Stabilization of lean premixed turbulent flames behind a backward-facing step: Numerical analysis of shear stretch-vorticity effects

Abstract

This study explored the stabilization of lean premixed turbulent flames in a backward-facing step combustor, using Large Eddy Simulation with the Artificially Thickened Flame model in OpenFOAM. Parametric variations in initial temperature (T₀ = 300–500 K), Reynolds number (Re = 5000–7000), and equivalence ratio (Φ₀ = 0.575–0.650) elucidated the interplay of hydrodynamic and chemical factors governing flame anchoring. The results demonstrate that the flame leading edge shifts upstream with increasing T₀-Re and Φ₀, a trend corroborated by experimental CH* chemiluminescence data. The x-distance of the flame leading edge was driven by a velocity balance mechanism wherein the turbulent flame speed, scaling with the square of the laminar burning velocity, aligned with the local flow velocity. Departing from the effect of turbulence intensity, this analysis for the flame leading edge points identified negative vorticity, induced by the shear-layer velocity gradient, as a key enhancer in the turbulent flame speed through shear stretch. The averaged characteristic rotating velocity exhibited a near-perfect linear relationship with the square of the laminar burning velocity, underscoring its pivotal role in flame stabilization. Morphological analyses indicated increased flame wrinkling under elevated T₀-Re and reduced Φ₀. Acoustically, the overall sound pressure level aligned closely with experimental trends, indicating that the noise generation is attributed to vortex shedding associated with the turbulence intensity at the flame leading edge points. Validated against a two-dimensional domain using the Paczko chemical mechanism, these findings highlight the dominance of shear-induced vorticity over turbulence intensity in flame stabilization, offering fresh insights into flame-turbulence interactions.