Stratification effects on jet-ignited methanol/air mixing layers

Understanding the interaction between hot jets and stratified methanol/air mixtures is important for interpreting jet-assisted ignition in gas-phase stratified mixtures formed by methanol spray evaporation. This study uses two-dimensional direct numerical simulations (2D-DNS) to investigate ignition and flame structure in a turbulent methanol/air stratified mixing layer. A baseline lean–rich–lean case is compared with homogeneous premixed flames, while the effects of jet velocity and jet reaction progress are examined. The results indicate that, relative to the corresponding homogeneous premixed flames, the conditioned displacement speed distributions of the stratified mixing layer shift toward lower values in the near-stoichiometric region and toward higher values in the lean region. These opposing shifts are consistent with a possible back-support effect under the present stratified configuration. The negative correlation between displacement speed and flame curvature is consistent with an effective Lewis number greater than unity and a thermo-diffusively stable flame response. Chemical explosive mode analysis (CEMA) shows that stronger stratification increases the occurrence of local extinction. Meanwhile, at low equivalence ratios, the CEMA-defined auto-ignition mode is nearly absent from the reaction front, where the local combustion state is dominated by diffusion-assisted ignition and post-ignition modes. Increasing the jet velocity promotes the mixing process in the lean zone, resulting in a higher reaction-rate contribution to the displacement speed, Sd,r. Finally, as the jet reaction progress increases, the ignition behavior evolves from shear-disrupted ignition failure, through a delayed intermediate state, to sustained diffusion-assisted flame propagation.