent (T/NT) interface layer in the mixture fraction field of the jet flow between ten and thirty nozzle diameters downstream. To this end, the LES—one with a coarse grid and one with a fine grid—are in a first step validated against the experimental data using the axial decay of the mean velocity and the mean mixture fraction as well as based on radial selfsimilar profiles of mean and root mean square values of these two quantities. Then, probability density functions (pdf) of the mixture fraction at various axial and radial positions are compared and the quality of the LES is discussed. In general, the LES results are consistent with the experimental data. However, in the flow region where the imprint of the T/NT interface layer is dominant in the mixture fraction pdf, discrepancies are observed. In a next step, statistics of the T/NT interface layer are studied, where a satisfactory agreement for the pdf of the location of the interface layer from the higher resolved LES with the experimental data is observed, while the one with the coarse grid exhibits considerable deviations. Finally, the mixture fraction profile across the interface is investigated where the same trend as for the pdf of the location is present. In particular, it is found that the sharp interface that is present in experimental studies (Gampert et al., J Fluid Mech, 2013; Westerweel et al., J Fluid Mech 631:199, 2009) is less distinct in the LES results and rather diffused in radial direction outside of the T/NT interface layer.

Khetan, A., Han, B. & Pitsch, H., Structure & Activity of Pt on CeO2 (111) Surface Using First Principles Thermodynamics. Fabrication, Structure and Reactivity of Anchored Nanoparticles: Faraday Discussion 162, April 10th12th, Berlin, Germany. 2013.

Doran, E.M., Pitsch, H. & Cook, D.J., A priori testing of a twodimensional unsteady flamelet model for threefeed combustion systems. Proceedings of the Combustion Institute, 34(1), pp.13171324. 2013.
A number of interesting combustion applications involve the mixing of multiple fuel or air streams, such as splitinjection and dualfuel internal combustion engines, which are the main focus of this work, or secondary air in gas turbines. In this study, a twodimensional flamelet formulation for modeling autoignition and combustion in multistream configurations is presented and validated using twodimensional direct numerical simulations (DNS). The DNS simulations were carried out for a multistream nheptane mixture in constant volume isotropic turbulence with finiterate chemistry according to a reduced chemical mechanism which considers 44 species and 185 reactions. The DNS represents two fuel streams and an oxidizer stream, where the second fuel stream is introduced at a time after the first fuel and oxidizer have started to mix. The resulting data are then used to carry out a priori testing of the twodimensional flamelet model. The flamelet formulation is found to accurately represent the interaction of the multiple stream mixing and chemistry, thus enabling the accurate prediction of the autoignition of each fuel stream.

Pitsch, H., High Fidelity Modeling of Pollutant Emissions from Real Combustion Systems. In COMBURA Symposium, October 9th10th, Maastricht, Netherlands. 2013.

Kleinheinz, K., Mittal, V., Trisjono, P. & Pitsch, H., LargeEddy Simulation of a Piloted Premixed Jet Burner, 2013.

Wada, T., Sudholt, A., Pitsch, H. & Peters, N., Analysis of first stage ignition delay times of dimethyl ether in a laminar flow reactor. Combustion Theory and Modelling, 17(5), pp.906934. 2013.
The combustion chemistry of the first stage ignition and chemistry/flow interactions are studied for dimethyl ether (DME) with a mathematical analysis of two systems: a plug flow reactor study is used to reduce the reaction chemistry systematically. A skeletal reaction mechanism for the low temperature chemistry of DME until the onset of ignition is derived on the basis of the detailed DME mechanism of the Lawrence Livermore National Laboratory  see Curran, Fischer and Dryer, Int. J. Chem. Kinetics, Vol. 32 (2000). It is shown that reasonably good results for ignition delay times can be reached using a simple system of three ordinary differential equations and that the resulting analytical solution depends only on two reaction rates and the initial fuel concentration. The stepwise reduction of the system based on assumptions yields an understanding on why these reactions are so important. Furthermore, the validation of the assumptions yields insight into the influence of the fuel and the oxygen concentration on the temperature during the induction phase. To investigate the influence of chemistry/flow interactions, a 2D model with a laminar HagenPoiseuille flow and 2Dpolynomial profiles for the radial species concentration is considered. For the 2D model, it is found that only the diffusion coefficients and the reactor radius need to be taken into consideration additionally to describe the system sufficiently. Also, the coupling of flow and chemistry is clarified in the mathematical analysis. The insight obtained from the comparison of the 2D model and the plug flow model is used to establish an average velocity for the conversion of ignition locations to ignition delay times in a laminar flow reactor. Finally, the 2D analytical solution is compared against new experimental data, obtained in such a laminar flow reactor for an undiluted DME/air mixture with an equivalence ratio of phi = 0.835 and a temperature range of 555 to 585 K at atmospheric pressure.

Han, B., Viswanathan, V. & Pitsch, H., FirstPrinciples Based Analysis of the Electrocatalytic Activity of the Unreconstructed Pt(100) Surface for Oxygen Reduction Reaction. The Journal of Physical Chemistry C, 116(10), pp.61746183. 2012.
We apply a rigorous computational procedure combining ab initio DFT calculations and statistical mechanics based methods to examine the electrocatalytic activity of the unreconstructed Pt(100) surface for oxygen reduction reaction. Using the cluster expansion formalism, we obtain stable interfacial water structures using Monte Carlo simulations carried out using parametrized interactions of water–water and water–metal. We find that both longrange and multibody interactions are important to describe the adsorbate interactions as a consequence of the mismatch between the preferred “hexagonal” water overlayer and the underlying square symmetry of the (100) surface. Our results indicate that the stable interfacial water structure is substantially different from that found on the Pt(111) surface. We compute the potentialdependent equilibrium coverages of oxygencontaining adsorbates, which shows that the surface is poisoned by strongly adsorbed OH. We construct the freeenergy diagram of intermediates for oxygen reduction reaction on the Pt(100) surface and find that the limiting step is the reduction of the strongly adsorbed OH. We also find that, at a given potential, a higher degree of poisoning by OH is the reason unreconstructed (100) surfaces are catalytically less active than (111) surfaces. This study shows the importance of accurately capturing atomistic interactions beyond the nearest neighbor pairs.

Ihme, M. & Pitsch, H., On the generation of direct combustion noise in turbulent nonpremixed flames. International Journal of Aeroacoustics, 11(1), pp.2578. 2012.
Generation of combustion noise in an unconfined turbulent nonpremixed flame is investigated. For this, a model is developed, combining Lighthill's acoustic analogy with a flameletbased combustion model to consistently express all thermochemical quantities by a set of reduced scalars. The model is applied in a largeeddy simulation, and the acoustic pressure in the far field is obtained from an integral solution. Three relevant acoustic source terms with different source characteristics and Mach number scaling are identified. The spatial distribution and spectral characteristics of the acoustic sources are analyzed, and it is shown that the acoustic source due to chemical reaction is the main noise contributor, and is located in the upper part of the flame. Contributions from the acoustic sources due to Reynolds stresses and fluctuating mass flux are found to be virtually insignificant at low frequencies. Discrepancies in the prediction of highfrequency sound pressure level in the jet forward direction were analyzed and are attributed to highfrequency acoustic refraction effects due to variations in sound speed. The directivity exhibits a weak directionality in the 30 degrees forward direction, and some phase cancellation between individual acoustic sources is evident.

Schaefer, L., Goebbert, J.H. & Schröder, W., Dissipation element analysis in experimental and numerical shear flow. European Journal of Mechanics B/Fluids. 2012.

Schaefer, P., Gampert, M. & Peters, N., Streamline segment topology in the vicinity of stagnation points in turbulent flows. 15th International Symposium on Flow Visualization, June 25th28th, Minsk, Belarus. 2012.

Gampert, M., Schaefer, P. & Peters, N., Dissipation element analysis via highspeed rayleigh scattering in a turbulent jet flow. 15th International Symposium on Flow Visualization, June 25th28th, Minsk, Belarus. 2012.
Based on the extreme points of turbulent scalar fields, Wang and Peters (2006, 2008) developed the theory of dissipation elements. Starting from every grid point, trajectories along the ascending and descending gradient directions can be calculated, which inevitably end in extreme points. All points that share the same two ending points define a finite volume called dissipation element, which is parameterized by its linear length l between and the scalar difference ∆θ at the extreme points. Based on this theory, space filling and nonarbitrary elements are identified, which allow the reconstruction of statistical properties of the field as a whole in terms of conditional statistics within the elements. In the present study, a turbulent round propane jet discharging from a nozzle with diameter d=12mm into surrounding CO2 has been chosen as the core of the experimental setup. The free shear flow, i.e. the mixture fraction of propane and CO2, is visualized via Rayleigh scattering of a diode pumped double cavity Nd:YLF laser at the molecules. As we need a threedimensional test section, in which dissipation elements can be identified, a laser sheet is used to illuminate a twodimensional plane perpendicular to the jet axis. The resulting signal is recorded with a high speed CMOS camera. In a next step, the recorded time series of the planar test section at a fixed downstream position is transformed into a spatial signal based on Taylor’s hypothesis so that we obtain a frozen threedimensional volume of the mixture fraction field. The latter can be postprocessed using the same algorithms as for the DNS and yields a good agreement of the statistical properties of dissipation elements on the one hand and allow their visualization on the other hand.

Gampert, M., Schaefer, P. & Peters, N., Scalar superlayer contributions to the mixture fraction pdf in a jet flow. 7th International Symposium on Turbulence, Heat and Mass Transfer, September 24th27th, Palermo, Italy. 2012.
Based on twodimensional highspeed measurements of the mixture fraction Z in a turbulent round jet with nozzle based Reynolds numbers between 3,600 and 8,000, we investigate the scalar superlayer of the flow. Since combustion occurs in the vicinity of the stoichiometric mixture fraction, which is around Z=0.06 for typical fuel/air mixtures, it is expected to take place largely within that layer. Therefore, profound understanding of this part of the flow is essential for an accurate modeling of turbulent nonpremixed combustion. To this end, we validate a composite model of Effelsberg & Peters 1 for the pdf P(Z) which takes into account the different contributions stemming from the fully turbulent as well as the superlayer part of the flow. We observe a very good agreement between the measurements and the model over a wide range of flow conditions.

Berr, N., Schmidl, D., Göbbert, J.H., Lankes, S., an Mey, D., Bemmerl, T. & Bischof, C., TrajectorySearch on ScaleMP's vSMP Architecture. In Applications, Tools and Techniques on the Road to Exascale Computing : proceedings of the 14th biennial ParCo conference ; ParCo2011 ; held in Ghent, Belgium / Ed. by Koen De Bosschere .. Advances in Parallel Computing ; 22. New York, NY: IOS Press: . 2012.

Mittal, V., Pitsch, H. & Egolfopoulos, F., Assessment of counterflow to measure laminar burning velocities using direct numerical simulations. Combustion Theory and Modelling, 16(3), pp.419433. 2012.
The laminar burning velocity is a fundamental property that is extensively used in the study and modelling of premixed combustion processes. A counterflow flame configuration is commonly used to measure this quantity for different combustion systems. In this procedure, the burning velocities are typically measured at various low stretch conditions and the unstretched burning velocity is extrapolated from these measurements. This extrapolation is done assuming a theoretically onedimensional system along the centreline. We analyse the validity of this assumption by performing DNS studies with finite rate chemistry of the experimental counterflow configuration. The extrapolation process using onedimensional computations is performed on the DNS data and the extrapolated value is compared to the computed laminar burning velocity for the chemical mechanism used. We show that the assumption works well if the nozzle exit velocity has a nearly tophat profile. For nonuniform velocity profiles, it is shown that the temperature curvature at the centreline becomes important. This effect cannot be captured by the onedimensional formulation. Thus, experimental studies measuring laminar burning velocity need to ensure that the nozzle velocity profile is very close to uniform. The extrapolation to zero stretch using 1D counterflow simulations can be performed in different ways. Based on the results obtained in this paper, a simple and accurate extrapolation method is proposed.

Berr, N., Schmidl, D., Göbbert, J.H., Lankes, S., an Mey, D., Bemmerl, T. & Bischof, C., TrajectorySearch on ScaleMP's vSMP Architecture. In Applications, Tools and Techniques on the Road to Exascale Computing. Advances in Parallel Computing. pp. 227234. 2012.
ScaleMP's vSMP software turns commodity Infiniband clusters with Intel's x86 processors into large shared memory machines providing a single system image at low cost. However, codes need to be tuned to deliver good performance on these machines. TrajSearch, developed at the Institute for Combustion Technology at RWTH Aachen University, is a postprocessing code for analyzing trajectories in three dimensional turbulent flow fields. The initial OpenMP version of TrajSearch has been tuned to run very efficiently on the vSMP cluster which exhibits a pronounced NUMA behavior. As a side effect, the code also performs nicely on a more expensive large SGI Altix UV shared memory machine.

Vannier, V., Schenk, M., KohseHöinghaus, K. & Bahlawane, N., Preparation and characterisation of chromiumdoped cobalt oxide spinel thin films. Journal of Materials Science, 47(3), pp.13481353. 2012.
Thin films of cobaltbased oxide spinel were prepared by pulsed spray evaporation chemical vapour deposition (PSECVD), and doping with chromium was systematically investigated up to a Cr/Co ratio of 0.096, corresponding to a stoichiometry of Co3−x Cr x O4 with x = 0.00–0.26. The effect of doping concentration on the structure, assessed by Xray diffraction and Raman scattering, and on the optical and electrical properties of the oxide films was investigated. Singlephase spinel could be preserved for stoichiometries below x = 0.22. The influence of Crdoping on the band gap energies and on the electrical conductivity was determined, and the obtained results were exploited to discuss the cationic site occupations. The influence of Cr doping was complemented by the investigation of the surface catalytic reactivity towards the oxidation of dichloromethane.

Varea, E., Modica, V., Vandel, A. & Renou, B., Measurement of laminar burning velocity and Markstein length relative to fresh gases using a new postprocessing procedure. Combustion and Flame, 159(2), pp.577–590. 2012.
The purpose of this study is to present a new tool for extracting the laminar burning velocity in the case of spherically outward expanding flames. This new procedure makes it possible to determine the laminar burning velocity directly based on the flame displacement speed and the global fresh gas velocity near the preheat zone of the flame front. It therefore presents a very interesting alternative to the standard method (commonly used in the literature), which is based on the flame front displacement and the ratio of unburned and burned gas densities. The influence of external flame stretching on the burning velocity can be characterized and the Markstein length relative to the unburned gases (i.e., fresh gases) can be deduced by using this new tool. Contrary to the standard procedure, the unstretched laminar burning velocity is determined directly without using the fuel mixture properties. The temporal evolution of the flame front is visualized by highspeed laser tomography and the algorithm, based on a tomographic image correlation method, makes it possible to accurately measure the fresh gas velocity near the preheat zone of the flame front. The measurements of laminar flame speeds are carried out in a highpressure and hightemperature constantvolume vessel over a wide range of equivalence ratios for methane, ethanol, and isooctane/air mixtures. To validate the experimental facility and the postprocessing of the flame images, fresh gas velocities and unstretched laminar burning velocities, as well as Markstein lengths relative to burned and unburned gases, are presented and compared with experimental and numerical results of the literature for methane/air flames. New results concerning ethanol/air and isooctane/air flames are presented for various experimental conditions (373 K, equivalence ratios range 0.7–1.5, pressure range 0.1–5 MPa).

Cai, L., Beeckmann, J., Pitsch, H. & Raman, V., Iterative Mechanism Optimization Using Modelbased Experimental Design. 34th International Symposium on Combustion, July 29thAugust 3rd, Warsaw, Poland. 2012.

Lodato, G., Ham, F. & Pitsch, H., Optimal Inclusion of Transverse Effects in the NonReflecting Outflow Boundary Condition. AIAA Journal, 50(6), pp.12911306. 2012.
The inclusion of transverse effects in designing characteristic boundary conditions for the Euler and NavierStokes equations was discussed by different authors and has proved to give some improvement in the perspective of reducing numerical perturbations generated at open boundaries. Based on the most general characteristic formulation using nonorthogonal and rotated reference frames, an analysis of the different terms involved in such an approach is carried out in the present work. To achieve the best performance for the numerical behavior of the boundary condition, it is then shown that different transverse terms need to be treated differently when included in the definition of the incoming wave amplitude variations. The analysis is supported by a series of numerical tests involving an inviscid vortex convected at an angle or a purely acoustic wave propagating radially. It is concluded that the optimal behavior in terms of acoustic reflection by outgoing vorticity can be achieved by minimizing the contribution of the transverse terms related to the material derivatives along the bicharacteristics, either by properly relaxing them or by selecting a characteristic direction which is not necessarily orthogonal to the boundary.

Attili, A. & Bisetti, F., Statistics and scaling of turbulence in a spatially developing mixing layer at Reλ = 250. Physics of Fluids, 24(3), p.035109. 2012.

Pecnik, R., Terrapon, V., Ham, F., Iaccarino, G. & Pitsch, H., ReynoldsAveraged NavierStokes Simulations of the HyShot II Scramjet. AIAA Journal, 50(8), pp.17171732. 2012.
The internal flow in the HyShot II scramjet is investigated through numerical simulations. A computational infrastructure to solve the compressible Reynoldsaveraged NavierStokes equations on unstructured meshes is introduced. A combustion model based on tabulated chemistry is considered to incorporate detailed chemical kinetics mechanics while retaining a low computational cost. Both nonreactive and reactive simulations have been performed, and results are compared with ground test measurements obtained at DLR, German Aerospace Center. Different turbulence models were tested, and the dependence on the mesh is assessed through grid refinement. The comparison with experimental data shows good agreement, although the computed heat fluxes at the wall are higher than measurements for the reactive case. A sensitivity analysis on the turbulent Schmidt and Prandtl numbers shows that the choice of these parameters has a strong influence on the results. In particular, variations of the turbulent Prandtl number lead to large changes in the heat flux at the walls. Finally, the inception of thermal choking is investigated by increasing the equivalence ratio, whereby a normal shock is created locally and moves upstream, leading to a large increase in the maximum pressure. Nevertheless, a large portion of the flow is still supersonic.

Schaefer, P., Curvature statistics of streamlines in various turbulent flows. In Journal of Turbulence. 2012.

