Broken reaction zone and differential diffusion effects in high Karlovitz n-C7H16 premixed turbulent flames

Abstract

A series of direct numerical simulations of a high Karlovitz number, n-C7H16, turbulent premixed flames performed previously in Savard et al. (2014) are further analyzed in this paper. Two flames are considered: one with unity Lewis numbers to isolate the effect of turbulence on the flame, and one with non-unity Lewis numbers to study the influence of turbulence on differential diffusion. In this paper, the focus is put on the reaction zone and how it is affected by turbulence. First, the reaction zone is shown to be thin for both flames, yet large chemical source term fluctuations are observed. In particular, for the non-unity Lewis number flame, while being thin, the reaction zone is also broken. Second, differential diffusion is shown to have limited effect on the distributions of strain rate and curvature at the reaction zone. Due to the high level of turbulence, the flame behaves more like a material (i.e. passive) surface than a propagating surface. Third, the fuel consumption rate is found to be correlated (yet weakly) with strain rate in the unity Lewis number flame, whereas a stronger correlation with curvature is found in the non-unity Lewis number flame. All these results explain the apparent turbulent flame speeds. It is found that the contribution of the fluctuations in the fuel consumption rate averages away in the unity Lewis number flame. On the other hand, for the non-unity Lewis number flame, the non-linear correlation between fuel consumption rate and curvature has a strong impact on the turbulent flame speed.