Flammability Metrics of Novel Low-Global Warming Potential Refrigerants

Because of their high global warming potential (GWP), hydrofluorocarbon refrigerants (HFCs) are now systematically phased out. Replacements with low GWP exist, but give rise to safety hazards as they are mildly flammable. The assessment of the safety hazards of such fluids is typically based on their laminar flame speeds, which are naturally below 10 cm/s. Flames propagating at this speed are sensitive to several effects, such as radiation-induced heat losses and gravity-induced buoyancy. Hence, refrigerant flames are challenging to measure. Methods to assess and reduce uncertainties caused by the slow-motion of refrigerant flames have to be developed to increase the validity of experimental data sets.

The present project is conducted as a collaborative study of the ITV and the National Institute for Standards and Technology (NIST), U.S.A. Flame speed experiments are performed in a spherical combustion vessel by recording Schlieren films of the outwardly propagating flame kernel. For safety reasons, an exhaust gas cleaning system has to be utilized to neutralize hydrogen fluoride, which is a primary product of hydrofluorocarbon combustion.

Multiple refrigerant flames, such as those of difluoromethane (CH2F2, R-32) and 2,3,3,3-Tetrafluoropropene (C3H2F4, R-1234yf) are measured and analyzed regarding their capabilities as targets in kinetic modeling studies. For instance, detailed investigations of the influence of radiation heat loss revealed that laminar flame speeds of difluoromethane with air could be underestimated by 20 %. Using such data as targets for kinetic mechanism validation is unsuitable. On one side, new data reduction methods need to be implemented to reduce experimental uncertainties. On the other side, measurements have to be conducted under conditions where those effects are negligible, e.g., under microgravity to suppress buoyant flame motion.