Won, H.W., Peters, N., Tait, N. & Kalghatgi, G., Sufficiently premixed compression ignition of a gasolinelike fuel using three different nozzles in a diesel engine. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 226(5), pp.698708. 2012.
Fuels that are more resistant to autoignition allow more time for mixing before combustion occurs and help to reduce nitrogen oxides (NOx) and smoke in a diesel engine. However, hydrocarbon (HC) and carbon monoxide (CO) emissions are high at low loads because combustion is more likely to take place in lean mixture packets with better mixing caused by longer ignition delays. These problems can be significantly alleviated by managing the mixture strength by changing the injection pressure and the nozzle geometries. A singlecylinder diesel engine is run on a mixture of gasoline and diesel with a research octane number of 91, at different speeds, loads, and exhaust gas recirculation levels using three different nozzles. It is much easier to obtain low NOx and low smoke emissions with this fuel than with a European diesel fuel using the standard nozzle. Larger injector holes and lower injection pressures help to reduce the HC and CO emissions at low loads and also enable the gasolinelike fuel to run at a higher speed of 4000 r/min at a reasonably high load (indicated mean effective pressure) of 10 bar.

Schaefer, P., Gampert, M. & Peters, N., The length distribution of streamline segments in homogeneous isotropic decaying turbulence. Physics of Fluids, 24(4). 2012.
Based on the profile of the absolute value u of the velocity field u(i) along streamlines, the latter are partitioned into segments at their extreme points as proposed by Wang J. Fluid Mech. 648, 183203 (2010). It is found that the boundaries of all streamline segments, i.e., points where the gradient projected in streamline direction. partial derivative(u)/partial derivative(s) vanishes, define a surface in space. This surface also contains all local extreme points of the scalar ufield, i.e., points where the gradient in all directions of the field of the absolute value of the velocity and thereby those of the turbulent kinetic energy (k = u(2)/2, where k is the instantaneous turbulent kinetic energy) vanishes. Such points also include stagnation points of the flow field, which are absolute minimum points of the turbulent kinetic energy. As local extreme points are the ending points of dissipation elements, an approach for spacefilling geometries in turbulent scalar fields, such elements in the turbulent kinetic energy field also end and begin on the surface and the temporal evolution of dissipation elements and streamline segments are intimately related. Streamline segments by construction evolve both morphologically and topologically. A morphological evolution of a streamline segment corresponds to a continuous deformation when it is subject to stretching or compression and thus also implies a continuous evolution of the arclength l with time. Such an evolution does not change the number of the overall streamline segments and hence does not involve counting. On the other hand a topological evolution corresponds to either a cutting of a large segment into smaller ones or a connection of two smaller ones to form a larger segment. Such a process changes the number of segments and thus involves counting. This change of integer variables (i.e., counting) yields discrete jumps in the length of the streamline segment which are discontinuous in time. Following the terminology by Schaefer et al. "Fast and slow changes of the length of gradient trajectories in homogenous shear turbulence," in Advances in Turbulence XII, edited by B. Eckhardt (SpringerVerlag, Berlin, 2009), pp. 565572 we will refer to the morphological part of the evolution of streamline segments as slow changes while the topological part of the evolution is referred to as fast changes. This separation yields a transport equation for the probability density function (pdf) P(l) of the arclength l of streamline segments in which the slow changes translate into a convection and a diffusion term when terms up to second order are included and the fast changes yield integral terms. The overall temporal evolution (morphological and topological) of the arclength l of streamline segments is analyzed and associated with the motion of the above isosurface. This motion is diffusion controlled for small segments, while large segments are mainly subject to strain and pressure fluctuations. The convection velocity corresponds to the first order jump moment, while the diffusion term includes the second order jump moment. It is concluded, both theoretically and from direct numerical simulations (DNS) data of homogeneous isotropic decaying turbulence at two different Reynolds numbers, that the normalized first order jump moment is quasiuniversal, while the second order one is proportional to the inverse of the square root of the Taylor based Reynolds number Relambda(1/2). Its inclusion thus represents a small correction in the limit of large Reynolds numbers. Numerical solutions of the pdf equation yield a good agreement with the pdf obtained from the DNS data. The interplay of viscous drift acting on small segments and linear strain acting on large segments yield, as it has already been concluded for dissipation elements, that the mean length of streamline segments should scale with Taylor microscale.

Won, H.W., Pitsch, H., Tait, N. & Kalghatgi, G., Some effects of gasoline and diesel mixtures on partially premixed combustion and comparison with the practical fuels gasoline and diesel in a compression ignition engine. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 226(9), pp.12591270. 2012.
If fuels that are more resistant to autoignition are injected near top dead centre in compression ignition engines, they ignite much later than diesel fuel does, and combustion occurs when the fuel and air have had more chance to mix. This helps to reduce nitrogen oxide and smoke emissions. Moreover, this can be achieved at much lower injection pressures than for a diesel fuel. However, it is preferable to have fuels with a lower research octane number than those of commonly available gasolines, because this makes lowload operation easier while retaining the advantages at higher loads. A practical approach to making such fuels is to blend the gasoline and diesel fuel available in the market. Such fuel blends have a wide volatility range since they contain highboilingpoint components from the diesel but have a lower research octane number than that of the gasoline used but have a much longer ignition delay than that of the diesel fuel. This work describes the results of running a singlecylinder diesel engine on three such fuel blends. The engine could be run on such blends with extremely low smoke and low nitrogen oxide emissions at speeds of up to 4000 r/min and loads (indicated mean effective pressures) of up to 10 bar with an injection pressure of only 400 bar. The smoke levels at comparable nitrogen oxide levels were extremely high with diesel fuel in these conditions, even with an injection pressure of 1100 bar. The engine could also be run at nearidle conditions on these blends but with higher hydrocarbon and carbon monoxide emissions but much lower nitrogen oxide emissions and maximum pressure rise rate compared with those of the diesel fuel. The wider volatility range might be of benefit in avoiding overmixing and overleaning, which could lead to poor combustion stability. The paper also considers the tradeoffs between the nitrogen oxide, smoke, hydrocarbon and carbon monoxide emissions and the maximum pressure rise rate and discusses approaches to optimise this type of combustion.

Raessi, M. & Pitsch, H., Consistent mass and momentum transport for simulating incompressible interfacial flows with large density ratios using the level set method. Computers & Fluids, 63, pp.7081. 2012.
There exists a sharp density jump at the fluid interfaces in most of interfacial flows encountered in engineering applications. Numerical treatment of the discontinuous fluid density in such flows is a great challenge. In particular, numerical errors can lead to nonphysical results at large density ratios, hence limit a flow solver to only small and usually unrealistic density ratios. We introduce a consistent mass and momentum transport method which allows for simulations of interfacial flows with large density ratios in the context of the level set method. Fluid momentum is transported by discretizing the conservative form of the convective term in the momentum equation. A new technique is introduced to compute the flux density based on the temporal evolution of the fluid interfaces and by using the level set function at two subsequent time levels. For consistency, we use the same flux density for both mass and momentum transport, and thereby establish a tight coupling between them. We assess the performance of the proposed method by using a set of benchmark test cases, in which the density ratio ranges from 650 to 10(6). The new method is stable and accurate even at extreme density ratios. The numerical results agree very well with the theoretical and experimental results. In contrast, the results of a nonconservative formulation show nonphysical fluid behavior, where the dense fluid is slowed down by the light fluid due to numerical errors.

Soliman, A.M., Mansour, M.S., Peters, N. & Morsy, M.H., Dissipation element analysis of scalar field in turbulent jet flow. Experimental Thermal and Fluid Science, 37, pp.5764. 2012.
For better understanding of turbulence, the geometry of turbulent structures in turbulent jet flow should be analyzed. The aim of the present work was to experimentally verify the dissipation element theory on highly resolved twodimensional measurements turbulent jets using Rayleigh scattering technique. The statistical analysis of the characteristic parameters of dissipation elements; namely the linear length connecting the extremal points and the absolute value of the scalar difference at these points, respectively was also investigated. Rayleigh scattering was used to topographically produce 2D images of turbulent mixing to obtain the concentration distribution of two gases in a turbulent shear flow. The scalar field obtained was subdivided into numerous finite size regions. In each of these regions local extremal points of the fluctuating scalar are determined via gradient trajectory method. Gradient trajectories starting from any point in the scalar field phi(x, y) in the directions of ascending and descending scalar gradients will always reach a minimum and a maximum point where del phi = 0. The dissipation element has two extremal points (one maximal and one minimal) and two saddle points at the boundaries.

Knudsen, E. & Pitsch, H., Capabilities and limitations of multiregime flamelet combustion models. Combustion and Flame, 159(1), pp.242264. 2012.
Flamelet combustion models typically assume that burning occurs in either a fully premixed or a fully nonpremixed mode. These assumptions tend to limit the applicability of the models to singleregime combustors. Efforts aimed at reducing this limitation have introduced multiregime approaches that account for different types of mixing and chemistry interactions. In this study a multiregime model is applied to two laminar nheptane flames in an effort to characterize the capabilities and limitations of the approach. Both a 2D laminar triple flame and a 2D laminar counterflow diffusion flame are numerically simulated using the multiregime model. Data for comparison is generated by additionally simulating the flames using finite rate chemistry, a purely premixed flamelet model, and a purely nonpremixed flamelet model. Simulations demonstrate that the multiregime approach functions as desired, and tends to access flamelets from the appropriate regime under both nonpremixed and premixed conditions. Some important differences between the flamelet solutions and finite rate solution are observed, however. These differences are caused by the finite rate solution deviating away from the assumed flamelet manifolds, rather than by inadequate regime predictions. In the analyses of these simulations, an emphasis is placed on understanding the formation of the pollutant species NO. It is shown that even when the local combustion regime is correctly predicted, small deviations from an assumed flamelet manifold can lead to changes in the NO production rate. The simulation results confirm that multiregime flamelet models are applicable to a wide variety of reacting flows, but the results also help to characterize the limitations of these models.

Viswanathan, V., Hansen, H.A., Rossmeisl, J., Jaramillo, T.F., Pitsch, H. & Nørskov, J.K., Simulating Linear Sweep Voltammetry from FirstPrinciples: Application to Electrochemical Oxidation of Water on Pt(111) and Pt3Ni(111). The Journal of Physical Chemistry C, 116(7), pp.46984704. 2012.
Cyclic voltaminetry is a fundamental experimental method for characterizing adsorbates on electrochemical surfaces. We present a model for the electrochemical solid liquid interface, and we simulate the linear sweep voltammogram of the electrochemical oxidation of H2O on Pt(111) and Pt3Ni(111), based on kinetic and thermodynamic parameters computed by Density Functional Theory (DFT) and the dynamics of the system solved through Monte Carlobased methods. The model predicts onset of OH and O formation in good agreement with voltammetric and ex situ XPS experiments.

Knudsen, E., Richardson, E.S., Doran, E.M., Pitsch, H. & Chen, J.H., Modeling scalar dissipation and scalar variance in large eddy simulation: Algebraic and transport equation closures. Physics of Fluids, 24(5). 2012.
Scalar dissipation rates and subfilter scalar variances are important modeling parameters in large eddy simulations(LES) of reacting flows. Currently available models capture the general behavior of these parameters, but these models do not always perform with the degree of accuracy that is needed for predictive LES. Here, two direct numerical simulations (DNS) are used to analyze LES dissipation rate and variance models, and to propose a new model for the dissipation rate that is based on a transport equation. The first DNS that is considered is a nonpremixed autoigniting C2H4 jet flame simulation originally performed by Yoo et al. Proc. Combust. Inst.33, 1619–1627 (2011)10.1016/j.proci.2010.06.147. A LES of this case is run using algebraic models for the dissipation rate and subfilter variance. It is shown that the algebraic models fail to adequately reproduce the DNS results. This motivates the introduction of a transport equationmodel for the LES dissipation rate. Closure of the equation is addressed by formulating a new adapted dynamic approach. This approach borrows dynamically computed information from LES quantities that, unlike the dissipation rate, do not reside on the smallest flow length scales. The adapted dynamic approach is analyzed by considering a second DNS of scalar mixing in homogeneous isotropic turbulence. Data from this second DNS are used to confirm that the adapted dynamic approach successfully closes the dissipation rate equation over a wide range of LES filter widths. The first reacting jet case is then returned to and used to test the LES transport equationmodels. The transport equationmodel for the dissipation rate is shown to be more accurate than its algebraic counterpoint, and the dissipation rate is eliminated as a source of error in the transported variance model.

Schaefer, P., Gampert, M. & Peters, N., On the scaling of the mean length of streamline segments in various turbulent flows. Comptes Rendus Mécanique, 340(11–12), pp.859866. 2012.
The geometrical properties of streamline segments (Wang, 2010 1) and their bounding surface (Schaefer et al., 2012 2) in direct numerical simulations (DNS) of different types of turbulent flows at different Reynolds numbers are reviewed. Particular attention is paid to the geometrical relation of the bounding surface and local and global extrema of the instantaneous turbulent kinetic energy field. Also a previously derived model equation for the normalized probability density of the length of streamline segments is reviewed and compared with the new data. It is highlighted that the model is Reynolds number independent when normalized with the mean length of streamline segments yielding that the mean length l(m) plays a paramount role as the only relevant length scale in the pdf. Based on a local expansion of the field of the absolute value of the velocity u along the streamline coordinate a scaling of the mean size of extrema of u is derived which is then shown to scale with the mean length of streamline segments. It turns out that l(m) scales with the geometrical mean of the Kolmogorov scale eta and the Taylor microscale lambda so that l(m) proportional to (eta lambda)(1/2). The new scaling is confirmed based on the DNS cases over a range of Taylor based Reynolds numbers of Relambda = 50300. (C) 2012 Published by Elsevier Masson SAS on behalf of Academie des sciences.

Mueller, M.E. & Pitsch, H., LES model for sooting turbulent nonpremixed flames. Combustion and Flame, 159(6), pp.21662180. 2012.
In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages stateoftheart developments in both soot modeling and turbulent combustion modeling and gives special consideration to the smallscale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gasphase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gasphase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.

Mittal, V., Cook, D.J. & Pitsch, H., An extended multiregime flamelet model for IC engines. Combustion and Flame, 159(8), pp.27672776. 2012.
Modeling internal combustion engines is challenging due to the various coupled multiphysics phenomena. With the advent of modern supercomputing and advanced modeling techniques, studying and designing these engines through detailed simulations is becoming tractable. Since the combustion process is the primary controlling feature in these engines, a high fidelity combustion model is essential. This model must be efficient and valid across different combustion regimes, since modern engines might operate in hybrid modes. The Representative Interactive Flamelet (RIF) combustion model is a possible choice. This model has been developed to describe ignition, combustion, and pollutant formation in directinjected diesel engines. However, it has recently been shown that the model has the correct asymptotic behavior for both diesel and Homogeneous Charge Compression Ignition (HCCI) regimes, and the model has been applied successfully for HCCI type combustion. In this study, the model is validated against twodimensional direct numerical simulation data with multistep finite rate chemistry to evaluate model performance in the diesel, the HCCI, and hybrid regimes. A wide range of temperature and mixture fraction stratification cases are simulated to evaluate the model performance across different modes. The model performs well for all cases considered, even when high levels of concurrent thermal and charge stratification are present.

Peters, N., Turbulence statistics along gradient trajectories. ZAMM  Journal of Applied Mathematics and Mechanics / Zeitschrift für Angewandte Mathematik und Mechanik, 92(1), pp.47. 2012.
By calculating gradient trajectories in direction of ascending and descending scalar gradients a local maximum and a local minimum point is reached. Dissipation elements may then be defined as the spatial region from which the same pair of maximum and minimum points in a scalar field is reached. By exploring the twopoint correlation of the scalar gradient along such trajectories it was found that for large elements the mean velocity increment scales linearly with the arclength distance along the trajectory. This is different from the classical Kolmogorov scaling and has consequences for the modeling of the length distribution of dissipation elements.

Bisetti, F., Blanquart, G., Mueller, M.E. & Pitsch, H., On the formation and early evolution of soot in turbulent nonpremixed flames. Combustion and Flame, 159(1), pp.317335. 2012.
A Direct Numerical Simulation (DNS) of soot formation in an nheptane/air turbulent nonpremixed flame has been performed to investigate unsteady strain effects on soot growth and transport. For the first time in a DNS of turbulent combustion, Polycyclic Aromatic Hydrocarbons (PAH) are included via a validated, reduced chemical mechanism. A novel statistical representation of soot aggregates based on the Hybrid Method of Moments is used M.E. Mueller, G. Blanquart, H. Pitsch, Combust. Flame 156 (2009) 11431155, which allows for an accurate stateoftheart description of soot number density, volume fraction, and morphology of the aggregates. In agreement with previous experimental studies in laminar flames. Damkohler number effects are found to be significant for PAH. Soot nucleation and growth from PAH are locally inhibited by high scalar dissipation rate, thus providing a possible explanation for the experimentally observed reduction of soot yields at increasing levels of mixing in turbulent sooting flames. Furthermore, our data indicate that soot growth models that rely on smaller hydrocarbon species such as acetylene as a proxy for large PAH molecules ignore or misrepresent the effects of turbulent mixing and hydrodynamic strain on soot formation due to differences in the species Dainkohler number. Upon formation on the rich side of the flame, soot is displaced relative to curved mixture fraction isosurfaces due to differential diffusion effects between soot and the gasphase. Soot traveling towards the flame is oxidized, and aggregates displaced away from the flame grow primarily by condensation of PAH on the particle surface. In contrast to previous DNS studies based on simplified soot and chemistry models, surface reactions are found to contribute barely to the growth of soot, for nucleation and condensation processes occurring in the fuel stream are responsible for the most of soot most generation. Furthermore, the morphology of the soot aggregates is found to depend on the location of soot in mixture fraction space. Aggregates having the largest primary particles populate the region closest to the location of peak soot growth. On the contrary, the aggregates with the largest number of primary particles are located much further into the fuel stream.

Viswanathan, V., Wang, F. & Pitsch, H., Monte CarloBased Approach for Simulating Nanostructured Catalytic and Electrocatalytic Systems. Computing in Science & Engineering, 14(2), pp.6068. 2012.
This geometrygeneration method within a Monte Carlobased approach enables the simulation of kinetics on nanostructured catalytic and electrocatalytic systems relevant for industrial applications. The method is applicable to realistic catalyst geometries, and an industrially relevant systemfuel cell is studied as an example.

