Rayleigh-Taylor Unstable Flames: A Transition to Distributed Burning?

Distributed burning is an extreme regime of turbulent, premixed combustion in which the flame's reaction zone is thickened when it burns through turbulent reactants. But what happens when the reactants are laminar and the turbulence is generated by an instability of the flame front itself? Will the flame still thicken? In this presentation, we will use a DNS parameter study of Boussinesq model flames to show that Rayleigh-Taylor unstable flames do undergo a thickening transition, but that its character is different from the transition to distributed burning. In particular, thickening starts on the products side of the flame instead of in the preheat region. Lowering the Prandtl number expands the thickened region towards the reactants side of the flame, but the front of the flame remains substantially thinner than laminar. We explore the behavior of these unusual 'thick and thin' transitional flames using detailed measurements of their internal structure.