Dynamics of slowly propagating flames: Role of the Rayleigh-Taylor instability

Abstract

Environmental concerns have motivated the development of alternate fuels and refrigerant working fluids that have low global warming potential. Ammonia (NH3) is a candidate zero-carbon fuel and hydrofluorocarbons (HFCs) such as R-32 and R-1234yf are being adopted as refrigerants. Upon mixing with air, these compounds can sustain flames that are slowly propagating i.e., with laminar flame speeds less than 10 cm/s. These flames are referred to as slow as they are affected by buoyancy-induced flow and radiation heat loss, in contrast to typical hydrocarbon-fueled flames. In this study, we investigate the flame dynamics of slowly propagating refrigerant/air flames, with a focus on the effect of the Rayleigh-Taylor (RT) instability. Dispersion relations, that characterize the range of unstable wavelengths and their growth rates during early stages of instability growth, are derived from two-dimensional direct numerical simulations (DNS, including detailed chemistry and transport) of RT-unstable R-32/air flames. These results are compared to predictions from reduced-order theoretical models. DNS are also performed to study the long-term evolution of such flames and to quantify flame speed enhancements. Results showed that RT-unstable flames can propagate up to 6–7 times the laminar flame speed. Although slow from a flamelet point of view, these flames propagate much faster at larger scales. These results have implications on developing metrics to assess explosion risk, and minimize it while storing, transporting, and utilizing these compounds. (225 words)