Modeling the boundary-layer flashback of premixed CH₄/H₂/air swirling flames

We model the boundary-layer flashback of CH₄/H₂/air swirling flames via large-eddy simulations with the flame-surface-density model (LES-FSD), in particular, at high pressures. A local displacement speed model depends on the flame stretch, curvature, and heat loss is applied. For the CH₄/air flames at 1 atm, the LES-FSD results agree with those in experiments on the shape and propagating speed of the turbulent flame brush during flashback. For lean CH₄/H₂/air flames at 2.5 bar, the LES-FSD simulations yield lower critical equivalence ratio for flashback with the increase of H₂ volume fraction, consistent with the experiments. This is due to the improved modeling of the flame stretch and heat loss effects on the local displacement speed. We also propose a simple theoretical model to predict the boundary-layer flashback limit of the swirling flames. The model estimates the critical bulk velocity for given reactants and swirl number, via the balance between the flame-induced pressure rise and adverse pressure for boundary-layer separation. We validate the model against 11 datasets of CH₄/H₂/air swirling flame experiments, with the H₂ enrichment in fuel varying from 50% to 85%. Results show that the proposed model well estimates the flashback limits over a wide range of conditions.