Dhanda, A., Pitsch, H. & O’Hayre, R., Diffusion Impedance Element Model for the Triple Phase Boundary. Journal of The Electrochemical Society, 158(8), pp.B877B884. 2011.
Triple Phase Boundary (TPB) properties at Pt/Nafion interfaces in a Polymer Electrolyte Membrane (PEM) fuel cell depend on the relative rates of reaction and diffusion processes. In this paper, electrochemical impedance analysis is performed to model the coupling between the reaction and diffusion phenomena in the TPB zone. The transfer function for the PEM fuel cell system is obtained for the oxygen reduction reaction (ORR) mechanism with two adsorbed species. An analytical expression is derived for the TPB diffusion impedance element based on the transient concentration profile and the ORR kinetics derived for Pt microelectrodes placed on Nafion electrolyte. For this arrangement, the coupled kineticdiffusion impedance is shown to have a distinct gain and phase relationships, which is closely related to the Gerischer element. The model is used to study the transient response of the coupled impedance element in terms of TPB parameters and operating voltages. The TPB impedance element, in conjunction with existing models of mass transport impedance in GDL and porous electrodes, provides a more comprehensive analysis of combined Faradaic and diffusion processes occurring at the Pt/Nafion interfaces in PEM fuel cells.

Peters, N., Hoffmann, K., Felsch, C. & Abel, D., A Dynamic Simulation Strategy for PCCI Combustion Control Design. Oil Gas Sci. Technol. – Rev. IFP Energies nouvelles, 66(4), pp.549562. 2011.
A Dynamic Simulation Strategy for PCCI Combustion Control Design  Subject of this work is a dynamic simulation strategy for PCCI combustion that can be used in closedloop control development. A detailed multizone chemistry model for the highpressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting stationary model is capable of describing PCCI combustion sufficiently well. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created. This paper summarizes an important outcome of the the Collaborative Research Centre "SFB 686  Modellbasierte Regelung der homgenisierten NiedertemperaturVerbrennung" at RWTH Aachen University and Bielefeld University, Germany.

Dahms, R.N., Drake, M.C., Fansler, T.D., Kuo, T.W. & Peters, N., Understanding ignition processes in sprayguided gasoline engines using highspeed imaging and the extended sparkignition model SparkCIMM. Part A: Spark channel processes and the turbulent flame front propagation. Combustion and Flame, 158(11), pp.22292244. 2011.
Recent highspeed imaging of ignition processes in sprayguided gasoline engines has motivated the development of the physicallybased spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray/vaporization model and a model for fully developed turbulent flame front propagation. Previously, both SparkCIMM and highspeed optical imaging data have shown that, in sprayguided engines, the spark plasma channel is stretched and wrinkled by the local turbulence, excessive stretching results in spark restrikes, large variations occur in turbulence intensity and local equivalence ratio along the spark channel, and ignition occurs in localized regions along the spark channel (based upon a Karlovitznumber criteria). In this paper, SparkCIMM is enhanced by: (1) an extended flamelet model to predict localized ignition spots along the spark plasma channel, (2) a detailed chemical mechanism for gasoline surrogate oxidation, and (3) a formulation of early flame kernel propagation based on the Gequation theory that includes detailed chemistry and a local enthalpy flamelet model to consider turbulent enthalpy fluctuations. In agreement with new experimental data from broadband spark and hot soot luminosity imaging, the model establishes that ignition prefers to occur in fuelrich regions along the spark channel. In this highlyturbulent highlystratified environment, these ignition spots burn as quasilaminar flame kernels. In this paper, the laminar burning velocities and flame thicknesses of these kernels are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelets over a wide range of stoichiometry and exhaust gas dilution. The criteria for flame propagation include chemical (crossover temperature based) and turbulence (Karlovitznumber based) effects. Numerical simulations using ignition models of different physical complexity demonstrate the significance of turbulent mixture fraction and enthalpy fluctuations in the prediction of early flame front propagation. A third paper on SparkCIMM (companion paper to this one) focuses on the importance of molecular fuel properties and flame curvature on early flame propagation and compares computed flame propagation with high speed combustion imaging and computed heat release rates with cylinder pressure analysis.<br />
The goals of SparkCIMM development are to (a) enhance our fundamental understanding of ignition and combustion processes in highlyturbulent highlystratified engine conditions, (b) incorporate that understanding into a physicallybased submodel for RANS engine calculations that can be reliably used without modification for a wide range of conditions (i.e., homogeneous or stratified, low or high turbulence, low or high dilution), and (c) provide a submodel that can be incorporated into a future LES model for physicallybased modeling of cycletocycle variability in engines.

Varea, E., Criner, K., Godard, G., Vervisch, P. & Cessou, A., Investigation of Flame Stabilization Enhancement by Electric Field using Simultaneous Measurements of OH, mixture fraction and Velocity Fields. Proceedings of the European Combustion Meeting, 27th June  1st July 2011, Cardiff, United Kingdom. 2011.

Schaefer, P., Gampert, M., Goebbert, J.H., Gauding, M. & Peters, N., Asymptotic analysis of homogeneous isotropic decaying turbulence with unknown initial conditions. Journal of Turbulence, 12(30), pp.120. 2011.
In decaying grid turbulence there is a transition from the initial state immediately behind the grid to the state of fully developed turbulence downstream, which is believed to be selfsimilar. This state is characterized by a power law decay of the turbulent kinetic energy with a timeindependent decay exponent n. The value of this exponent, however, depends on the initial distribution of the velocity, about which we have only general information at the very best. For homogeneous isotropic decaying turbulence, the evolution of the twopoint velocity correlation is described by the von KarmanHowarth equation. In the nondimensionalized form of this equation a decay exponent dependent term occurs, whose coefficient will be called delta. We exploit the fact that delta vanishes for n > 2, which is shown to correspond to the limit d > infinity, where d denotes the dimensionality of space to formulate a singular perturbation problem. It is shown that a distinguished limit exists for d > infinity and delta > 0. We obtain in the limit of infinitely large Reynolds numbers an outer layer of limited, but a priori unknown extension, as well as an inner layer of the thickness of the order O(delta(3/2)), where the Kolmogorov scaling is valid. To leading order, we obtain an algebraic balance in the outer layer between the twopoint correlation and the thirdorder structure function. In the inner layer the analysis yields the emergence of higher order terms to the classical K41 scaling. All leading order solutions are shown to be subject to a band of uncertainty of the order O(delta), which is argued to be due to intrinsically unknown initial conditions.

Gampert, M., Schaefer, P. & Peters, N., Experimental investigation of small scale geometries in a turbulent round jet. Journal of Physics: Conference Series, 318(3). 2011.
In the present work, we present a method to gather highly accurate threedimensional measurements of a scalar field in order to experimentally validate the theory of dissipation elements as developped by Wang & Peters (2006, 2008). Combining a twodimensional highspeed Rayleigh scattering technique with Taylor's hypothesis allows to resolve the concentration field of gaseous propane discharging into ambient air from a turbulent round jet at a Reynolds number (based on nozzle diameter and exit velocity) of 2,800 down to the Kolmogorov scale in every spatial direction. Based on the acquired data, the normalized probability density function of the length of dissipation elements (P) over tilde((l) over tilde) is investigated at various downstream positions x/d = 15  40 and an excellent agreement with the theoretically derived model equation is obtained.

Hemchandra, S., Peters, N. & Lieuwen, T., Heat release response of acoustically forced turbulent premixed flamesrole of kinematic restoration. Proceedings of the Combustion Institute, 33(1), pp.16091617. 2011.
Predicting the ensemble averaged heat release response of a turbulent premixed flame to acoustic forcing is a fundamental problem associated with understanding combustion instabilities. This paper describes an analysis of this problem, by modeling the response of a flame that is simultaneously perturbed by broadband, turbulent fluctuations and narrowband, acoustic fluctuations of amplitude epsilon(a) and epsilon(T), respectively. It is shown that the response of the flame surface to coherent forcing and turbulent fluctuations are coupled even at linear order in coherent forcing amplitude, epsilon(a), due to flame propagation (kinematic restoration). This coupling effectively causes the local consumption and displacement speeds of the flame to vary in time over a forcing cycle. Turbulent fluctuations also provide a mechanism for destruction of coherent flame surface wrinkling at first order in epsilon(a), an effect which only occurs at O(epsilon(2)(a)) for laminar flames.

Heufer, K.A., Fernandes, R.X., Olivier, H., Beeckmann, J., Roehl, O. & Peters, N., Shock tube investigations of ignition delays of nbutanol at elevated pressures between 770 and 1250 K. Proceedings of the Combustion Institute, 33(1), pp.359366. 2011.
The ignition delays of nbutanol, a potential biofuel candidate, have been determined in a highpressure shock tube. Conditions behind the reflected shock are approximately between 1042 bar and 7701250 K. To our knowledge, the ignition delay measurements of butanol at these high pressures are the first of their kind. CH emission and pressure time histories have been probed to determine ignition delay times for all experiments. For stoichiometric fuelairmixtures the influence of the temperature and pressure has been characterized. Interestingly the experimental data deviate from the Arrhenius behavior for temperatures lower than 1000 K. This is in contrast to simulation results which have been obtained by employing the simulation tool CANTERA with different reaction mechanisms applying the typical assumption of isochoric conditions. It has been found out that a positive pressure and temperature gradient behind the reflected shock has a significant influence on the ignition delay below 1000 K causing a pronounced decrease in the ignition delay times. This change of the conditions behind the reflected shock is attributed to the shock attenuation and probably from preignition. Including the measured pressure gradients and assuming an isentropic compression behind the reflected shock, the simulation data and the experimental results show a same trend in the temperature dependence of the ignition delay. Nevertheless, striking differences between experiment and simulation persist especially for higher pressures. By performing sensitivity analysis at different temperatures some critical reactions could be identified and their role under our experimental conditions is discussed. In summary it can be stated that the employed reaction mechanisms may not be fully applicable to highpressure conditions and it seems plausible that the lack of more detailed low temperature fuel specific reactions could be the probable cause for the discrepancies which calls for detailed investigations at elevated pressures.

Jochim, B., Felsch, C., Drews, P., Vanegas, A., Hoffmann, K., Abel, D., Peters, N. & Pitsch, H., A multizone combustion model with detailed chemistry including cycletocycle dynamics for diesel engine control design. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 225(9), pp.12351252. 2011.
This paper reviews the research activities within the subproject B1 Model Reduction for LowTemperature Combustion Processes through CFDSimulations and MultiZone Models of the Collaborative Research Centre SFB 686  ModelBased Control of Homogenized LowTemperature Combustion. The SFB 686 is carried out at RWTH Aachen University, Germany and Bielefeld University, Germany, and is funded by the German Research Foundation (DFG). This paper thereby summarizes the outcome of various publications by the authors, with the appropriate references given in the individual sections. Additionally, some new results are introduced. The particular subject of this work is a dynamic simulation strategy for premixed charge compression ignition (PCCI) combustion that can be used in closedloop control development. A detailed multizone chemistry model for the highpressure part of the engine cycle is extended by a mean value gas exchange model accounting for the lowpressure part. Thus, an efficient model capable of describing PCCI combustion is sufficiently well established. In order to capture cycletocycle dynamics, identified system dynamics influencing the input parameters are incorporated. For this, a Wiener model is set up that uses the combustion model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created. The model is validated against transient experimental engine data.

Luo, K., Pitsch, H., Pai, M.G. & Desjardins, O., Direct numerical simulations and analysis of threedimensional nheptane spray flames in a model swirl combustor. Proceedings of the Combustion Institute, 33(2), pp.21432152. 2011.
Threedimensional nheptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variabledensity, lowMach number NavierStokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive onestep irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a richburn/quickquench/leanburn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.

Gampert, M., Goebbert, J.H., Schaefer, P., Gauding, M., Peters, N., Aldudak, F. & Oberlack, M., Extensive strain along gradient trajectories in the turbulent kinetic energy field. New Journal of Physics, 13(4). 2011.
Based on direct numerical simulations of forced turbulence, shear turbulence, decaying turbulence, a turbulent channel flow as well as a Kolmogorov flow with Taylorbased Reynolds numbers Relambda between 69 and 295, the normalized probability density function of the length distribution (P) over tilde((l) over tilde) of dissipation elements, the conditional mean scalar difference <Delta kl > at the extreme points as well as the scaling of the twopoint velocity difference along gradient trajectories <Delta u(n)> are studied. Using the field of the instantaneous turbulent kinetic energy k as a scalar, we find good agreement between the model equation for (P) over tilde ((l) over tilde) as proposed by Wang and Peters (2008 J. Fluid Mech. 608 11338) and the results obtained in the different direct numerical simulation cases. This confirms the independence of the model solution from both the Reynolds number and the type of turbulent flow, so that it can be considered universally valid. In addition, we show a 2/3 scaling for the mean conditional scalar difference. In the second part of the paper, we examine the scaling of the conditional twopoint velocity difference along gradient trajectories. In particular, we compare the linear s/tau scaling, where tau denotes an integral time scale and s the separation arclength along a gradient trajectory in the inertial range as derived by Wang (2009 Phys. Rev. E 79 046325) with the s . a(infinity) scaling, where a(infinity) denotes the asymptotic value of the conditional mean strain rate of large dissipation elements.

Dahms, R.N., Drake, M.C., Fansler, T.D., Kuo, T.W. & Peters, N., Understanding ignition processes in sprayguided gasoline engines using highspeed imaging and the extended sparkignition model SparkCIMM: Part B: Importance of molecular fuel properties in early flame front propagation. Combustion and Flame, 158(11), pp.2245  2260. 2011.
Recent highspeed imaging of ignition processes in sprayguided gasoline engines has motivated the development of the physicallybased spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray and vaporization model and a model for fully developed turbulent combustion. Previously, both SparkCIMM and highspeed optical imaging data have shown that, in sprayguided engines, large variations in turbulence intensity, equivalence ratio, and enthalpy along the stretched and wrinkled spark plasma channel favor localized ignition spot formations in richmixture regions. In combination with strong local flow velocity, multiple successful ignition events along the restriking spark lead to early nonspherical turbulent flame fronts. In this paper, SparkCIMM is enhanced by: (1) criteria to capture localized flame extinction phenomena, (2) a formulation of early flame kernel propagation based on the Gequation theory that includes effects of nonunity Lewis numbers, and (3) an extended equation to compute turbulent burning velocities of stretched flames in stratified mixtures. Localized rich ignition along the spark leads to early flames, whose propagation is, due to initially small turbulent Damkohler numbers, significantly influenced by molecular fuel properties. The analysis reveals that nonunity Lewis number curvature effects, intensified by heavy dilution by exhaust gas recirculation, strongly affect the early flamekernel development in sprayguided gasoline engines. In particular, these effects significantly bias the flammability limit of flame kernels towards richmixtures while inhibiting their propagation in lean regions. Favorable initial conditions for combustion are found in richmixture regions, albeit in the presence of substantial equivalence ratio fluctuations and scalar dissipation rates. This paper demonstrates that the full complexity of the model equations developed here is required to reproduce the characteristic experimental features (spark channel stretching, multiple restrikes, localized flame kernel formation, and early turbulent flame front corrugation) of sprayguided ignition phenomena.

Schaefer, P., Gampert, M., Gauding, M., Peters, N. & Treviño, C., The secondary splitting of zerogradient points in a scalar field. Journal of Engineering Mathematics, 71(1), pp.8195. 2011.
The mechanisms related to the secondary splitting of zerogradient points of scalar fields are analyzed using the twodimensional case of a scalar extreme point lying in a region of local strain. The velocity field is assumed to resemble a stagnationpoint flow, cf. Gibson (Phys Fluids 11:2305–2315, 1968), which is approximated using a Taylor expansion up to third order. The temporal evolution of the scalar field in the vicinity of the stagnation point is derived using a series expansion, and it is found that the splitting can only be explained when the thirdorder terms of the Taylor expansion of the flow field are included. The nondimensional splitting time turns out to depend on three parameters, namely the local Péclet number Pe δ based on the initial size of the extreme point δ and two parameters which are measures of the rate of change of the local strain. For the limiting casePe δ → 0, the splitting time is found to be finite but Pécletnumber independent, while for the case of Pe δ → ∞ it increases logarithmically with the Péclet number. The physical implications of the twodimensional mathematical solution are discussed and compared with the splitting times obtained numerically from a Taylor–Green vortex.

Luckhchoura, V., Peters, N. & Diwakar, R., Computational analysis of injectionrate shapes in a smallbore directinjection diesel engine. International Journal of Engine Research, 12(2), pp.145168. 2011.
The purpose of this study is to analyse the differences in the combustion process, and pollutants formation (especially soot) due to tophat and boot injectionrate shapes at one specific highload point of a singlecylinder smallbore diesel engine using multidimensional engine simulations. The simulations are performed using a response interactive flamelet model with detailed chemical kinetics. A detailed chemistrybased soot model is used for the prediction of soot emissions. The heights of the first and second stages of the boot shape are varied to observe the effect of the injected fuel mass distribution. In addition, results of boot shapes are compared with a trapezoidal (tophat) shape. A detailed analysis of soot formation and oxidation is also presented for some selected rate shapes. Through computational analysis it is shown that the boot shapes have the potential to decrease combustiongenerated noise and to lower emissions at the investigated load point compared to the tophat shape. Variations in the temporal distribution of the injected fuel mass show that a lower height of the first stage and a higher height of the second stage of the boot shape result in a relatively slower rise of heat release (lower combustiongenerated noise) in the early part of combustion and enhanced soot oxidation as a result of higher spray momentum near the end of injection.

Gampert, M., Goebbert, J.H., Schaefer, P., Gauding, M., Peters, N., Aldudak, F. & Oberlack, M., Compressive and extensive strain along gradient trajectories. Journal of Physics: Conference Series, 318(5), p.052029. 2011.
Based on direct numerical simulations of forced turbulence, shear turbulence, decaying turbulence, a turbulent channel flow as well as a Kolmogorov flow with Taylor based Reynolds numbers Reλ between 69 and 295, the normalized probability density function of the length distribution P̃(l̃) of dissipation elements, the conditional mean scalar difference at the extreme points as well as the scaling of the twopoint velocity difference along gradient trajectories are studied. Using the field of the instantanous turbulent kinetic energy k as a scalar, we find a good agreement between the model equation for P̃(l̃) as proposed by Wang and Peters (2008) and the results obtained in the different DNS cases. This confirms the independance of the model solution from both, the Reynolds number and the type of turbulent flow, so that it can be considered universally valid. In addition, we show a 2/3 scaling for the mean conditional scalar difference. In the second part of the paper, we examine the scaling of the conditional twopoint velocity difference along gradient trajectories. In particular, we compare the linear s/τ scaling, where τ denotes an integral time scale and s the separation arclength along a gradient trajectory in the inertial range as derived by Wang (2009) with the s · a∞ scaling, where a∞ denotes the asymtotic value of the conditional mean strain rate of large dissipation elements.

