Structure of a high Karlovitz n-C7H16 premixed turbulent flame

Abstract

Results from a series of direct numerical simulations (DNS) of a high Karlovitz, slightly lean (ϕ=0.9), n-C7H16/air premixed turbulent flame are presented. The flame is statistically flat and is subjected to an inflow of homogeneous isotropic turbulence. A 35-species and 217-reaction mechanism (Bisetti et al., 2012) [17] is used to represent the chemistry. Two simulations have been performed: one with unity Lewis number to asses the effects of turbulence on the flame structure in the absence of differential diffusion, and the other with non-unity Lewis numbers to analyze how turbulence affects differential diffusion. The Karlovitz numbers are 280 and 220 respectively. The first simulation reveals that the flame is strongly affected by turbulence as enhanced mixing largely thickens the preheat zone. However, the turbulent flame structure (i.e. the correlation between species and temperature) is similar to that of a one-dimensional flat flame, suggesting that turbulence has limited effet on the flame in temperature space, in the absence of differential diffusion. In the second simulation, the flame structure is affected by turbulence, as differential diffusion effects are weakened. It is suggested that this result is attributed to the fact that turbulence drives the effective species Lewis numbers towards unity through an increase in effective species and thermal diffusivities. Finally, the reaction zones of both the unity and the non-unity Lewis number turbulent flames remain thin, and are locally broken (only to some extent for the unity Lewis number flame, and more strongly for non-unity).