Displacement speed, flame surface density, and burning rate in highly turbulent premixed flames characterized by low Lewis numbers

Journal article, 2023

Direct Numerical Simulation data obtained earlier from four pairs of moderately or highly turbulent, lean hydrogen-air, complex-chemistry flames are analyzed to explore influence of molecular diffusion on differently averaged Flame Surface Densities (FSD), displacement speeds Sd, and various terms in the FSD transport equation. For this purpose, each pair involves (i) a flame where mixture averaged molecular diffusivities are adopted and Lewis number Le is significantly less than unity and (ii) an equidiffusive flame where all molecular diffusivities are set equal to molecular heat diffusivity of the mixture and Le=1, with other things being equal in the two flames. Reported results show, in particular, that significantly higher turbulent burning rates simulated in the former flames result mainly from an increase in the local fuel consumption rate, whereas an increase in flame surface area plays a secondary role, especially in more intense turbulence. The rate increase stems from (i) an increase in the peak local fuel consumption rate and (ii) an increase in a width of a zone where the rate is significant. The latter phenomenon is of more importance in richer flames and both phenomena are most pronounced in the vicinity of the flame leading edges, thus, further supporting a crucial role played by the leading edge of a premixed turbulent flame in its propagation. Moreover, mean displacement speed differs significantly from the laminar flame speed even in the equidiffusive flames, vary substantially across flame brush, and may be negative at the leading edges of highly turbulent flames.