Chakravarty, H.K., Reddy, K.P.J. & Arunan, E., Thermal Decomposition of 2Bromoethanol: Single Pulse Shock Tube Experiments, Modeling, DFT and TST Calculations. Proceedings of 28th International Symposium on Shockwaves, 33. 2011.

ElAsrag, H.A., Pitsch, H., Kim, W., Do, H. & Mungal, M.G., Damköhler Number Similarity for Static Flame Stability in GaseousFueled Augmentor Flows. Combustion Science and Technology, 183(7), pp.718737. 2011.
Afterburners (or augmentors) are used to increase thrust in aircraft engines. Static flame stability, or the robustness to flame blowoff, is an important metric in the performance assessment of combustion in aircraft engine afterburners, where bluffbodytype flame holders are typically used to stabilize the flame. The design of such flame holders is complicated by the operating conditions, which involve flows at high speed, high temperature, and low pressure. In this paper, experimental and computational studies of Damkohler (Da) number similarity are presented with relevance to augmentor flame stability. The Da number describes the ratio of flow and chemical time scales. Hence, as long as a reference Da number is kept constant, similar characteristics of static stability should be expected of the bluffbody stabilized flame at low and high speeds. Flame stability in highspeed vitiated flows could then be studied at low speed if the chemical time scale is reduced, for instance by lowering the oxidizer flow temperature. However, each chemical reaction has its own Da number and not all can be kept constant at the same time. Since different stabilization mechanisms are governed by different chemical reactions, it is not necessarily clear what the relevant Da number is. Here, a consistent method for defining the Da number is provided based on the analysis of the modeled governing equations. Numerical simulations are performed for three different velocities with the inflow temperature adjusted to keep the Da number constant. Results are validated by comparison with experimental PIV data and the reported flame liftoff height. For the same characteristic Da number and constant momentum ratio between the fuel jet and the vitiated cross flow, the three flames show similar mean features for the recirculation zone and the flame shape. The flow field is found to exhibit von Karman vortex shedding with the same Strouhal number for all cases. The average nondimensional flame liftoff height is also found to be the same for all cases. These results suggest a method to properly define the Da number to test augmentor stability features in lowspeed test facilities under the conditions of similarity.

Varea, E., Vandel, A., Modica, V. & Renou, B., Laminar Burning Velocity and Markstein length relative to fresh gases determination applied on ethanol air flames. In Proceedings of the European Combustion Meeting, June 27thJuly 1st, Cardiff, United Kingdom. 2011.

Mueller, M.E. & Pitsch, H., Large eddy simulation subfilter modeling of sootturbulence interactions. Physics of Fluids, 23(11), p.115104. 2011.
The smallscale interactions between turbulence, chemistry, and soot have a profound effect on the soot formation, growth, and destruction processes in turbulent reacting flows. In this work, the smallscale subfilter interactions between turbulence and soot are modeled using a presumed subfilter PDF for the statistical moments of the soot number density function. Due to a very large (infinite) Schmidt number, soot is confined to very thin structures. In addition, soot is formed initially from Polycyclic Aromatic Hydrocarbons, which exhibit a strong sensitivity to the local scalar dissipation rate in the flow field. These interactions of soot with the gasphase chemistry, molecular transport, and turbulence result in very high spatial and temporal intermittency. Therefore, the soot subfilter PDF is presumed to be a pair of delta distributions with a sooting mode and a nonsooting mode. In addition to the mean values of the scalars, one additional parameter is needed to specify this distribution. The presumed soot subfilter PDF approach is evaluated a priori using a recent twodimensional direct numerical simulation (DNS) database of soot evolution in a nonpremixed flame. The analysis shows that predictions of the soot intermittency as well as the soot source terms are improved with this presumed soot subfilter PDF approach compared to simply using the mean values of the soot scalars. Several choices for specifying the additional parameter of the PDF are also evaluated with the database. The results show that the parameter is best specified using the second moment of the soot number density. In addition, the transport equation for the second moment of the number density is briefly discussed. A model for the lone unclosed term is proposed and evaluated with the DNS data.

Beeckmann, J. & Pitsch, H., Investigation of the Laminar Burning Velocities of C1C4 alcohols. Proceedings of the 5th European Combustion Meeting, June 28thJuly 1st, Cardiff, Wales. 2011.

Gauding, M., Goebbert, J.H. & Peters, N., Scalebyscale Statistics of Passive Scalar Mixing with Uniform Mean Gradient in Turbulent Flows. Proceedings of the 6th AIAA Theoretical Fluid Mechanics Conference, June 27th30th, Honolulu, Hawaii, USA, 20114024. 2011.
The mixing of passive scalars with imposed mean gradient in stationary homogeneous turbulence is studied by means of direct numerical simulation. The scalar field is characterized by strong intermittency and anisotropy due to the mean shear gradient. Statistics in direction of the mean shear gradient are different from statistics normal to the mean shear. A generalized Yaglom equation is derived to take the anisotropy and the mean scalar shear gradient into account. This equation incorporates the balance between production, transport and dissipation of turbulent scalar energy. It is insensitive to smallscale intermittency and only describes the turbulent scalar energy transfer through scales. Various Schmidt numbers between 0.11 and 14.4 are investigated, while the Taylormicroscale Reynolds number is fixed between 119 and 148. A comparison with Yaglom’s 2/3law is performed and limiting cases for large and small scales are discussed.

Dahms, R., Felsch, C., Roehl, O. & Peters, N., Detailed chemistry flamelet modeling of mixedmode combustion in sparkassisted HCCI engines. Proceedings of the Combustion Institute, 33(2), pp.30233030. 2011.
A detailed chemistry mixedmode flamelet model for the prediction of combustion in sparkassisted homogeneous charge compression ignition (HCCI) engines is presented in this paper. The complex phenomena of sparkchannel processes (turbulent corrugation, multiple restrikes) and of early flame kernel propagation (localized flame kernel formation, nonspherical early flame shapes), both induced by sprayguided sparkignition combustion initiation, are captured by the recently introduced SparkCIMM ignition model. In this paper, laminar burning velocities and flame thicknesses are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelet calculations to appropriately consider locally rich, highly diluted, and autoigniting stratified mixtures. Flame extinction criterions are formulated and incorporated into an extended Gequation flame front tracking scheme. Autoignition processes in the unburnt mixture, controlled by the flameinduced pressure and temperature increase, are captured by a recently developed multizone flamelet model that accounts for detailed chemistry and effects of scalar mixing on turbulent combustion. The laminar burning velocity is shown to increase significantly as the flame propagates into the chemically reacting mixture within the first stage of autoignition. After the initiation of the thermal runaway, however, flame extinction occurs rapidly. These interactions of mixedmode combustion processes are captured by the presented flamelet model. It is developed based on a time and length scale analysis, revealing a scale separation between the ignition delay of diffusion combustion and the flame time of flame propagation. The analysis of the mixture preparation process, along with the simulation of turbulent flame front propagation and its extinction, demonstrates that the prediction of combustion in sparkassisted HCCI engines requires the on hand comprehensive mixedmode combustion model. A comparison of simulation results from this new model with data from experiments and combustion models of reduced physical complexity proofs its qualification.

Gampert, M., Schaefer, P. & Peters, N., Threedimensional measurements of a turbulent scalar field in a round jet. 41st AIAA Fluid Dynamics Conference and Exhibit, June 27th30th, Honolulu, Hawaii. 2011.

Drews, P., Albin, T., Heßeler, F.J., Peters, N. & Abel, D., FuelEfficient ModelBased Optimal MIMO Control for PCCI Engines. Proceedings of the 18th IFAC World Congress, August 18thSeptember 2nd, Milano, Italy, pp.1299813003. 2011.
Recent research in modern combustion technologies, like partial homogeneous charge compression ignition (PCCI), demonstrates the capability of reducing pollutant emissions, e.g. soot and NOx. In addition to this advantage, a possibility to reduce fuel consumption and noise production by modelbased optimal control is presented in this paper. In order to understand the basic properties of the PCCI mode, process measurements were conducted using a slightly modified series diesel engine. Control variables are engine combustion parameters: the indicated mean effective pressure, the combustion average and the maximum gradient of the cylinderpressure. Control inputs are the parameters: quantity of injected fuel, start of injection and the intake manifold fraction of recirculated exhaust gas. The process has very fast, almost proportional behaviour over the engine`s working cycles. Focusing on the static behaviour of the process, a nonlinear neural network model is used for identification. Successive linearization of the nonlinear network is used to build an affine internal controller model for the actual operating point. The presented controller structure is able to consider constraints by individual formulation of the cost function. With this configuration the closedloop process is able to track the combustion setpoints with high control quality with minimal possible fuel consumption and combustion noise.

Goebbert, J.H. & Peters, N., Analysis of Turbulent Premixed Flames using Gradient Trajectories. Proceedings of the 5th European Combustion Meeting, June 27thJuly 1st, Cardiff, Wales. 2011.

Gampert, M. & Madlener, R., PanEuropean Management of Electricity Portfolios: Risks and Opportunities of Contract Bundling. Energy Policy, 39(5), pp.28552865. 2011.
Due to the liberalization of energy markets in the European Union, today's European utilities not only focus on electricity supply, but also offer exchangetraded “structured products” or portfolio management for unbundling financial and physical risk positions. Many utilities are only able to provide these services in their domestic markets. In a globalized economy, the need for a centrally organized panEuropean portfolio management has arisen, as it allows a simplified commodity sourcing in combination with an optimized risk management. In this paper, we examine the challenges to be overcome for establishing a Europeanwide bundling of electricity contracts. For this purpose, a case study based on the business perspective of RWE Supply & Trading in Central and Eastern Europe is carried out. In a first step, we analyze general requirements for a panEuropean bundling of electricity contracts. Then, RWE's situation in Europe is examined, based on which we finally propose a concept to meet customer demands in Central and Eastern Europe.

Schaefer, P., Gampert, M. & Peters, N., On The Dissipation Rate Coefficient In Homogeneous Isotropic Decaying And Forced Turbulence. Seventh International Symposium on Turbulence and Shear Flow Phenomena, July 28th31st, Ottawa, Canada. 2011.
The normalized nondimensional von KármánHowarth equation for isotropic homogeneous decaying and forced steady turbulence is integrated to obtain expressions for the dissipation rate coefficient Cϵ=(Lϵ)/<u2>3/2, where L denotes the longitudinal integral length scale, ϵ the mean dissipation rate and <u2> the mean variance of the longitudinal velocity fluctuations. For decaying turbulence the final exact expressions for Cϵ for the low and high Reynolds number limit depend on the decay exponent n, which is known to depend on the initial velocity structure at the turbulence production. The dependence on n leads to a nonuniversal coefficient. The expressions for the steady forced case depend on the forcing mechanism and thus are not universal either. Nonetheless, a lower value and considerably less scatter as compared to the decaying turbulence case should be expected when similiar forcing algorithms are employed.

Aye, M.M., Beeckmann, J.L., Vanegas, A., Peters, N. & Pitsch, H., Experimental Investigation of Diesel and Surrogate Fuels: Spray and Ignition Behavior. SAE Technical Paper Series, 2011011921. 2011.
In this work, surrogate fuels composed of ndecane and alphamethylnaphthalene (AMNL) with different compositions according to the reference cetane numbers 53, 45, 38, and 23 are investigated. In addition to the twocomponent mixtures, we examine a threecomponent mixture composed of ndecane, AMNL, and dinbutyl ether (DNBE) corresponding to a reference cetane number of 53. Spray characteristics of liquid and fuel vapor phase and the relationship between ignition quality and liftoff length are investigated. The experimental results show, first of all, that for these mixtures, the cetane number is a good indicator for the ignition delay. Diesel and surrogate fuels have different liquid penetration lengths, which depend on the evaporation rate, and hence vapor pressure and boiling point of the fuels. Spray tip penetration of all investigated fuels except diesel are similar for the vaporphase, because the differences in fuel properties are too small to considerably affect the momentum flux. Furthermore, we discuss the overlap number (OL) of the fuels to estimate the soot formation tendency under compression ignition (CI) engine like conditions. It is found out that the OL generally increases with decreasing cetane number, although there are substantial variations for the studied fuels with the same cetane number. Most interestingly, the low cetane number fuel, which has a substantially higher content of a tworing aromatic, is shown to have diminished soot formation, which is consistent with its higher OL number.

Kerschgens, B., Vanegas, A. & Pitsch, H., Numerical Assessment of Emission Sources for a Modified Diesel Engine Running in PCCI Mode on a Mixture of Gasoline and Diesel. SAE Technical Paper Series, 2011240014. 2011.
Premixed charge compression ignition (PCCI) is an interesting alternative to conventional diesel combustion, as it allows very low emission levels for part load operation. The difficult control of the onset of combustion is an obstacle to the implementation of PCCI. In a recent study, different mixtures of gasoline and diesel fuel have been used in a modified diesel engine to delay the ignition and thus to allow for a substantial premixing time. For these cases, very low levels of particulate emissions have been reported. However, the emissions of CO and NOx were considerably high. In this study, combustion and pollutant formation in the abovementioned modified diesel engine are simulated using the representative interactive flamelet (RIF) approach. A detailed chemical reaction mechanism for a mixture of nheptane, isooctane, toluene, and ethanol, serving as surrogate fuel for the dieselgasoline blend, is used for the simulations. By systematic comparison of experimental and numerical results, an improved understanding of PCCI combustion is achieved and the origins of the CO and NOx emissions are identified. Finally, measures to reduce these emissions while keeping the low PM levels are suggested.

Wang, L., Pitsch, H., Yamamoto, K. & Orii, A., An efficient approach of unsteady flamelet modeling of a crossflowjet combustion system using LES. Combustion Theory and Modelling, 15(6), pp.849862. 2011.
Steady flamelet models have been widely used in turbulent combustion simulations because of their simplicity, efficiency, yet physicsbased nature. They are, however, unable to handle slow chemical and physical processes such as pollutant formation. Unsteady flamelet models have been shown to be able to provide accurate predictions especially for pollutants, but their implementations are usually not as straightforward as for the steady models, and additional assumptions are involved. One relatively straightforward approach of implementing the unsteady flamelet model is to tabulate the time history of unsteady flamelet solutions. This often leads to flamelet libraries of large sizes because of increased dimensions for the new physics. The purpose of this paper is to introduce a new and efficient approach of tabulating unsteady flamelet solutions in the LES of complex systems, here demonstrated in simulations of a crossflowjet combustion system. This approach employs Taylor series expansions to represent the time history of unsteady flamelet solutions. Compared with other approaches, the new approach retains the efficiency and simplicity benefits of steady flamelet models but possesses the accuracy of unsteady flamelet models. Various issues associated with the formulation and implementation of this approach are discussed, which include the selection of the base solution, the order of accuracy of the expansion, and the treatment of simultaneous wall heat losses and heat transfer through thermal radiation. This approach is validated in large eddy simulations of a crossflowjet combustion system. Good agreement with experiments is obtained for both temperature and NO concentration, as well as for major species.

Cessou, A., Varea, E., Criner, K., Godard, G. & Vervisch, P., Simultaneous Measurements of OH, mixture fraction and Velocity Fields to Investigate Flame Stabilization Enhancement by Electric Field. Experiments in Fluids, 52(4), pp.905917. 2011.
Simultaneous stereoscopic PIV, OH and acetone planar laserinduced fluorescence measurements are performed to analyze the processes involved in the enhancement of flame stabilization by electric field. Instantaneous velocity and mixture fraction fields are measured simultaneously at the base of a lifted flame to analyze whether the flow properties in front of the flame when electric field is applied are compatible with a mechanism involving ionic wind. The measurements conditioned on the instantaneous flame bases with and without the electric field are compared. The velocity in front of the flame decreases with electric field what is in agreement with the assumption involving ionic wind. To analyze the mixture in front of the flame, a joined analysis of velocity and mixture fraction is required to show the mixture stays near stoichiometry when the electric field is applied. The need of a joined analysis illustrates the interest of performing the three laser diagnostics simultaneously.

Mueller, M.E., Blanquart, G. & Pitsch, H., Modeling the oxidationinduced fragmentation of soot aggregates in laminar flames. Proceedings of the Combustion Institute, 33(1), pp.667674. 2011.
While the formation and growth of soot particles has received much attention, the subsequent destruction of the particles is less well understood. Soot particles are destroyed though two parallel processes: oxidation and fragmentation. Oxidation is the removal of mass from particles due to chemical reactions with molecular oxygen and hydroxyl radicals. Fragmentation is the breakup of large aggregates into smaller aggregates. Here, a new model for fragmentation inspired by previous experimental investigations is proposed and formulated within the Hybrid Method of Moments (HMOM). With the formulation, the rate of particle loss due to oxidation is closed, resolving a longstanding problem with the Method of Moments. Less important, secondary unclosed terms are introduced, and models for these terms are proposed. The oxidation and fragmentation models are validated using a set of laminar premixed methane flames and then applied to a series of laminar counterflow diffusion acetylene flames. The role of fragmentation is distinctly different in the two flame types. In the premixed flames, fragmentation only occurs in lean flames with a high oxygen concentration. In the diffusion flames, fragmentation is virtually absent, for soot passes through an OH radical layer and is completely oxidized before reaching high oxygen concentrations.

Varea, E., Vandel, A., Modica, V. & Renou, B., Laminar Burning Velocity and Markstein length relative to fresh gases determination for IsooctaneEthanol air flames. In International Colloquium on the Dynamics of Explosions and Reactive Systems, Irvine, California, USA. 2011.

Gampert, M., Schaefer, P. & Peters, N., Experimental investigation of small scale geometries in a turbulent round jet. 7th Symposium on Turbulence and Shear Flow Phenomena, July 28th31st, Ottawa, Canada. 2011.
In the present work, we present a method to gather highly accurate threedimensional measurements of a scalar field in order to experimentally validate the theory of dissipation elements as developped by Wang & Peters (2006, 2008). Combining a twodimensional highspeed Rayleigh scattering technique with Taylor's hypothesis allows to resolve the concentration field of gaseous propane discharging into ambient air from a turbulent round jet at a Reynolds number (based on nozzle diameter and exit velocity) of 2,800 down to the Kolmogorov scale in every spatial direction. Based on the acquired data, the normalized probability density function of the length of dissipation elements () is investigated at various downstream positions x/d = 15 − 40 and an excellent agreement with the theoretically derived model equation is obtained.

