Abstract
Direct numerical simulations of turbulent boundary layer flashback (BLF) for a lean premixed hydrogen–air flame in a high-pressure, diffusive-thermally (DT) unstable condition were conducted using mixture-averaged (MA) and unity-Lewis-number (UL) molecular transport models. With identical friction Reynolds and Damköhler numbers, and near-identical velocity and length scale ratios, flashback occurs only in the MA case, while the UL flame undergoes continuous extinction and is convected downstream, suggesting that differential diffusion strongly enhances flashback. Further analysis focuses on the MA case. The most upstream leading flame bulges exhibit remarkably consistent features, with high curvature and strongly enhanced flame speed. Their spanwise spacing is consistent with the spacing of low-velocity streaks in the boundary layer, suggesting a dominant role of the underlying turbulent structures in determining flame bulge structure. Flame speeds show significant enhancement in the region from 5 to 25 viscous units from the wall, with a peak turbulent flame speed 17.5 times the laminar flame speed, while quenching occurs closer to the wall. Enhanced displacement speed and enhanced surface area production by positively curved flame elements are observed in the same 5 to 25 viscous units region. Local fuel enrichment, evidenced by elevated equivalence ratios, is also observed near the wall, consistent with enhanced flame–turbulence interaction. Notably, the highest local equivalence ratios are associated with negative local displacement speeds, resulting from low-probability extinction events in which progress-variable isosurfaces retreat under high strain rates into previously enriched, partially burned regions. Significant super-adiabatic temperatures and wall heat fluxes exceeding six times the peak value of a corresponding one-dimensional head-on quenching configuration are observed, consistent with previous studies of DT-unstable flame–wall interactions. Overall, the results demonstrate strong enhancement of DT instability in the near-wall region where leading edges form, showing that it is a leading-order effect in flashback under strongly DT-unstable conditions.
Novelty and significance statement: We present the first direct numerical simulation (DNS) of turbulent boundary layer flashback (BLF) of lean premixed hydrogen–air flame in a diffusive-thermally (DT) unstable condition. The chosen high-pressure thermochemical conditions are characteristic of realistic heavy-duty hydrogen-fuelled gas turbines. We report several novel findings for flame–turbulence interactions in the vicinity of leading flame bulges near to the wall, including enhanced fuel enrichment, burning velocity, and surface-area production by DT effects, and connect the spanwise structure of bulges to low streamwise velocity streaks. We also report the occurrence of local extinction at retreating flame bulges, and markedly elevated wall heat flux at the leading flashback fronts. This study is significant as it provides new insights into how DT instability influences flashback, thereby advancing the understanding of hydrogen BLF and suggesting modelling implications for hydrogen flames undergoing flashback, such as the necessity of considering differential diffusion.