Fundamental mechanisms in premixed turbulent flame propagation via vortex-flame interactions part II: Numerical simulation

Fundamental phenomena occurring in turbulent premixed combustion are invest igated via direct numerical simulations (DNS) of two-dimensional vortex-pre mixed flame interactions. Different strengths Of vortices and fuels are con sidered in order to analyze the separate effects of strain, preferential di ffusion, unsteadiness, and radiative heat losses. One- and two-step chemica l models are utilized to study the effect of multistep chemistry, involving an intermediate species, in the flame-vortex interaction. The two-step mec hanism consists of a first-order chain branching reaction between reactant A and radical X, A + X --> 2X, and a second-order termination reaction wher e radicals recombine to form product P, via X + X --> P. Two lean premixed flames (propane- and methane-air) leading to two different Lewis numbers (r espectively 1.7 and 0.95) and two Damkohler numbers are investigated. It ap pears that the Lewis number is a first-order parameter controlling the inte raction. For the propane-air flame (Le = 1.7), multistep chemistry effects are negligible and a one-step chemical model is sufficient to well describe the interaction. Unsteady effects are pronounced, especially for the propa ne-air flame, even for moderate Damkohler numbers. This observation can hav e a significant impact on the validity of flamelet libraries used in turbul ent combustion models based on the flamelet concept. From the present simul ations, a transition criteria which separates flamelet and non-flamelet reg imes is proposed for propane- and methane-air flames. Radiative heat losses do not play a significant role during the interaction and can be neglected . Most of the conclusions derived from simulations are supported and confir med by the experimental data obtained in the first part of this study. (C) 1999 by The Combustion Institute.