Effects of strain rate and curvature on surface density function transport in turbulent premixed flames in the thin reaction zones regime

Abstract

Strain rate and curvature effects on Surface Density Function (SDF) transport in the thin reaction zones regime are studied using a three-dimensional direct numerical simulations (DNS) with a single-step Arrhenius type chemistry. It is shown that if the tangential strain rate on a flame isosurface exceeds a critical value, then a negative normal strain rate is induced, which acts to bring the isoscalar lines closer to each other and hence leads to a higher value of SDF. This is reflected in a positive correlation between SDF and tangential strain rate. Curvature is also found to affect SDF through the correlation between tangential strain rate and curvature on a given flame isosurface. Strain rate and curvature are found to have an appreciable effect on various terms of the SDF transport equation. The SDF straining term is correlated positively with tangential strain rate as expected and is also correlated negatively with the curvature. The combined SDF curvature and propagation terms operate as a source near the fresh gas side of the flame and as a sink towards the burned gas side. The SDF propagation term is found to correlate negatively with flame curvature towards the fresh gas side and positively towards the burned gas side. The variation of the SDF curvature term with local flame curvature is found to be nonlinear due to the additional stretch induced by the tangential diffusion component of the displacement speed.