Role of Molecular Diffusion in Turbulent Flames: Two Examples.

In modeling turbulent flames, especially for high turbulent Reynolds number, it is often believed that diffusion is primarily controlled by turbulent diffusivity; and as such the effects of molecular diffusion can be ignored by artificially assuming unity Lewis number (Le) defined as the ratio of thermal diffusivity to mass diffusivity. Based on our recent experiments with expanding turbulent flames, we will present two examples where \textit{Le} significantly alters the flame dynamics even in strong turbulent environments. In the first example, we show that ignition of a combustible mixture by a high-energy kernel can be facilitated by turbulence for \textit{Le\textgreater 1} mixture while it is commonly believed to be more difficult in turbulence due to the increased dissipation rate of the deposited energy. In the second example, we show that nominally nonflammable mixtures with low adiabatic flame temperatures can burn strongly in turbulence for \textit{Le\textless 1} mixtures. In both cases, turbulence morphs the positively stretched spherical flame into a multitude of wrinkled flamelets subjected to both positive and negative stretches. Mechanistically, these effects are consequences of the coupling between differential molecular diffusion and these positively or negatively stretched flamelets.