Goebbert, J.H., Gauding, M., Gampert, M., Schaefer, P., Peters, N. & Wang, L., A new View on Geometry and conditional Statistics in Turbulence. Inside: Innovatives Supercomputing in Deutschland, 9(1), pp.3037. 2011.

Schaefer, P., Gampert, M. & Peters, N., The local dynamics of streamlines in turbulence. Journal of Physics: Conference Series, 318(5). 2011.
Based on a coordinate transformation, similiar to the one used in the flamelet approach in nonpremixed combustion, we introduce a coordinate locally tangent to streamlines and transform the NavierStokes equations to an equation for the variation of the absolute value of the velocity u i . Different from previous approaches, the unsteady term splits into two terms, one of which contains all information about the time and spacedependence of the streamline coordinate. Based on the DNS of homogeneous isotropic decaying turbulence at a Taylor based Reynolds number of Re λ = 71 we first discuss the temporal variation of the streamline coordinate field, reflecting nonlocal fast and slow changes of the latter. Then, based on the transformed equation we discuss the balance of the different terms along streamlines and discern terms of different orders of magnitude.

Sudholt, A., Wada, T., Pitsch, H. & Peters, N., First Stage Ignition of Dibutyl Ether. Proceedings of the 5th European Combustion Meeting, June 28thJuly 1st, Cardiff, Wales. 2011.

Varea, E., Criner, K., Godard, G., Vervisch, P. & Cessou, A., Simultaneous Measurements of OH, mixture fraction and Velocity Fields to Investigate Flame Stabilization Enhancement by Electric Field. In International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal. 2010.

Wada, T., Jarmolowitz, F., Abel, D. & Peters, N., An Instability of Diluted Lean Methane/Air Combustion: Modeling and Control. Combustion Science and Technology, 183(1), pp.119. 2010.
In order to study possibilities of Model Predictive Control (MPC) to low temperature combustion, the authors performed a numerical study of combustion with a lean highly diluted methane/air mixture in a perfectly stirred reactor using a detailed chemical kinetic model. Chosen conditions are the following: equivalence ratio 0.6, residence time of mixture in reactor 0.5 s; molar fraction of Nitrogen 0.9, temperature of incoming unburned gases and reactor 1100 K, and heat loss coefficient 2 × 10−3 cal/(cm2Ks). At these conditions strong oscillations are predicted in agreement with previous experimental findings of M. de Joannon et al. (200411.<br />
de Joannon , M. , Sabia , P. , A. Tregrossi , A. , and Cavaliere , A. 2004 . Dynamic behavior of methane oxidation in premixed flow reactor . Combust. Sci. Tech. , 176 , 769 . Taylor & Francis Online, Web of Science ®View all references). It is found that, at low temperatures (<1300 K), reactions forming CH3O and CH3O2 from CH3 had a strong influence on the oscillations. Once these reactions were subtracted from the kinetic model, no oscillations were observed. A virtual MPC suppressed the oscillations at low temperatures successfully and demonstrated extremely low NO and N2O emissions (<0.1 ppm).

Zeng, P., Binninger, B., Peters, N., Pitsch, H. & Herrmann, M., Modeling and Simulation of Primary Atomization with Phase Transition. Bulletin of the American Physical Society, 55(16), p.1 S. 2010.

Varea, E., Vandel, A., Modica, V., Corbin, F., Godard, G. & Renou, B., Measurement of laminar flame speed for high pressure and high temperature conditions: validation of the facility and developement of a new tool for postprocessing. In International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal. 2010.

Schäfer, P., Gampert, M., Göbbert, J.H., Wang, L. & Peters, N., Testing of Model Equations for the Mean Dissipation using Kolmogorov Flows. Flow, Turbulence and Combustion, 85(2), pp.225243. 2010.

Beeckmann, J., Cai, L., Roehl, O., Pitsch, H. & Peters, N., A Reduced Kinetic Reaction Mechanism for the Autoignition of Dimethyl Ether. In 33rd International Symposium of Combustion, August 2nd, Bejing, China. 2010.

Knudsen, E.W., Kim, S.H. & Pitsch, H., An analysis of premixed flamelet models for large eddy simulation of turbulent combustion. Physics of Fluids, 22(11), p.115109. 2010.
When premixed flamelet models are applied in the context of large eddy simulation, a number of assumptions are implicity made. The validity of these assumptions depends on, for example, the simulated flame’s location within the premixed regime diagram, the accuracy of the presumed subfilter flamelet coordinate distributions, and the extent to which the asymptotic flamelets capture the turbulenceperturbed chemistry. Here, the errors that arise due to these assumptions are considered, analyzed, and compared using a direct numerical simulation of a premixed turbulentflame propagating in the thin reaction zones regime. Flamelet representations of the progress variable source term are formed in an a priori fashion. Level set flamelet methods in particular are considered because, although they offer a number of advantages, they make some of the most stringent flame structure assumptions. Errors due to the level set model are evaluated relative to other flamelet error sources, such as the shape of the presumed probability density function and the influence of the variance model. The results provide guidance on the importance of the individual modeling assumptions, and are used to propose a new modeling strategy in an effort to improve the level set framework.

Knudsen, E.W. & Pitsch, H., LargeEddy Simulation for Combustion Systems: Modeling Approaches for partially premixed flows. The Open Thermodynamics Journal, 4(2), pp.7685. 2010.

Luckhchoura, V., Chivite, A., Vogel, S., Peters, N., Rottmann, M. & Pischinger, S., Modeling the InjectionRate Shapes in Diesel Engines. In Thermo and fluid dynamic processes in diesel engines: THIESEL 2010, September 14th17th, Valencia, Spain. 2010.

Schäfer, P., Gampert, M., Göbbert, J.H., Gottschalk, D. & Peters, N., Geometrical Dissipation element analysis if inhomogenous turbulence. In NIC Symposium 2010. Schriften des Forschungszentrums Jülich: IAS Series Vol. 3. Jülich. 2010.

Vanegas, A., Kerschgens, B. & Peters, N., The PCCI Combustion Process for Diesel Engine. A Key Element for upcoming Euro 6. In FISITA 2010 World Automotive Congress: Automobiles and sustainable mobility, May 30thJune 4th, Budapest, Hungary. 2010.
The Premixed Charge Compression Ignition, a lowtemperature combustion strategy, is one of the most important ways to achieve very low pollutant emissions without sacrificing power and fuel consumption. By using PCCI it is possible to drastically reduce NOx and soot emissions simultaneously. Before this combustion process can be fully implemented into the serial diesel engine, it is necessary to solve some difficulties like the higher unburned HC and CO emissions, as well as the early combustion phasing. In the present study, some strategies to solve this problem are introduced. The first strategy is to find the optimum spray cone angle experimentally and with numerical methods; the second strategy is to find the optimum piston bowl geometry; and the third strategy is to find a good injection strategy. The combination of these strategies shows that it is possible to drastically reduce the pollutant emissions through the PCCI combustion concept, which is used only for lowload conditions. The highload conditions are covered by the conventional diffusive combustion process. The transition from low to highload conditions should be done through the injection strategy by using a combustion closedloop control, which was developed by the Institute of Automatic Control at RWTH Aachen University. This control, however, will not be discussed in this paper.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Modeling primary atomization and interface evaporation in turbulent twophase flows by means of levelset method and secondorder boundary layer theory. In 5th workshop 'MicroMacro Modelling and Simulation of LiquidVapour Flows', April 14th 16th, IRMA, Strassbourg. 2010.

Luckhchoura, V., Pardeshi, R., Sharma, A., Vogel, S., Peters, N., Cardénas, M. & Kneer, R., Investigation of the Opening Angle between Two Sprays in Diverging Nozzles. In ILASS Europe 2010: 23rd Annual Conference on Liquid Atomization and Spray Systems, September 6th8th, Brno, Czech Republic. p. 129. 2010.

Peters, N., Wang, L., Mellado, J.P., Goebbert, J.H., Gauding, M., Schaefer, P. & Gampert, M., Geometrical Properties Of Small Scale Turbulence. In NIC Symposium 2010, February 24th25th, Jülich, Germany. IAS series. pp. 365373. 2010.
In order to extract smallscale statistical information on turblulent flows from passive scalar fields obtained by direct numerical simulation (DNS), a new method of analysis is introduced. It consists of determining local minimum and maximum points of the fluctuating scalar field via gradient trajectories starting from every grid point in the directions of ascending and descending scalar gradients. The ensemble of grid cells from which the same pair of extremal points is reached determines a spatial region which is called dissipation element. Dissipation elements are highly convoluted and have on average an elongated shape. Their diameter is nearly constant and of the order of a few Kolmogorov scales while the length is highly variable and has a mean of the Taylor length. Since dissipation elements are spacefilling, it is possible to reproduce the overall statistics of the turbulent fields based on the comparably simple statistics of the elements.

Wang, L., Conditional Statistics Along Gradient Trajectories in Fluid Turbulence. In High Performance Computing in Science and Engineering '09. pp. 265272. 2010.
To investigate turbulent flows, direct numerical simulations (DNS) is playing more and more important roles benefitting from the modern computing technologies. Once the DNS data are obtained, according to different theory and purposes, the data analysis will be the core of the work at next stages. From the sizable capacity data for homogeneous shear turbulence, the conditional statistics along gradient trajectories have been investigated. It has been derived and also proved numerically that the twopoint velocity difference structure functions along the same gradient trajectories have a linear scaling with respect to the arclength between the two points, different from the classical Kolmogorov scaling. In addition, the performance of the OpenMP parallelized code is satisfactory.

Wang, L., On properties of fluid turbulence along streamlines. Journal of Fluid Mechanics, 648, pp.183203. 2010.

Mellado, J.P., The evaporatively driven cloudtop mixing layer. Journal of Fluid Mechanics, 660, pp.536. 2010.

Luckhchoura, V., Kupiek, C., Jochim, B., Won, H.W. & Peters, N., Effects of the ClusterNozzles on the Engine Performance and Soot Formation in a Direct Injection Diesel Engine: Experiments and Numerical Simulations. In FISITA 2010 World Automotive Congress: Automobiles and sustainable mobility, May 30thJune 4th, Budapest, Hungary. 2010.

Desjardins, O. & Pitsch, H., Detailed Numerical Investigation of Turbulent Atomization of
Liquid Jets. Atomization and Sprays, 20(4), pp.311336. 2010.

Gauding, M., Goebert, J.H. & Peters, N., The effect of filtering on gradient trajectories of scalar flow fields. In Proceedings of the 8th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, June 9th11th, Marseille, France. pp. 588592. 2010.

Gampert, M., Schaefer, P. & Peters, N., Testing of a new model equations for the mean dissipation via Kolmogorov flows. In Proceedings of the 8th International Symposium on Engineering Turbulence Modelling and Measurements, June 9th11th, Marseille, France. pp. 7378. 2010.

Pitsch, H., Shedding New Light on a Burning Question. Journal of Fluid Mechanics, 658, pp.14. 2010.
Only recently, threedimensional direct numerical simulations (DNS) with detailed chemistry have become possible. These types of simulations will have an important impact on combustion science because of the data they provide. Obtaining similar data sets through experimental characterization is very challenging and requires joint measurements of a multitude of chemical species concentrations in addition to temperature and velocity fields. Gruber et al. (J. Fluid Mech., vol. 658, 2010, pp. 5–32) have performed a detailed chemistry DNS to understand the physics of flame/wall interactions. They found that flames interact with streamwise vortical structures close to the wall, and that these structures can push the flame towards or away from the wall. The flames get extinguish when they are too close to the wall. Interestingly, the extinction process actually causes an orderofmagnitude increase in heat release, and therefore, strongly amplifies wall heat transfer.

Beeckmann, J., Kruse, S. & Peters, N., Effect of Ethanol and nButanol on Standard Gasoline Regarding Laminar Burning Velocities. SAE Technical Paper Series, 2010011452. 2010.
Ethanol is frequently used as a blending component in standard gasoline, with blend rates up to 10%vol liq . nButanol has received recent interest as an alternative fuel instead of ethanol for use in spark ignition engines. Similar to ethanol, nbutanol can be produced via the fermentation of sugars, starches, and lignocelluloses obtained from agricultural feedstock.<br />
<br />
It is of great interest to modern engine development to understand the effect of ethanol and nbutanol as blending components on the laminar burning velocity of standard gasoline. The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes.<br />
<br />
Tested fuel components are ethanol, nbutanol, and standard marked gasoline without any oxygen content. Fuel blends consist of standardmarked gasoline containing ethanol and butanol. The maximum blend rate of oxygenates is 10%vol liq . Experiments were done at different equivalence ratios between 0.7 and 1.3. Test conditions in this work are pressures of 10 bar and temperatures of 373 K.<br />
<br />
The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. Experimental results are discussed and compared with the numerical simulations, as far as models are available, and references from the literature.

Wang, L. & Peters, N., Mean velocity increment conditioned on gradient trajectories of various scalar variables in turbulence. Physica Scripta, 2010(T142), p.014004. 2010.
This paper focuses on the generality of the velocity structure along the gradients of several field variables in scalar form, such as the passive scalar, the Cartesian projections of the velocity vector, kinetic energy and energy dissipation. These scalar variables behave differently, which can be explained mathematically by the fact that the source terms in their governing equations are different. Consequently, it is reasonable to expect some generalities among different scalars if the respective source terms are instantaneously small. The validity of this expectation has been discussed for individual cases, especially at high Reynolds numbers. It is found that the pointwise mean strain rates along gradients of various scalars are always negative. By analyzing the scalar gradient orientations, it is seen that statistically the gradient vectors with large magnitudes align with each other, whereas those with small magnitudes tend to be randomly organized. In terms of the twopoint velocity increment, for the passive scalar it has recently been derived that there exists a nearly linear scaling of the velocity difference with respect to the arc length of gradient trajectories (Wang 2009 Phys. Rev. E 79 046325). Theoretically, this scaling can also be extended to other scalars with satisfactory agreement with numerical results.

Narayanaswamy, K., Blanquart, G. & Pitsch, H., A consistent chemical mechanism for oxidation of substituted aromatic species. Combustion and Flame, 157(10), pp.18791898. 2010.
Computational studies of combustion in engines are typically performed by modeling the real fuel as a surrogate mixture of various hydrocarbons. Aromatic species are crucial components in these surrogate mixtures. In this work, a consistent chemical mechanism to predict the high temperature combustion characteristics of toluene, styrene, ethylbenzene, 1,3dimethylbenzene (mxylene), and 1methylnaphthalene is presented. The present work builds on a detailed chemical mechanism for high temperature oxidation of smaller hydrocarbons developed by Blanquart et al. Combust. Flame 156 (2009) 588–607. The base mechanism has been validated extensively in the previous work and is now extended to include reactions of various substituted aromatic compounds. The reactions representing oxidation of the aromatic species are taken from the literature or are derived from those of the lower aromatics or the corresponding alkane species. The chemical mechanism is validated against plug flow reactor data, ignition delay times, species profiles measured in shock tube experiments, and laminar burning velocities. The combustion characteristics predicted by the chemical model compare well with those available from experiments for the different aromatic species under consideration.

Won, H.W. & Peters, N., Optimizing the Injection Pressure for Cluster Nozzle Concepts in DI Diesel Engines. International Journal of Engine Research, 11(2), pp.163174. 2010.
A combination of highpressure injection and small orifices will be one of the strategies to achieve lean combustion. However, equispaced small orifices tend to increase soot under highload condition because the spray tip penetration becomes exceedingly inadequate. For this reason, the cluster concept was chosen as a means to realize lean combustion. Six clusters were investigated with different injection pressures under partload condition and highload condition in a singlecylinder diesel engine, and the results were compared with a reference nozzle. The clusters tend to produce higher smoke than the reference nozzle for low injection pressures under conventional injection timing because the spray from the clusters with a shorter spray tip penetration loses momentum near the piston bowl. However, the clusters show improved smoke emissions with higher injection pressures. The combination of highpressure injection and cluster concepts can be seen as one of the solutions to achieve lean combustion for diesel engines. Clusters with highpressure injection have improved emissions, as better fuel atomization and evaporation are achieved while holding momentum near the piston bowl and maintaining the penetration of the spray.

Luckhchoura, V., Modeling of injectionrate shaping in diesel engine combustion. PhD dissertation. Aachen: Aachen, Techn. Hochsch., Diss., 2010: . . 2010.

Schaefer, P., Gampert, M., Göbbert, J.H., Gottschalk, D. & Peters, N., Geometrical Dissipation element analysis of inhomogenous turbulence. In NIC Symposium 2010, February 24th25th, Jülich, Germany. 2010.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Numerical investigation of spray primary breakup with phase transition. In Proceedings of the 8th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, June 9th11th, Marseille, France. pp. 782787. 2010.

Mellado, J.P., Wang, L. & Peters, N., Investigation of the Conditional Scalar Dissipation Rate Across a Shear Layer Using Gradient Trajectories. In Progress in turbulence III: ITi Conference in Turbulence 2008. Springer Proceedings in Physics. pp. 2124. 2010.
The dependence on the lateral distance y to the centerplane and on time t of the average of the scalar dissipation rate chi = 2D vertical bar del Z vertical bar(2) conditioned on the scalar Z has been investigated in a temporallyevolving shear layer using a direct numerical simulation. As the inviscid scaling of the unconditional mean dissipation rate is approached, the conditional mean (chi) over barz follows also an approximate selfsimilar behavior within the time interval considered in the simulations and (chi) over barz shows two strong peaks at a distance of about one vorticity thickness from the centerplane, with dissipation values about 3 times the one in the central region. It is showed that this spatial variation is introduced by the nonturbulent/turbulent transition regions, which are identified by means of gradient trajectories of the scalar field.

Struckmeier, U., Lucassen, A., Hansen, N., Wada, T., Peters, N. & KohseHöinghaus, K., Demonstration of a burner for the investigation of partially premixed lowtemperature flames. Combustion and Flame, 157(10), pp.19661975. 2010.

Mellado, J.P., Stevens, B., Schmidt, H. & Peters, N., Twofluid formulation of the cloudtop mixing layer for direct numerical simulation. Theoretical and Computational Fluid Dynamics, 24(6), pp.511536. 2010.
A mixture fraction formulation to perform direct numerical simulations of a disperse and dilute twophase system consisting of water liquid and vapor in air in local thermodynamic equilibrium using a twofluid model is derived and discussed. The goal is to understand the assumptions intrinsic to this simplified but commonly employed approach for the study of twolayer buoyancy reversing systems like the cloudtop mixing layer. Emphasis is placed on molecular transport phenomena. In particular, a formulation is proposed that recovers the actual nondiffusive liquidphase continuum as a limiting case of differential diffusion. Highorder numerical schemes suitable for direct numerical simulations in the compressible and Boussinesq limits are described, and simulations are presented to validate the incompressible approach. As expected, the Boussinesq approximation provides an accurate and efficient description of the flow on the scales (of the order of meters) that are considered.

