Comparison of displacement speed statistics of turbulent premixed flames in the regimes representing combustion in corrugated flamelets and thin reaction zones

The curvature and strain rate dependence of local displacement speed Sd is compared between two three-dimensional simplified chemistry and transport based direct numerical simulation (DNS) datasets of turbulent premixed flames with comparable turbulent Reynolds number. One of these datasets has attributes of the corrugated flamelets regime (Karlovitz number Ka<1), and the combustion situation in the other belongs to the thin reaction zones regime (Ka>1). It has been found that the probability of finding negative displacement speed increases for the flame in the thin reaction zones regime in comparison to the flame in the corrugated flamelets regime. However, the mean displacement speed remains positive throughout the flame brush in both the flames. It is shown that the curved flame contribution in displacement speed St is principally responsible for the negative displacement speed in the flame representing the thin reaction zones regime. The alignment characteristics of scalar gradient with principal strain rates is found to be different in these two flames, which leads to significant differences in tangential strain rate and curvature dependence of the magnitude of the reaction progress variable gradient ∣∇c∣. This difference in ∣∇c∣ statistics leads to marked differences in the statistics of the combined reaction and normal diffusion component of displacement speed SLS=(Sr+Sn), which in turn leads to significant differences in Sd statistics between the corrugated flamelets and the thin reaction zones regimes. This difference has important implications in the context of flame surface density based reaction rate closure.