Influence of Lewis number on strain rate effects in turbulent premixed flame propagation

The effects of tangential strain rate on the displacement speed of turbulent premixed flames in the thin reaction zones regime are studied for three different Lewis numbers (Le = 0.8, 1.0 and 1.2) using three-dimensional compressible direct numerical simulation (DNS) of statistically planar flames. For non-unity Lewis numbers it is shown that the variations of temperature and scalar gradient in response to tangential strain rate on a given reaction progress variable isosurface have a profound influence on displacement speed behaviour. In the case of Le = 0.8, temperature and tangential strain rate are found to be positively correlated at locations of zero curvature whereas the opposite behaviour is apparent for the case of Le = 1.2. It is demonstrated that the effects of the temperature-curvature and tangential strain rate-curvature correlations are implicitly present in the response of the temperature to local strain rate. The temperature-curvature correlation and strain rate-curvature correlation are found to be in agreement with previous experimental results. Displacement speed and strain rate are found to be weakly correlated in general, but their conditional joint pdf at zero curvature locations shows a negative correlation.