Direct numerical simulations are performed to investigate the transient upstream flame propagation (flashback) through homogeneous and fuel-stratified hydrogen-air mixtures transported in fully developed turbulent channel flows. Results indicate that, for both cases, the flame maintains steady propagation against the bulk flow direction, and the global flame shape and the local flame characteristics are both affected by the occurrence of fuel stratification. Globally, the mean flame shape undergoes an abrupt change when the approaching reactants transition from an homogeneous to a stratified mixing configuration. A V-shaped flame surface, whose leading-edge is located in the near-wall region, characterizes the nonstratified, homogeneous mixture case, while a U-shaped flame surface, whose leading edge propagates upstream at the channel centerline, distinguishes the case with fuel stratification (fuel-lean in the near-wall region and fuel-rich away from the wall). The characteristic thickness, wrinkling, and displacement speed of the turbulent flame brush are subject to considerable changes across the channel due to the dependence of the turbulence and mixture properties on the distance from the channel walls. More specifically, the flame transitions from a moderately wrinkled, thin-flamelet combustion regime in the homogeneous mixture case to a strongly wrinkled flame brush more representative of a thickened-flame combustion regime in the near-wall region of the fuel-stratified case. The combustion regime may be related to the Karlovitz number, and it is shown that a nominal channel-flow Karlovitz number, ${\text{Ka}}_{\text{in}}^{\text{ch}}$, based on the wall-normal variation of canonical turbulence ($t_{\eta }={(\nu /\epsilon )}^{1/2}$) and chemistry ($t_l={\delta }_l/S_l$) timescales in fully developed channel flow, compares well with an effective Karlovitz number, ${\text{Ka}}_{\text{fl}}^{\text{ch}}$, extracted from the present DNS datasets using conditionally sampled values of $t_{\eta }$ and $t_l$ in the immediate vicinity of the flame ($0.1<C<0.3$).

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• Combustion
• Compressible boundary layers
• Compressible flows
• Reacting flows
• Structure & turbulence of boundary layers
• Turbulence
• Complex fluids
• First-principles calculations