New pathways to stability and instability in non-premixed Rayleigh-Taylor flames

We report on detailed numerical simulations of a non-premixed, turbulent Rayleigh-Taylor (RT) flame, where multimode perturbations were imposed on the initial interface. In a previous article, the stability of this configuration to single-mode perturbations was investigated, which we now extend to multimode disturbances. The sharp initial interface separating the fuel and oxidizer evolved under the influence of the RT instability, while the instability-driven mixing in turn enhanced burning at the flame site. Simulations at two different Atwood numbers were performed. At an Atwood number greater than 0.5, the growth of the two layer RT flame was enhanced by bubble expansion and the additional buoyancy due to combustion. At lower Atwood numbers, burning within the flame region resulted in the formation of a third layer with a lower density than either the air or fuel streams. The behavior of the resulting three-layer RT problem depended on the direction of the applied acceleration. When the applied acceleration was such that the initial configuration was globally unstable, burning within the reaction zone resulted in the formation of an active third layer which caused the fuel-flame interface to be stable near the spikes. If the initial configuration was globally stable, burning and formation of the reaction zone resulted in a fuel-flame surface that was RT-unstable. The problem configuration and our findings are relevant to ultra compact combustors. In addition, the novel flow configuration investigated here can potentially provide a fundamentally new framework for understanding the properties of several non-premixed flames.