Background

Withing the Cluster of Excellence - The Fuel Science Center, significant efforts have been made to develop e-fuels from renewable electricity and carbon sources for enabling highly efficient and advanced propulsion systems. Compared to conventional fuels, such fuels can have very different thermo-physical properties depending on their molecular structure. Particularly, fuels with high vapor pressures are highly susceptible to flash boiling depending on boundary conditions, which can significantly alter the spray formation and mixing behavior. Thus, it becomes imperative to develop a fundamental understanding of the underlying physics associated with the flash boiling of these fuels in a single droplet configuration to accurately quantify its effect on the macroscopic spray structure. In this work, short-chain oxymethylene ethers (OMEx) are chosen as a generic example to study the flashing behavior of newly developed e-fuels.

Simulation of single droplet flash boiling

A two-way coupled Eulerian–Lagrangian framework is used in this study. The compressible Navier–Stokes equations are solved using the in-house code CIAO, which is a structured, high-order, finite-difference code. Staggering of the flow variables is performed spatially in order to improve the discretization accuracy for a given stencil size. It uses a low-storage five-stage, explicit Runge–Kutta scheme for the time advancement.

The flash boiling of OMEx single droplets with a diameter of 200 micrometer is investigated at low ambient pressures with varying superheating degrees. A square box of size 0.54 m × 0.54 m × 0.54 m with no-slip adiabatic walls with a temperature of 323 K is considered as the computational domain. The stationary single droplet is placed at the center of the box. Nitrogen is used as ambient gas, and the chemistry of the gas phase is neglected. To initiate the bubble growth process, the critical radius of the bubbles is perturbed by 0.0001% over a time interval of 1 ns.

Unlike in previous studies that characterized the dynamics of bubble growth into four phases for high enough superheating degrees, it is here observed that only three growth phases are present: (1) surface tension-controlled phase (ST), (2) transition phase (T), and (3) inertia-controlled phase (IC). The thermal diffusioncontrolled (TD) growth phase is not observed here, since the droplets explode near the end of the inertia-controlled phase.

Influence of molecular structure on bubble dynamics

It is found that decreasing the number of -CH2O- groups in the OME-fuel significantly increases the bubble growth rate because of the stronger thermal feedback from the surrounding superheated liquid. OMEx with increasing chain length is found to have a higher hydrodynamic pressure. Additionally, the DME droplet is found to burst faster compared with the other molecules. Bubble radius evolutions of OMEx fuels are also shown in non-dimensional form, which demonstrates that a generalized curve can be used to predict the characteristics of bubble growth for OMEx with lower number of -CH2O- groups such as x = 0 and 1. As the number of -CH2O- groups is increased to four, the non-dimensional growth characteristics of the vapor bubbles show a strong deviation, thus illustrating the requirement of further investigation of the non-dimensional parameters.