Lewis number effects on premixed flame dynamics in spatially periodic shear flows

This study explores the influence of the Lewis number Le on the dynamics of laminar premixed flames subjected to spatially periodic shear flow. The periodic shear flow serves as a simplified model to study flame–flow interactions, resembling the oncoming elongated streamwise velocity streaks with alternating low and high velocities observed in near-wall boundary layer flashback. Both one-step Arrhenius kinetics and detailed chemistry for lean hydrogen–air flames are employed to investigate the influence of Le , shear amplitude A , gas expansion ratio Θ and domain width D on flame propagation. The results demonstrate that increasing Le leads to a monotonic decrease in flame velocity, indicating the suppression of thermo-diffusive (TD) instability. Shear amplitude A and gas expansion ratio Θ are found to enhance flame speeds across all Le , consistent with the behavior of flames governed primarily by Darrieus–Landau (DL) instability. For flames with non-unity Le , a two-stage propagation regime is generally observed, comprising an initial metastable stage and a subsequent stable stage. However, persistent oscillations in flame speed are observed at low A , low Le , high Θ, or large domain widths D , suggesting potential thresholds where intrinsic instabilities dominate and interact strongly with imposed shear. These findings highlight the complex interplay between TD and DL instabilities, whose competition or synergy governs flame structure and stability. The study further suggests that non-monotonic flame behavior may occur in wider domain.