Won, H.W. & Peters, N., Modified bowl geometry for cluster nozzles in directinjection diesel engines. Proceedings of the Institution of Mechanical Engineers / Part D, Journal of automobile engineering, 224(7), pp.953968. 2010.

Schaefer, P., Gampert, M. & Peters, N., Secondary splitting of zero gradient points in turbulent scalar fields. In Progress in Turbulence IV. pp. 35  39. 2010.
The mechanisms related to the secondary splitting of zero gradient points of scalar fields are analyzed using the two dimensional case of a scalar extreme point lying in a region of local strain. The velocity field is assumed to resemble a stagnation point flow, cf. Gibson 1, which is approximated using a Taylor expansion up to third order. It is found that the splitting can only be explained when the third order terms of the Taylor expansion of the flow field are included. The nondimensional splitting time turns out to depend on three parameters, namely the local Péeclet number Peδ based on the initial size of the extreme point δ and two parameters which are measures of the rate of change of the local strain. For the limiting case Peδ → 0, the splitting time is found to be finite but Péclet number independant, while for the case of Peδ → ∞ it increases logarithmically with the Péclet number.

Kerschgens, B., Gauding, M., Felsch, C., Peters, N. & Hasse, C., A consistent flamelet model to describe the interaction of combustion chemistry and mixing in the controlled auto ignition regime. SAE Technical Paper Series, 2010010181. 2010.
In internal combustion engines operating in Controlled Auto Ignition (CAI) mode, combustion phasing and heatrelease rate is controlled by stratification of fuel, fresh air, and hot internally recirculated exhaust gases. Based on the Representative Interactive Flamelet (RIF) model, a twodimensional flamelet approach is developed. As independent parameters, firstly the fuel mixture fraction and secondly the mixture fraction of internally recirculated exhaust gases are considered. The flamelet equations are derived from the transport equations for species mass fraction and total enthalpy, employing an asymptotic analysis. A subsequent coordinate transformation leads to the phase space formulation of the twodimensional flamelet equations. By the use of detailed chemical reaction mechanisms, the effects of dilution, temperature, and chemical species composition due to the internally recirculated exhaust gases are represented. The new approach is tested in phase space, using different variations of operating conditions (i.e., variations of temperature, inhomogeneity, degree of turbulence) typical of CAI engines. Effects of temperature and the composition of the recirculated exhaust gases on combustion, combustion timing, and the location of auto ignition in phase space are individually studied and discussed. The effect of the chemical composition of recirculated exhaust gases is investigated by employing different amounts of oxygen and other chemical species in the internal EGR.

Schaefer, P., Gampert, M., Goebbert, J.H. & Peters, N., Dissipation element analysis of inhomogenous turbulence. In Computational Fluid Dynamics. pp. 711716. 2010.

Luckhchoura, V., Pardeshi, R., Sharma, A., Vogel, S., Peters, N., Cardénas, M. & Kneer, R., Influence of the opening angle between two sprays in diverging nozzles. 23rd Annual Conference on Liquid Atomization and Spray Systems, September 6th9th, Brno, Czech Republic. 2010.
In the cluster nozzle concept, each orifice of a conventional nozzle is replaced by two smaller orifices yielding the same flow rate. The opening angle between two sprays was positive, thus the nozzles were called diverging. In this study, numerical simulations of three cluster nozzles were performed in a chamber at 800 K ambient temperature and 50 bar ambient pressure conditions. First, the computed liquid and vapor penetration lengths were compared with the measured penetrations to validate the model. Then, the computed results were further analyzed to examine the influence of sprayspray interaction due to varying opening angle between the sprays on droplet breakup, spray shape and evaporation leading to mixture preparation.

Won, H.W., Investigation of cluster nozzle concepts for direct injection diesel engines. PhD dissertation. . 2010.
For last several decades, there has been a lot of research about DI (Direct Injection) diesel engines because they have a clear advantage in terms of CO2 (Carbon Dioxide) emissions compared to gasoline engines. Furthermore, because of its ability to operate at low equivalence ratios, the diesel engine produces lower CO (Carbon Monoxide) and HC (HydroCarbon) emissions. Unfortunately, the diesel engine suffers from relatively high NOx (Nitrogen Oxides) and PM (Particulate Matter) emissions. In diesel engine research, keeping low fuel consumption in all operating conditions and reduction of PM and NOx emissions, are both critically important problems which need to be addressed. For this reason, research about diesel engines is being actively pursued in recent times. PM and NOx could be considerably reduced using low temperature combustion under part load conditions. But, further investigation is also necessary for fuel consumption and emission reduction under high load conditions. Reduction of orifice diameter, multiple injections and high pressure injection are avenues of rigorous research with the injection systems evolving rapidly. Fuel consumption and emission of PM are improved by using high pressure injection due to better airfuel mixing while there are still problems with NOx under all operating conditions and insufficient reduction in PM emissions under high load conditions. Diesel engine noise and up to some extent, PM and NOx emissions are reduced by the use of multiple injections. However, the reduction achieved by multiple injections is not enough to satisfy the regulations on emission which are becoming more and more stringent. Reduction in diameter of orifices has some positive effects, such as significant reduction of PM and improvement in airfuel mixing through better atomization and evaporation. On the other hand, it also has a negative effect under high load conditions, which is the reduced spray tip penetration with high temperature and pressure in combustion chamber. Based on prior knowledge available on cluster nozzles, several cluster nozzles were designed for the present work. They were comprehensively investigated by varying various parameters. Cluster nozzles consist of many groups (pairs in this case) of small orifices, and are aimed to achieve improved emission by low temperature combustion under part load conditions and lower emission levels and fuel consumption levels by high pressure injection under high load conditions. The nozzles were designed with different numbers of holepairs and different geometric configurations. Among the parameters varied during the engine experiments were swirl, injection pressure and piston geometry. The study helped in improving the understanding about characteristics of incylinder combustion phenomena, performance and emissions were also improved as a consequence. Results from the cluster nozzles were compared with those for a conventional nozzle, henceforth mentioned as the reference nozzle, to check the improvement. The cluster nozzles, having small orifice sizes, were operated with low temperature combustion under part load conditions. Emissions and fuel consumption were clearly improved together for this condition. Also at medium and high loads, it was possible to achieve lower PM emission level and fuel consumption by using proper combination of nozzle type, incylinder geometry and high injection pressure. Optimized nozzle design and incylinder geometry made small improvements under high speed full load condition which requires rapid combustion speed. As a conclusion of this work, it could be said that though there are some difficulties in the application, especially at high load and high speed condition, the cluster nozzle concept can be implemented as a reliable solution for the current problem of achieving better combustion and emission reduction.

Gauding, M., Goebbert, J.H. & Peters, N., The effect of filtering on gradient trajectories of scalar flow fields. Proceedings of the 8th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, pp.588592. 2010.

Beeckmann, J., Cai, L., Roehl, O., Pitsch, H. & Peters, N., A Reduced Kinetic Reaction Mechanism for the Autoignition of Dimethyl Ether. SAE Technical Paper Series, 2010012108. 2010.
A reduced kinetic reaction mechanism for the autoignition of dimethyl ether is presented in this paper. Dimethyl ether has proven to be one of the most attractive alternatives to traditional fossil fuels for compression ignition engines. It can either be produced from biomass or from fossil oil. For dimethyl ether, Fischer et al. (Int. J.Chem. Kinet. 32 ( 12 ) (2000) 713740) proposed a detailed reaction mechanism consisting of 79 species and 351 elementary reactions. In the present work, this detailed mechanism is systematically reduced to 31 species and 49 reactions. The reduced mechanism is discussed in detail with special emphasis on the high temperature thermal decomposition of dimethyl ether and on the fuel specific depleting reactions, which produce the methoxymethyl radical. In addition, a reaction pathway analysis for low temperature combustion is applied, where hydroperoxymethylformate is found to be the dominating parameter for the low temperature regime. A validation of the reduced mechanism is performed by calculating the ignition delay times in a pressure range from 1 bar to 40 bar, as well as at different equivalence ratios varying from 0.5 to 2.0 and temperatures from 625 K to 1650 K. These results are compared with experimental findings and values obtained using the detailed mechanism. Overall, very good agreement is achieved.

Won, H.W., Moon, S.E., Luckhchoura, V., Lee, S.W. & Peters, N., An Experimental Investigation into the Effects of Injection Pressures and Swirl Ratio using Cluster Nozzle Configurations in a DI Diesel Engine. In FISITA 2010 World Automotive Congress: Automobiles and sustainable mobility, May 30thJune4th, Budapest, Hungary. 2010.
One of the most important parameter that affects diffusion controlled combustion process in diesel engine is air fuel mixing which is mainly controlled by injection. Based on the diesel concept, it is practically impossible to avoid fuel rich and stoichiometric pockets, but the formation of soot and NOx are also time dependent. If the mixing time is sufficiently small, both pollutant could be reduced simultaneously without getting into the well known sootNOx trade off. In order to develop a low emission engine, research is necessary to come up with a new combustion strategy for diesel engine which includes the use of cluster nozzle. The concept of cluster nozzle is that it is possible to improve the condition of air fuel mixture by changing nozzle configuration. The cluster nozzle, grouping multi smaller holes together, has the potential to improve air fuel mixing in the center of spray. The cluster nozzle is characterized by high number of degree of freedom relate to spray figure, number of spray holes and configuration of holes in a group. Three cluster nozzles were tested with a CRI 3.3 piezo injector in a singlecylinder Duramax engine and compared with a conventional nozzle. The experiment was conducted under four test points and various rail pressures were applied to find an optimal rail pressure at each test point. Compared to conventional nozzle, ignition delay of cluster nozzle is shorter and the initial rate of heat release and the maximum rate of heat release of diffusion combustion are higher. Also cluster nozzles combined with high injection pressure shows better fuel consumption rate and emission level. The cluster nozzles achieved improved smokeNOx trade off while maintaining same NOx level with higher EGR. And three different mean swirl ratios were tested because the swirl is an important factor of spray formation and combustion process in combustion chamber. Swirl ratio variation was conducted to find an optimal swirl ratio for cluster nozzle at each test point. Among three different swirl ratio, 0.5, 1.5 and 3.0, better smoke emission was achieved with 1.5 and 3.0 swirl ratio compared with 0.5 swirl ratio. Between 1.5 and 3.0 swirl ratio, even though both resulted similar in smoke emission, 1.5 swirl ratios has lower BSFC and HC level than 3.0 swirl ratio. The findings form those experiments could be helpful for conceptualizing cluster nozzle for application in the whole operation range of diesel engine.

Wang, L., Two Point Velocity Difference Scaling along Scalar Gradient Trajectories in Turbulence. Progress in Turbulence III, pp.4548. 2010.
In the context of dissipation element analysis of scalar fields in turbulence 1, the elongation of elements by the velocity difference at the minimum and maximum points was found to increase linearly with the length of an element. To provide a theoretical basis for this finding by analyzing twopoint properties along the gradient trajectories, an equation for the mean product of the scalar gradient at two points along the same trajectory is derived. In the inertial range a balance similar to that from which Kolmogorov’s 4/5 law can be derived.While that law leads to a 1/3 scaling for the velocity difference, by conditioning on gradient trajectories we obtain a linear relation between the velocity difference and the twopoint’s arclength on the same trajectory. Results from DNS show satisfactory agreement with the theoretical prediction.

Drews, P., Albin, T., Hoffmann, K., Vanegas, A., Felsch, C., Peters, N. & Abel, D., ModelBased Optimal Control for PCCI Combustion Engines. In AAC 2010: 6th IFAC Symposium Advances in Automotive Control, July 12th14th, Munich, Germany. pp. 288  293. 2010.
New combustion methods for engines have been recently researched very intensively. In diesel engines, the homogenisation of the airfuel mixture by early fuel injection has significant effects on emission reduction. The paper presents a modelbased optimal control strategy for premixed charge compression ignition (PCCI) low temperature combustion in diesel engines. In order to understand the basic properties of the PCCI mode, static and dynamic measurements were conducted using a real conventional diesel engine. The main inputs of the combustion process are the exhaust gas recirculation rate and injection parameters. Outputs are the indicated mean effective pressure and the fuel mass conversion balance point. The process has very fast, almost proportional dynamics over the engine`s working cycles. Focusing on the static behaviour of the process, a nonlinear neural network model is used for identification. Successive linearisation of the nonlinear network is used as predictive controller model. The presented controller structure is able to consider constraints and can be computed very fast. Finally, the controller is validated under real time conditions by experimental tests at the engine test bench. Although the controller structure contains a model and a convex optimisation step with regards to constraints, its implementation is very simple, as no observer is used, and the linearised model consists of static gains only.

Beeckmann, J., Aye, M.M., Gehmlich, R. & Peters, N., Experimental Investigation of the Spray Characteristics of DinButyl Ether (DNBE) as an Oxygenated Compound in Diesel Fuel. SAE Technical Paper Series, 2010011502. 2010.
Increasing concern for the environment and the impending scarcity of fossil fuels requires continued development in hydrocarbon combustion science. For compressionignition engines, adding oxygenated compounds to the fuel can reduce noise, soot formation, and unburned hydrocarbons while simultaneously increasing thermal efficiency.<br />
<br />
In order to reliably model and design compressionignition engines to use new fuel blends, accurate spray characteristic data is required. In this study, the spray characteristics of various blends of the oxygenated compound dinbutyl ether (DNBE) with standard EN590 Diesel fuel are presented, including spray cone angle and spray penetration length for both liquid and gas phases. The experiments were conducted in a spray chamber at ambient conditions of 50 bar and 800 K, simulating TDC conditions in a Diesel engine. Injection pressures were varied from 7001600 bar. Miescatter and shadowgraphy techniques were used to measure the liquid phase and gas phase spray characteristics, respectively. The liftoff length and ignition delay is investigated using OHmeasurement technique. The spray structure of the DNBE alone and its blends with Diesel fuel have shown a larger spray angle and shorter penetration length than with the pure Diesel fuel, providing improvements to atomization behaviour, and thus expected increases in performance.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Direct Numerical Simulation of Spray Primary Breakup with Phase Transition. In Spray 2010: 9. Workshop über Sprays, Techniken der Fluidzerstäubung und Untersuchungen von Sprühvorgängen, May 3rd5th, Heidelberg, Germany. pp. 309  316. 2010.
We perform direct numerical simulation on large distributed memory parallel computers in order to investigate the primary breakup process of diesel spray direct injection. Local refinement algorithm– Refined level–set method has been used to reduce the memory requirement, and we analyze the performance by experiments on a 1024– processor parallel computer.

Mellado, J.P., Stevens, B., Schmidt, H. & Peters, N., Probability density functions in the cloudtop mixing layer. New Journal of Physics, 12(8), p.085010. 2010.
The cloudtop mixing layer is an idealized configuration often employed in the literature to study local aspects (over length scales of the order of 10 m) of the top of stratocumulustopped mixed layers. Latent heat effects are further investigated here by means of direct numerical simulations, discussing the probability density functions of the horizontal and vertical velocities, as well as the mixture fraction (equal to a normalized enthalpy and total waterspecific humidity). The focus is on the turbulent convection layer that develops from the buoyancy reversal instability as a consequence of the evaporative cooling at the upper cloud boundary. An approximately selfsimilar behavior is found, where the convection scales based on the molecular buoyancy flux at the cloud top characterize the distributions at different times, at least to leading order and within the statistical convergence achieved in the simulations. However, a very strong vertical variation in the density functions across the turbulent convection layer is found, which is of relevance to possible models. NonGaussian behavior is often observed, even in the horizontal component of the velocity vector. In particular, large values of skewness and flatness are measured at the lower end of the turbulent zone, where external intermittency is very strong.

Spiekermann, P., Jerzembeck, S., Felsch, C., Vogel, S., Gauding, M. & Peters, N., Experimental data and numerical simulation of commonrail ethanol sprays at diesel enginelike conditions. Atomization and Sprays, 19(4), pp.357386. 2009.
CommonRail ethanol sprays are experimentally investigated in a highpressure vessel for pressures up to 50 bar and temperatures up to 800 K. Mie, Shadow, and Raman optical measurement techniques are used for the spray investigations. The experimental setup and the measurement techniques are described in detail. Detailed experimental results are presented for various vessel conditions. This includes liquid and gaseous penetration, spray angle of liquid and gaseous phase, liquid and vapor fuel mass fraction, liquid and vapor temperature, as well as Sauter mean radius. The experimental results provide a considerable dataset for numerical validation purposes. In addition to the experimental investigations, numerical simulations are carried out applying the commonly used discrete droplet model (DDM). A short review of the DDM including its inherent submodels is presented along with the simulation setup. Selected experimental results (liquid and gaseous penetration, liquid and vapor fuel mass fraction, liquid and vapor temperature, and Sauter mean radius) are compared to corresponding results obtained from the spray simulations to evaluate the performance of the DDM approach.

Jerzembeck, S., Matalon, M. & Peters, N., Experimental investigation of very rich laminar spherical flames under microgravity conditions. Proceedings of the Combustion Institute, 32(1), pp.11251132. 2009.
Very rich premixed outward propagating spherical flames of nheptane (ϕ = 3.5) and isooctane (ϕ = 3.9) spherical flames were experimentally investigated under microgravity conditions at 420 K and initial pressures of up to 30 bar. Inert gases with significantly different molecular weights were used as diluents leading to mixtures with varying effective Lewis numbers. The experimental setup consisted of a spherical closed pressurized vessel that enabled optical access. The experiments were performed in a drop tower under microgravity conditions to prevent the influence of buoyancy on the expanding flames. A Schlieren measurement technique combined with a highspeed CCD camera was used to track the expanding flames. During the entire experiment, the flames were laminar and smooth with no wrinkles due to combustion instabilities observed. This allowed for accurate measurements of the propagation speed and its dependence on stretch, which was then compared to theoretical predictions based a hydrodynamic model for relatively thin flames. The experimental and theoretical results show good agreement with each other. Investigation of such rich mixtures that are close to the upper flammability limit has not been reported before.

