Effects of Soret and differential diffusion on boundary layer flashback of H2/CH4 swirling flames

Eliminating the risk of flashback is one of the most important aspects when designing hydrogen-fueled combustors. In this work, a series of well-resolved large-eddy simulations have been performed for the flashback of premixed H2/CH4 bluff-body swirling flames, to evaluate the roles of Soret and differential diffusion on flame dynamics, overall flashback speed and structure of flame tongue. The thermal condition of the wall determines the flashback mode, while Soret and differential diffusion affect the flame dynamics by generating more flame wrinkles and ramp-cliff-like structures of the flame-tip ring. Analysis based on the 1D counterflow flame suggests that both factors can increase the flame displacement speed, thereby accelerating the overall flashback process. It is shown that the combined effects of these two factors lead to a large variation of local equivalence ratio by +/- 0.1, and the predicted flashback speed is only about 30 % of the experimental measurement if both two factors are ignored. The analysis of flame structure reveals the similarities between turbulent flame tongue and laminar counterflow flame. However, the effect of Soret and differential diffusion on the enhancement of the overall combustion process is found to be more pronounced in turbulent flame tongue than in laminar counterflow flame. This study highlights the importance to account for Soret and differential diffusion when predicting the boundary layer flashback of hydrogen-enriched flames.