Deflagrations of mildly flammable refrigerant-air mixtures in closed volumes

With current concerns around global climate change, new hydrofluorocarbons with low Global Warming Potential (GWP) are being evaluated as alternatives. These alternative refrigerants, however, are mildly flammable and pose safety concerns for the heating, ventilation, air conditioning, and refrigeration industry. Consequently, careful assessment of different flammability characteristics and risks for these refrigerants is essential for their safe use in different applications. In this study, we performed tests and numerical simulations for two refrigerants, difluoromethane (R-32) and 2,3,3,3-tetrafluoropropene (R-1234yf), in a closed 3m container to identify important characteristics related to deflagration propagation in closed volumes. The thermodynamic property data as well as chemical kinetic and transport descriptions for these reaction systems were calibrated based on available experimental flame speed data as well as our detailed chemical mechanism available in the literature. Large Eddy Simulations (LES) were performed to analyze global flame structure and propagation differences in uniformly mixed, stoichiometric R32-air and stoichiometric R-1234yf/air mixtures. For the R-1234yf case, results demonstrate important buoyancy-induced effects whereby initial deflagration "bubbles" form upon ignition from a hot surface point and propagate vertically upwards with relatively larger velocities than in the lateral direction. Unlike the R-32/air case, which produces a fireball propagating almost uniformly in all directions, the R-1234yf/air case initially develops jet-like flame features that propagate vertically upwards, impinges upon the enclosure ceiling and transitions to rapidly consume the entire remaining flammable mixture within the empty enclosure. The stages of flame propagation along with the comparisons with the experimental data are discussed.