Gauding, M., Brands, T., Felsch, C., Hottenbach, P., Hasse, C., Pauls, C., Grünefeld, G. & Peters, N., Experimental and Numerical Investigation of Ignition Mechanisms for Multiple Injection Strategies at Diesel EngineLike Conditions. In 4th European Combustion Meeting: 14th  17th April 2009, Vienna, Austria. 2009.
Split injections in DI Diesel engines and combustion vessels exhibit different ignition phenomena for the first and for the second injection pulse. In this work, the ignition phenomena of a split injection into a combustion vessel are investigated experimentally and numerically. The first pulse of the split injection is injected into quiescent air; combustion is an autoignition process. The ignition mechanism of the second pulse is fundamentally different. As soon as the mixture fields of the first and the second pulse get into contact, a strained premixed flame between the mixture fields is established. The premixed flame propagates along the region of stoichiometric mixture towards the mixture field of the second pulse. This leads to a triggered ignition of the second pulse.

Wada, T. & Peters, N., Investigation of Chemical Kinetics in ChemicalInduced Oscillations with Diluted Lean CH4/Air Mixture at Low Temperatures. In 7th AsiaPacific Conference on Combustion: National Taiwan Univ. Hospital, Intern. Convention Center, 24th  27th May, 2009, Taipei, Taiwan. 2009.
Numerical simulations of oscillation phenomena are performed using a perfectly stirred reactor model. Reactions were carried out at low temperatures under at mospheric pressure, and diluted lean CH4/air mixtures were used. To specify a predominant reaction path, a skeletal kinetic model was created from a detailed model and evaluated using reaction flow analyses. The results show that reactions from CH3 to CH3O and CH3O2 are the most important reactions with respect to oscillations and indicate the critical temperature of low and hightemperature chemistries. On the basis of the obtained results, we describe the mechanism of all the oscillations in the reactor and extend our fundamental understanding of hightemperature chemistry to chemical phenomena observed below the critical temperature.

Mellado, J.P., Schmidt, H., Stevens, B. & Peters, N., DNS of the turbulent cloudtop mixing layer. In Advances in turbulence XII: proceedings of the 12th EUROMECH European Turbulence Conference, 7th  10th September 2009, Marburg, Germany. Springer Proceedings in Physics. pp. 401404. 2009.
The turbulent cloudtop mixing layer is studied using direct numerical simulation (DNS). This configuration models the top of stratocumulus clouds and is employed to investigate the role of latent heat effects. A partial description of the turbulent flow that develops when the cloud and the cloudfree air mix under buoyancy reversal conditions is presented in this paper.

Dahms, R. & Peters, N., On Nitrogen Oxide Formation in SprayGuided SparkIgnited Gasoline Engines. In Sixth Mediterranean Combustion Symposium, Corsica, France. 2009.

Jerzembeck, S., Glawe, C. & Peters, N., Development and Experimental Evaluation of a High Temperature Mechanism for blended NHeptaneIsooctaneEthanolAirMixtures and GasolineEthanolAirMixtures. In Recent Advances in Energy and Environment: 4th IASME/WSEAS International Conference on Energy and Environment, 24th  26th March, 2009, Cambridge, England. Energy and Environmental Engineering Series. pp. 7883. 2009.
Laminar burning velocity measurements using the closed vessel bomb method have been done for fuel blendairmixtures at 373 K initial temperature and up to 20 bar initial pressure. The two experimentally investigated fuel blends consist, on the one hand, of 78.3 % vol. isooctane, 11.7 % vol. nheptane, and 10 % vol. ethanol, and on the other hand of 90 % vol. standard gasoline and 10 % vol. ethanol. A detailed hightemperature chemical kinetic model has been developed with the focus on calculating laminar burning velocities of these mixtures. The reduced high temperature mechanism developed by Jerzembeck et al. 1 for PRFs (Primary Reference Fuels consisting of isooctane and nheptane) is the basis for the present work. The dominant reactions of ethanol were added to this mechanism. These reactions were taken from the mechanism of Marinov et al. 2. Laminar burning velocities calculated with the newly developed mechanism are in good agreement with the experimental results for low and high pressure mixtures, as well as for single fuel and blended fuel air mixtures.

Won, H.W., Sharma, A., Hottenbach, P., Gauding, M., Roberts, F.X., Peters, N. & Gruenefeld, G., Investigation of Particulate Emissions for ClusterNozzle Concepts in DI Diesel Engines. In ICLASS 2009: 11th International Annual Conference on Liquid Atomization and Spray Systems, 26th  30th July, Vail, Colorado USA. 2009.
In a conventional Diesel engine, air is gradually drawn into the fuel spray from the surrounding area. The ignition delay period is short, so combustion starts before the fuel has thoroughly mixed with the air. Consequently, the center of the spray is overly rich, resulting in smoke, while stoichiometric mixture is formed in the surrounding area, resulting in a high NOx concentration. Based on the Diesel concept, it is practically impossible to totally avoid fuelrich and stoichiometric pockets, but the formation of soot and NOx are also timedependent. If the mixing time is sufficiently small, both pollutants could be reduced simultaneously without getting into the well known sootNOx tradeoff. In order to develop a lowemission engine, research is necessary to come up with a new combustion strategy for Diesel engines which includes the use of cluster nozzles. Decreasing the holesize improves mixing in the center of the spray and therefore the soot production is lowered tremendously. Based on this experience, three cluster designs were developed for the present work. The basic strategy of the cluster nozzles is to provide a better primary breakup and therefore a better mixture formation caused by the smaller nozzle holes, while keeping a comparable penetration length of the vapor phase due to merging of the sprays. In this study, two different clusternozzle designs were investigated in a combustion vessel and compared to a conventional nozzle with the same flow rate. Two different measurement techniques are employed to investigate the combustion process. The local soot concentration during combustion is measured semiquantitatively using Laser Induced Incandescence (LII). The natural soot luminosity is recorded simultaneously using a double frame camera. The results indicate that soot formation can be reduced by using cluster nozzles, at least in the early combustion phase under the investigated conditions. Three cluster designs similar to the ones used in the combustion vessel, were investigated through engine measurements. The nozzles used in this study were designed for improved homogenization of incylinder charge. They were tested<br />
in a singlecylinder engine with CRI 3.3 piezoinjectors under partload conditions for a partly homogenous mode of Diesel combustion, and also under high load conditions for conventional Diesel combustion. Numerical simulations were also carried out to explain the observations of the engine experiments. Certain test cases were simulated to get some detailed information on performance of cluster nozzles, giving more insight into soot formation. Another nozzle was designed based on the results from the three clusters. Engine experiments with the nozzle show<br />
improvements in soot emissions under highload conditions.

Felsch, C., Dahms, R., Glodde, B., Vogel, S., Peters, N., Barths, H., Sloane, T., Wermuth, N. & Lippert, A., An Interactively Coupled CFDMultiZone Approach to Model HCCI Combustion. Flow, Turbulence and Combustion, 82(4), pp.621641. 2009.
The objective of this work is to present an innovative interactively coupled CFDmultizone approach. In a consistent manner, the approach combines detailed flow field information obtained from CFD with detailed chemical kinetics solved in a multizone model. Combustion and pollutant formation in an HCCI engine with recompressing VVA strategy are numerically investigated using the interactively coupled CFDmultizone approach. A surrogate fuel for gasoline is used in the simulation that consists of nheptane (18% liquid volume fraction) and isooctane (82% liquid volume fraction). The underlying complete reaction mechanism comprises 482 elementary reactions and 115 chemical species. The interactively coupled CFDmultizone approach shows to be accurate enough to describe HCCI chemistry, and is at the same time economical enough to allow application in an industrial environment. For the test case investigated, the simulation results are compared to experimental data that has been obtained using real gasoline. The overall agreement between simulation and experiment is found to be very good.

Honnet, S., Seshadri, K., Niemann, U. & Peters, N., A surrogate fuel for kerosene. Proceedings of the Combustion Institute, 32(1), pp.485492. 2009.
Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular JetA1, JetA, and JP8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of ndecane 80% and 1,2,4trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene.<br />
<br />
A chemicalkinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemicalkinetic mechanisms for the components: ndecane and 1,2,4trimethylbenzene. Improvements are made to the previously developed chemicalkinetic mechanism for ndecane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemicalkinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemicalkinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene.

Kerschgens, B., Felsch, C., Vanegas, A. & Peters, N., Applying an Interactively Coupled CFDMultiZone Approach to Study the Effects of Piston Bowl Geometry Variations on PCCI Combustion. SAE International Journal of Engines, 2(1), pp.17931810. 2009.
Recently, a consistent mixing model for the twoway coupling of a CFD code and a zerodimensional multizone code was developed. This work allowed for building an interactively coupled CFDmultizone approach that can be used to model HCCI combustion.<br />
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In this study, the interactively coupled CFDmultizone approach is applied to PCCI combustion in a 1.9l FIAT GM Diesel engine. The physical domain in the CFD code is subdivided into multiple zones based on one phase variable (fuel mixture fraction). The fuel mixture fraction is the dominant quantity for the description of nonpremixed combustion. Each zone in the CFD code is represented by a corresponding zone in the zerodimensional multizone code. The zerodimensional multizone code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The thermodynamic state of each zone, and thereby the phase variable, changes in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer). An elaborated mixing model and an appropriate treatment of the source terms keep the thermodynamic state of the zones in the CFD code and the zerodimensional multizone code identical.<br />
<br />
The results obtained with the interactively coupled CFDmultizone approach are compared to experimental results for five selected operating conditions, showing very good agreement. In a following step, the engine geometry used in the numerical simulations is modified, yielding two different pistonbowl geometries which vary from the original shape in regard to bowl diameter and bowl depth. The volume of the bowl and thus the compression ratio are kept at the original value. Simulations of PCCI combustion using these modified geometries show the influence of the pistonbowl shape on mixture formation and pollutant emissions, leading the way to numerically optimise the pistonbowl design for PCCI combustion.

Mellado, J.P., Stevens, B., Schmidt, H. & Peters, N., Buoyancy reversal in cloudtop mixing layers. Quarterly Journal of the Royal Meteorological Society, 135(641), pp.963978. 2009.
A theoretical and numerical smallscale study of the evaporative cooling phenomenon that might appear in the stratocumulustopped boundary layers is presented. An ideal configuration of a cloudtop mixing layer is considered as defined by two nonturbulent horizontal layers, stably stratified and with buoyancy reversal within a certain range of mixture fractions due to the evaporative cooling. Linear stability analysis of the shearfree configuration is employed to provide a new interpretation of the buoyancy reversal parameter, namely in terms of a timescale ratio between the stable and the unstable modes of the system. An incompressible highorder numerical algorithm to perform direct numerical simulation of the configuration is described and twodimensional simulations of singlemode perturbations are presented. These simulations confirm the role of the different parameters identified in the linear stability analysis and show that convoluted flow patterns can be generated by the evaporative cooling even for the low levels of buoyancy reversal found in stratocumulus clouds. They also show that there is no enhancement of turbulent entrainment of upperlayer fluid in the shearfree configuration, and turbulent mixing enhancement by the evaporative cooling is restricted to the lower layer.

Won, H.W. & Peters, N., Investigation of Clusternozzle Concepts for Direct Injection Diesel Engines. Atomization and Sprays, 19(10), pp.983996. 2009.

Barths, H., Felsch, C. & Peters, N., Mixing models for the twoway coupling of CFD codes and zerodimensional multizone codes to model HCCI combustion. Combustion and Flame, 156(1), pp.130139. 2009.
The objective of this work is the development of a consistent mixing model for the twowaycoupling of a CFD code and a multizone code based on multiple zerodimensional reactors. The twowaycoupling allows for a computationally efficient modeling of HCCI combustion. The physical domain in the CFD code is subdivided into multiple zones based on three phase variables (fuel mixture fraction, dilution, and total enthalpy). Those phase variables are sufficient for the description of the thermodynamic state of each zone, assuming that each zone is at the same pressure. Each zone in the CFD code is represented by a corresponding zone in the zerodimensional code. The zerodimensional code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The difficulty in facing this kind of methodology is to keep the thermodynamic state of each zone consistent between the CFD code and the zerodimensional code after the initialization of the zones in the multizone code has taken place. The thermodynamic state of each zone (and thereby the phase variables) will change in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer). The focus of this work lies on a consistent description of the mixing between the zones in phase space in the zerodimensional code, based on the solution of the CFD code. Two mixing models with different degrees of accuracy, complexity, and numerical effort are described. The most elaborate mixing model (and an appropriate treatment of the source terms) keeps the thermodynamic state of the zones in the CFD code and the zerodimensional code identical. The models are applied to a test case of HCCI combustion in an engine.

Vanegas, A. & Peters, N., Experimental analysis of the effect of very early pilot injection on pollutant formation for a PCCI Diesel Engine. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June, 2009, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. 2009.

Felsch, C., Hoffmann, K., Vanegas, A., Drews, P., Barths, H., Abel, D. & Peters, N., Combustion model reduction for diesel engine control design. International Journal of Engine Research, 10(6), pp.359387. 2009.
The subject of this work is the derivation of a simulation model for premixed charge compression ignition (PCCI) combustion that can be used in closedloop control development. For the highpressure part of the engine cycle, a detailed threedimensional computational fluid dynamics model is reduced to a standalone multizone chemistry model. This multizone chemistry model is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary exactness, and is at the same time very economic with respect to computational costs. The model is further extended by the identified system dynamics that influence the stationary inputs. For this purporse, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant diesel engine is created.

Mellado, J.P., Wang, L. & Peters, N., Gradient trajectory analysis of a scalar field with external intermittency. Journal of Fluid Mechanics, 626(1), pp.333365. 2009.
The passive scalar field of a temporally evolving shear layer is investigated using gradient trajectories as a means to analyse the scalar probability density function and the conditional scalar dissipation rate in the presence of external intermittency. These results are of significance for turbulent combustion, where improved predictions of the statistics of the conditional dissipation rate are needed in several models. First, the variation of the conventional first and second moments of the conditional dissipation rate across the layer is quantitatively documented in detail. A strong dependence of the conditional dissipation rate on the lateral position and on the conditioning value of the scalar is observed. The dependence on the transverse distance to the centreplane partially explains the doublehump profile usually reported when this dependence is ignored. The variation with the scalar observed in the ratio between the second and first moments would invalidate certain assumptions commonly done in turbulent combustion. It is also seen that conditioning on the scalar does not reduce the fluctuation of the dissipation rate with respect to unconditional values. Next, the role of external intermittency in these results is investigated. For that purpose, the flow is partitioned into different zones based on different types of gradient trajectories passing through each point, thereby introducing nonlocal information in comparison with the standard turbulent/nonturbulent separation based on the conventional intermittency function. In addition to the homogeneous outer regions, three zones are identified: a turbulent zone, a turbulence interface and quasilaminar diffusion layers. The relative contribution from each of these zones to the conventional intermittency factor is reported. The statistics are then conditioned on each of these zones, and the spatial variation of the scalar distribution and of the conditional scalar dissipation rate is explained in terms of the observed zonal statistics. For the Reynolds numbers of the present simulation, between 1500 and 3000 based on the vorticity thickness and the velocity difference, and a Schmidt number equal to 1, it results that the major contribution to both statistics is due to the turbulence interfaces. At the same time, the turbulent zone shows a distinct behaviour, being approximately homogeneous but anisotropic.

Felsch, C., Kerschgens, B., Vanegas, A., Glawe, C., Peters, N., Hoffmann, K., Drews, P., Abel, D. & Barths, H., Systematische Reduktion eines interaktiv gekoppelten CFDMehrzonenmodells zu einem rechenzeitoptimierten eigenständigen Mehrzonenmodell für die PCCI Verbrennung. In Motorische Verbrennung  aktuelle Probleme und moderne Lösungsansätze: 19./20. März 2009, Haus der Technik, München. Berichte zur Energie und Verfahrenstechnik. pp. 381393. 2009.

Jarmolowitz, F., Abel, D., Wada, T. & Peters, N., Control of a homogeneous stirred reactor: Trajectory piecewiselinear model for NMPC. In Proceedings of the European Control Conference 2009, August 23rd26th, Budapest, Hungary. pp. 23012306. 2009.
As a consequence of increasing computational power and improved algorithms, model predictive control (MPC) nowadays is becoming applicable to highdynamic processes. If using nonlinear MPC strategies (NMPC), e.g. sequential quadratic programmingsolvers (SQP), it is reasonable, if not mandatory to reduce the prediction model in order to cope with realtime. In this work the trajectory piecewiselinear approach (TPWL) as a model order reduction (MOR) scheme is investigated in simulation on its applicability on controlling combustion instabilities in an open homogeneous reactor using a SQP strategy. The corresponding nonlinear model exhibits, like the experiment, temperature oscillations during combustion. The temperature oscillations are due to chemical kinetics and typical for homogeneous lowtemperature combustion. Since chemical kinetics are highly nonlinear, NMPC is a promising approach to damp these oscillations and to control the system in different operating points.

Schäfer, P., Gampert, M., Wang, L. & Peters, N., Fast and slow changes of the length of gradient trajectories in homogenous shear turbulence. In Advances in Turbulence XII: proceedings of the 12th EUROMECH European Turbulence Conference, 7th  10th September, 2009, Marburg, Germany. Springer Proceedings in Physics. pp. 565572. 2009.
Gradient trajectories in scalar fields have recently received attention in the context of dissipation elements 1, 2 which in turn are of interest for the flamelet concept in nonpremixed combustion 3. Dissipation elements are space filling regions in a scalar field defined such that gradient trajectories starting from any point within the element in ascending and descending directions reach the same minimum and maximum points. Gradient trajectories advance preferentially through regions of the scalar field that have been smoothed by the combined action of diffusion and extensive strain. Since the extensive strain in these regions is of the order of the inverse of the integral time scale T, dimensional analysis predicts the mean length l m of dissipation elements to be of the order of the Taylor length 4.

Wada, T., Mizomoto, M., Yokomori, T. & Peters, N., Extinction of methane/air counterflow partially premixed flames. Proceedings of the Combustion Institute, 32(1), pp.10751082. 2009.
Flame interactions of partially premixed flames (PPFs) were experimentally and numerically studied using counterflow configurations with CH4/air mixtures under atmospheric condition. The main aims of this study were to describe the flame interactions in terms of variations in global strain rate (K) and to understand flame extinction. Two premixedfuel streams were characterized by equivalence ratios (ϕlean and ϕrich). Experiments were conducted under the following conditions: ϕlean + ϕrich = 2, 0.5 < ϕlean < 0.7 (i.e., 1.3 < ϕrich < 1.5), and several K values. To describe the interactions, twin flames (TFs), also examined under corresponding ϕ, were used as a criterion, because the ϕ values were within the flammability limits. Experimental results of PPFs showed a different behavior than those of TFs: first, three different luminous layers were observed; second, thin singlelayered flames, with a nearly constant flame width, were observed near extinction for several combinations of ϕ; third, PPFs could exist for significantly higher K than TFs. Numerical results supported the experimental results and showed that two reaction paths, CH4 + OH → CH3 + H2O and CH3 + O → CO + H2 + H, were remarkably intensified by flame interaction of PPF near extinction; the significance of the reaction paths was determined. In addition, it was found that excess CO production plays an important role in extinction of PPFs.

