Revisiting the relation between premixed flame brush thickness and turbulent burning velocities from Ken Bray’s notes

Abstract

Applying the Bray-Moss-Libby (BML) bi-modal asymptotic limit to a progress variable signal in a premixed turbulent flame, a relation is established between the variation rate of the mean turbulent flame brush thickness, the flame surface wrinkling and the turbulent burning velocity. This scaling suggests that the link between the amplification of the flame surface by turbulence and the increase of the turbulent burning velocity may need to be modulated by a corrective term proportional to the rate of variation of the turbulent flame brush thickness. This correction is expected to be driven by a short relaxation time scale, which is representative of the turbulent flame brush adjusting to its continuously varying turbulence environment. The analysis is grounded on the usual exact expressions for the various characteristic displacements and consumption speeds, widely used in turbulent combustion modeling. Prior to this analysis, the validity of the implication on these characteristic speeds of the thin flamelet framework, introduced to perform the derivation of the relation between the turbulent flame brush dynamics and the turbulent burning velocity, is evaluated from direct numerical simulation (DNS) of a turbulent premixed jet-flame.