Broken reaction zone and differential diffusion effects in high Karlovitz n-C_7H_(16) premixed turbulent flames
A series of direct numerical simulations of a high Karlovitz number, n-C_7H_(16), 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.
© 2015 The Combustion Institute. Published by Elsevier Inc. Received 13 December 2014, Revised 30 December 2014, Accepted 31 December 2014, Available online 22 January 2015. In Press, Corrected Proof. The authors gratefully acknowledge funding from the Air Force Office of Scientific Research (Award FA9550-12-1-0144) under the supervision of Dr. Chiping Li, and from the Natural Sciences and Engineering Research Council of Canada (NSERC Postgraduate Scholarship D).