Drews, P., Hoffmann, K., Beck, R., Gasper, R., Vanegas, A., Felsch, C., Peters, N. & Abel, D., Fast model predictive control for the air path of a turbocharged diesel engine. In Proceedings of the European Control Conference 2009, August 23rd26th, Budapest, Hungary. pp. 33773382. 2009.

Felsch, C., Gauding, M., Hasse, C., Vogel, S. & Peters, N., An extended flamelet model for multiple injections in DI Diesel engines. Proceedings of the Combustion Institute, 32(2), pp.27752783. 2009.
Combustion modeling using the Representative Interactive Flamelet (RIF) model has proven successful in predicting Diesel engine combustion. The RIF model was previously used for Diesel engine combustion processes with not more than two consecutive injections into the combustion chamber. In this study, the RIF model is extended allowing for any number of injection events.<br />
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First, the twodimensional laminar flamelet equations, which can describe the transfer of heat and mass between twointeracting mixture fields, are introduced. This is followed by a description of the various mixture fraction and mixture fraction variance equations that are required for the model extension accounting for multiple injection events. Finally, the modeling strategy for multiple injection events is derived: Different phases of combustion and interaction between the mixture fields resulting from different injections are identified. Based on this, the extension of the RIF model to describe any number of injections is put forward.<br />
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Simulation results using the extended RIF model are compared against experimental data for a CommonRail DI Diesel engine that was operated with three injection pulses. For the pilot injection and the main or post injection, respectively, different ignition phenomena are pointed out and the influence of the scalar dissipation rate on these ignition phenomena is investigated in detail.

Vogel, S. & Peters, N., Simulation of Lifted Diesel Sprays using a newly developed Combined LevelSet Flamelet Model. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. p. 3. 2009.

Dahms, R., Drake, M.C., Fansler, T.D., Kuo, T.W. & Lippert, A.M., Modeling Ignition Phenomena in SprayGuided SparkIgnited Engines. Proceedings of the Combustion Institute, 32(2), pp.2743–2750. 2009.
An ignition model for sprayguided (SG) directinjection gasoline engines called SparkCIMM—Spark Channel Ignition Monitoring Model—is presented in this paper. The model concept is motivated by highspeed imaging data showing complex processes for spark (formation, turbulent stretching and wrinkling, and multiple restrikes) and ignition (localized flame kernel formation and growth). Turbulent fluctuations occurring on the scale of turbulent sparkchannel wrinkling (∼0.05–0.1 mm) and local ignition (∼0.1–0.5 mm) are analyzed using expressions for the subgrid turbulent kinetic energy, eddy turnover velocity and mixturefraction variance. Computational particles are introduced along the spark channel to monitor its motion and ignitability. Ignition is determined by a local Karlovitznumber criterion that incorporates effects of turbulence, detailed chemical kinetics, and continuous energy deposition from the spark. Wherever suitable ignition conditions are found along the elongated spark channel, a small flame kernel is launched and tracked as it grows and merges with other flame kernels to form the turbulent flame surface. When the flame surface is sufficiently large, a Gequation flamelet combustion model tracks the turbulent flame front. Both the early flamekernelgrowth and Gequation combustion models include effects of local mixturefraction variance. Results from the SparkCIMM ignition model and Gequation combustion model are compared with experiments in an SG directinjection engine. The model reproduces the general experimental features of sparkchannel stretching, corrugation, and localized ignition; and flame front location probabilities are in good agreement.

Peters, N. & Won, H.W., A cluster nozzle concept with high injection pressures for DI Diesel engine. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. p. 3. 2009.

Zeng, P., Sarholz, S., Iwainsky, C., Binninger, B., Peters, N. & Herrman, M., Simulation of Primary Breakup for Diesel Spray with Phase Transition. In Recent advances in parallel virtual machine and message passing interface: 16th European PVM/MPI Users' Group Meeting, 7th  10th September 2009, Espoo, Finland. Lecture Notes in Computer Science. pp. 313320. 2009.
We perform direct numerical simulation on large distributed memory parallel computers in order to investigate the primary breakup process of diesel spray direct injection. Local refinement algorithmRefined levelset method has been used to reduce the memory requirement, and we analyze the performance by experiments on a 1024processor parallel computer.

Gauding, M., Pawlowski, A., Felsch, C., Sonntag, B., Kneer, R. & Peters, N., Fundamental Investigation of Diesel SpraySpray Interaction for ClusterHole Nozzles. In 11th ICLASS International Conference on Liquid Atomization and Spray Systems, 26th  30th July 2009, Vail, Colorado. ILASS International. 2009.

Jerzembeck, S., Peters, N., PepiotDesjardins, P. & Pitsch, H., Laminar burning velocities at high pressure for primary reference fuels and gasoline: Experimental and numerical investigation. Combustion and Flame, 156(2), pp.292301. 2009.
Spherical flames of n heptane, isooctane, PRF 87 and gasoline/air mixtures are experimentally investigated to determine laminar burning velocities and Markstein lengths under enginerelevant conditions by using the constant volume bomb method. Data are obtained for an initial temperature of 373 K, equivalence ratios varying from ϕ=0.7ϕ=0.7 to ϕ=1.2ϕ=1.2, and initial pressures from 10 to 25 bar. To track the flame front in the vessel a dark field He–Ne laser Schlieren measurement technique and digital image processing were used. The propagating speed with respect to the burned gases and the stretch rate are determined from the rate of change of the flame radius. The laminar burning velocities are obtained through a linear extrapolation to zero stretch. The experimentally determined Markstein numbers are compared to theoretical predictions. A reduced chemical kinetic mechanism for nheptane and isooctane was derived from the Lawrence Livermore comprehensive mechanisms. This mechanism was validated for ignition delay times and flame propagation at low and high pressures. In summary an overall good agreement with the various experimental data sets used in the validation was obtained.

Peters, N., Multiscale combustion and turbulence. Proceedings of the Combustion Institute, 32(1), pp.125. 2009.
Multiscale physics is the interaction of different physical processes occurring at largely separated scales. In combustion, many elementary reactions combine to only a few, but still have separated time scales. In flames, owing to the presence of diffusion, time scales manifest themselves as length scales, i.e. thicknesses of reaction layers embedded within each other. For premixed flames there results a single velocity scale, the laminar burning velocity, which in turn defines a flame thickness and a flame time as global length and time scales, respectively. The laminar burning velocity represents the simplest microscale model to be used at a premixed combustion interface.<br />
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While combustion is a multiscale process, this is not so evident for turbulence. Based on the picture of a cascade process traditional turbulent closure approximations treat turbulence as a singlescale problem. Attempts to model turbulent combustion in the same way by using methods developed for nonreacting turbulent flows therefore must fail, because they ignore the multiscale nature of combustion.<br />
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There is, however, a long tradition and much progress in multiscale modeling of combustion, both on the macroscale as well as on the microscale level. Unfortunately much of that work is conceived only in its particular context, not as part of a multiscale approach. For instance, papers in the TURBULENT FLAMES Colloquium and the FIRE RESEARCH Colloquium at this and at previous Combustion Symposia often take the viewpoint of macroscale modeling only, while REACTION KINETICS and LAMINAR FLAMES concentrate on microscale aspects. What seems to be needed is a more explicit reference to the needs of models developed in the other parts of the community. Furthermore, research is needed to develop suitable definitions of the interface between macroscale and microscale models.

Luckhchoura, V., Robert, F.X., Peters, N., Rottmann, M. & Pischinger, S., Experimental and Numerical Investigation of Injection Rate Shaping in a SmallBore DirectInjection Diesel Engine. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June, AGIT Technology Center, Aachen, Germany. CEUR Workshop Proceedings. 2009.

Dahms, R., Fansler, T.D., Drake, M.C., Kuo, T.W., Lippert, A.M. & Peters, N., Modeling ignition phenomena in sprayguided sparkignited engines. Proceedings of the Combustion Institute, 32(2), pp.27432750. 2009.
An ignition model for sprayguided (SG) directinjection gasoline engines called SparkCIMMSpark Channel Ignition Monitoring Modelis presented in this paper. The model concept is motivated by highspeed imaging data showing complex processes for spark (formation, turbulent stretching and wrinkling, and multiple restrikes) and ignition (localized flame kernel formation and growth). Turbulent fluctuations occurring on the scale of turbulent sparkchannel wrinkling (similar to 0.050.1 mm) and local ignition (similar to 0.10.5 mm) are analyzed using expressions for the subgrid turbulent kinetic energy, eddy turnover velocity and mixturefraction variance. Computational particles are introduced along the spark channel to monitor its motion and ignitability. Ignition is determined by a local Karlovitznumber criterion that incorporates effects of turbulence, detailed chemical kinetics, and continuous energy deposition from the spark. Wherever suitable ignition conditions are found along the elongated spark channel, a small flame kernel is launched and tracked as it grows and merges with other flame kernels to form the turbulent flame surface. When the flame surface is sufficiently large, a Gequation flamelet combustion model tracks the turbulent flame front. Both the early flamekernelgrowth and Gequation combustion models include effects of local mixturefraction variance. Results from the SparkCIMM ignition model and Gequation combustion model are compared with experiments in an SG directinjection engine. The model reproduces the general experimental features of sparkchannel stretching, corrugation, and localized ignition; and flame front location probabilities are in good agreement. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Zeng, P., Binninger, B., Peters, N. & Herrmann, M., Simulation of Primary Breakup for Diesel spray with phase transition. In Towards Clean Diesel Engines: 7th International Symposium, 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. p. 1. 2009.

Felsch, C., Hoffmann, K., Vanegas, A., Drews, P., Albin, T., Abel, D. & Peters, N., A dynamic PCCI combustion model for Diesel engine control design. In 7th International Symposium Towards Clean Diesel Engines: 4th  5th June 2009, AGIT Technology Center, Aachen, Germany. p. 3. 2009.
The subject of this work is the derivation of a simulation model for premixed charge compression ignition (PCCI) combustion that can be used in closedloop control development. For the highpressure part of the engine cycle, a detailed threedimensional computational fluid dynamics model is reduced to a standalone multizone chemistry model. This multizone chemistry model is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary exactness, and is at the same time very economic with respect to computational costs. The model is further extended by the identified system dynamics that influence the stationary inputs. For this purporse, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant diesel engine is created.

Zeng, P., Binninger, B., Peters, N. & Herrman, M., Simulation of Primary Breakup for Diesel Spray with Phase Transition. In 11th ICLASS International Conference on Liquid Atomization and Spray Systems, 26th  30th July 2009, Vail, Colorado. 2009.

Won, H.W., Sharma, A., Moon, S.E., Vanegas, A. & Peters, N., An Experimental Study of Cluster Nozzles for DI Diesel Engine. 9th International Conference on Engines and Vehicles, 13th  18th September 2009, Capri, Naples, Italy. 2009.
In a conventional Diesel engine, air is gradually drawn into the fuel spray from the surrounding area. The ignition delay period is short, so combustion starts before the fuel has thoroughly mixed with the air. Consequently, the center of the spray is overly rich, resulting in smoke, while a stoichiometric mixture is formed in the surrounding area, resulting in a high NOx concentration. Based on the Diesel concept it is practically impossible to totally avoid fuel rich and stoichiometric pockets, but the formation of soot and NOx are also time dependent. If the mixing time is sufficiently small both pollutants could be reduced simultaneously without getting into the well known sootNOx tradeoff. In order to develop a low emission engine, research is necessary to come up with a new combustion strategy for Diesel engines, which includes the use of cluster nozzles. Conditions for low raw particulate emission are: suitable start of injection, lean air/fuel mixture, sufficiently high temperatures in the combustion chamber and a sufficient retention time with these conditions. This could be achieved by use of cluster nozzles and a careful tuning of the air/fuel mixing parameters of the engine piston geometry. Decreasing the holesize improves mixing in the center of the spray and therefore the soot production goes down tremendously. It has been observed, that an optimum number of holes exists (6 to 8) and that a certain swirl level is necessary. Based on this experience the cluster concept was developed. The cluster nozzles can also be used for improved homogenization of incylinder charge so as to enable a partly homogenous mode of Diesel combustion. In this study, three cluster designs are investigated through engine measurements. They were tested in a singlecylinder engine with CRI 3.3 piezo injectors under partload conditions for a partly homogenous mode of Diesel combustion, and also under high load conditions for conventional Diesel combustion. The experimental studies give some detailed information on how cluster nozzles could help in lowering soot formation and thereby provide more freedom to reduce other pollutants.

Peters, N., Hoffmann, K., Felsch, C. & Abel, D., A Dynamic Simulation Strategy for PCCI Combustion Control Design. In ECOSM'09: IFAC Workshop on Engine and Powertrain Control, Simulation and Modeling, 30th November  2nd December, 2009, RueilMalmaison, France. 2009.
A Dynamic Simulation Strategy for PCCI Combustion Control Design  Subject of this work is a dynamic simulation strategy for PCCI combustion that can be used in closedloop control development. A detailed multizone chemistry model for the highpressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting stationary model is capable of describing PCCI combustion sufficiently well. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created. This paper summarizes an important outcome of the the Collaborative Research Centre "SFB 686  Modellbasierte Regelung der homgenisierten NiedertemperaturVerbrennung" at RWTH Aachen University and Bielefeld University, Germany.

Röhl, O. & Peters, N., A Reduced Mechanism for Ethanol Oxidation. In 4th European Combustion Meeting: Vienna University of Technology, 14th  17th April, Vienna, Austria. 2009.
To account for the increasing share of alternative fuels, reliable chemical kinetic mechanisms are needed. For ethanol, as one of the most important alternative fuels, Marinov 1 proposed a hightemperature mechanism consisting of 57 chemical species and 383 elementary reactions. In the present work, this mechanism is systematically reduced, nally leading to 38 species and 228 reactions. For validation, numerical results using the reduced mechanism are compared to experimental ignition delay times and laminar burning velocities, as well as results obtained with the original mechanism.

Wang, L., Scaling of the twopoint velocity difference along scalar gradient trajectories in fluid turbulence. Physical Review, 79(4), p.046325, 5 S. 2009.

Beeckmann, J., Keppner, J., Glatz, T. & Peters, N., Laminar Burning Velocities of Dimethyl Ether, nHeptane and isoOctane at High Pressure. SAE Technical Paper Series, 2009012656. 2009.
Oxygenates, such as methanol or ethanol, are frequently used as blending components in standard gasoline. One oxygenate, dimethyl ether (DME), is also used as a fuel component in some regions of the world, for example in Asia. In addition, patent reviews show the potential of DME as a blending component in liquefied petroleum gas (LPG) or mixed with propane. The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes. Therefore, it is of great interest for modern engine development to understand the effect of oxygenates on the laminar burning velocity.<br />
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The experimental results have been conducted under enginelike conditions with elevated initial pressures of up to 20 bar and initial temperatures of 373 K. Experiments were done at equivalence ratios between 0.8 and 1.3. The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. The filling procedure and filling setup have been significantly improved compared to previous work done in the same apparatus. Schlieren measurements coupled with a highspeed camera are used for image acquisition to track the expanding flame front. A post processing tool is used to extrapolate the measurements to zero stretch. Tested fuel components were DME, nheptane and isooctane. Nheptane and isooctane results are new data with minimised standard deviations achieved by an optimised filling procedure and therefore presented in this study. Additionally, the experimental results obtained are compared to numerical simulations. Hightemperature chemical kinetic models have been used for calculating laminar burning velocities of the considered fuel components. Finally, the experimental and numerical results are discussed in detail in conjunction with references found in literature.

Wada, T., Paczko, G. & Peters, N., Numerical Investigation of Highly Diluted Lean CH4/Air Oscillations at Low Temperatures. In 4th European Combustion Meeting, 14th  17th April, Vienna, Austria. 2009.

Röhl, O., Jerzembeck, S., Beeckmann, J. & Peters, N., Numerical Investigation of Laminar Burning Velocities of High Octane Fuel Blends Containing Ethanol. SAE Technical Paper Series, 2009010935. 2009.
Recently, fuels containing ethanol have become more and more important for spark ignition engines. Fuels with up to 10 vol.% ethanol can be used in most spark ignition engines without technical modification. These fuels have been introduced in many countries already. Alternatively, for fuels with higher amounts of ethanol so called flex fuel vehicles (FFV) exist.<br />
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One of the most important quantities characterizing a fuel is the laminar burning velocity. To account for the new fuels with respect to engine design, reliable data need to be existent. Especially for engine simulations, various combustion models have been introduced which rely on the laminar burning velocity as the physical quantity describing the progress of chemical reactions, diffusion, and heat conduction. However, there is very few data available in the literature for fuels containing ethanol, especially at high pressures.<br />
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A detailed chemical kinetic mechanism for isooctane, nheptane, and ethanol is used to calculate laminar burning velocities under engine relevant conditions. The results are validated against data from literature and new experimental measurements using the closedvessel bomb method. Finally, based on the results obtained, the effect of ethanol blended to standard fuels is analyzed.

Wada, T., Peters, N. & Mellado, J.P., Linear Stability Analysis of OneStep Model with Diluted Lean CH4/Air Oscillations at Low Temperatures. In Clear Air 2009  10th International Conference on Energy for a Clean Environment, 7th  10th July 2009, Lisbon, Portugal. 2009.
Chemically induced oscillations in a diluted lean CH4 /air mixture were studied using an extended onestep model for lowtemperature conditions (<1300 K) in a perfectly stirred reactor. This extended onestep model was used to find analytical solutions. In this study, linear stability analysis was used to evaluate the system. The results showed the following: First, there was a good agreement between the simulation results done with detailed chemical kinetic models and experimental observations. In addition, these results prove that the onestep model is valid both at highand lowtemperature conditions. Second, the oscillations occur only when the steadystate points of the system are linearly unstable. This proves that linear stability analysis is an effective method for studying not only irreversible phenomena such as ignition or extinction but also the dynamic and reversible behavior of a nonlinear system.