Effects of compositional fluctuations on premixed flames

Abstract

Fully compressible direct numerical simulation (DNS) is used to study the behaviour of a stratified turbulent premixed flame in the wrinkled flamelet regime (ReΛ = 38, u′/S L = 0.7, Da = 2.3) with reduced multi-step chemistry. A laminar premixed flame in an initially quiescent domain propagates into a turbulent flow so that the scalar and velocity fields are correctly coupled. Comparison of a stratified scenario (0.4 < φ < 1.2) with an otherwise identical homogeneous case indicates that in this regime the turbulent flame speed is increased by enhanced flame front wrinkling as a result of differential propagation speeds. The preferential propagation into more combustible mixture results in a correlation between the mixture fraction and progress variable, particularly at the leading edge of the flame brush. In front of the flame, the composition gradient preferentially aligns with the most compressive strain. Within the flame front, the progress variable gradient preferentially aligns with the most extensive turbulent strain, and in the latter portion of the flame the alignment between the mixture fraction gradient and the most compressive principal strain deteriorates. Despite this, the convenient approximation of the joint pdf by assuming statistical independence of the mixture fraction and progress variable does not introduce a significant error in the approximation of the reaction rates.