Numerical investigation of boundary layer flashback of CH₄/H₂/air swirl flames under different thermal boundary conditions in a bluff-body swirl burner

One of the major challenges of applying hydrogen (H₂) enriched fuels in industrial combustion systems is the risk of flashback that can cause considerable damage on combustors and affect pollutant emissions. While flashback has been extensively investigated, comparatively less analysis is made to understand the impact of thermal boundary conditions on flashback of bluff-body swirling H₂ enriched flames. The aim of this work is to fill a part of this gap through large-eddy simulations of premixed 95%H₂/5%CH₄-air flame flashback in a bluff-body swirl burner. The results show that flashback characteristics are very sensitive to the bluff-body thermal boundary condition. At T_wall=350 K, flashback is led by a large-scale swirling flame tongue (Mode I) that can cause the deflection of streamlines ahead of the flame front and is responsible for flashback Mode I, while a small region of negative velocity induced by small-scale flame bulges cannot lead to a net upstream propagation of the flame front. However, when treating the bluff-body as an adiabatic wall, the flashback is led by multiple small-scale flame bulges, and the circumferential motion of the lowest flame tip is negligible (Mode II). These small-scale bulges can cause large reverse flow pockets and formation of a stagnation point ahead of the preheat zone that facilitates flashback, similar to non-swirling channel flashback. This is the main mechanism responsible for flashback Mode II. At T_wall=500 K, flashback is led by different structures, switching between Mode I (upstream propagation of swirling flame tongue) to Mode II (upstream propagation of non-swirling flame bulges). Furthermore, it is found that as the boundary heat loss increases, the axial flashback speed decreases, while the azimuthal flashback speed increases due to the presence of Mode I. The results open up the possibility of extending the flashback limit of H₂ enriched swirling flames by controlling the thermal condition of bluff body.