Synergy of turbulence and thermo-diffusive effects on the intermittent boundary-layer flashback of swirling flames

TL;DR

This paper addresses intermittent boundary-layer flashback in hydrogen-enriched swirling flames using large-eddy simulation with the flame-surface-density method. By resolving flow–flame interactions and incorporating thermo-diffusive effects through a displacement-speed model, it reveals two BLF regimes that evolve with hydrogen content: small, outlet-near flame bulges and a deep, rotating flame tongue at higher enrichment. The results show qualitative agreement with experiments for low and moderate hydrogen levels, while underprediction occurs at the highest enrichment, pointing to areas for model improvement such as flame–wall and thermo-acoustic interactions. Overall, the study demonstrates that turbulence–thermo-diffusion synergy critically shapes intermittent BLF and provides a pathway for more accurate predictive tools in H2-enriched turbulent flames.

Abstract

We simulated the intermittent boundary-layer flashback (BLF) of hydrogen-enriched swirling flames using large-eddy simulation (LES) with the flame-surface-density (FSD) method. Three cases of intermittent BLF, characterized by periodic flame entry and exit of the mixing tube, are presented. The intermittent BLF characteristics varied with the hydrogen volume fraction. Small flame bulges entered and exited the mixing tube in low hydrogen-enrichment cases. The duration of intermittent BLF events and BLF depth increased as the hydrogen content increased. Meanwhile, a large flame tongue penetrating deeply upstream characterised the highest hydrogen-enrichment case.The mean BLF peak depths and standard deviations obtained through simulations aligned well with experimental data for low and moderate hydrogen-enrichment cases. However, LES-FSD underestimated the average BLF peak depth for the highest hydrogen-enrichment case.Analysis of the flow-flame interaction revealed two mechanisms underlying the intermittent BLF phenomena. The flame bulges' oscillation near the outlet is caused by the reverse flow induced by the recirculation zone. At the same time, the deep intermittent BLF occurrs due to the boundary layer separation induced by the large turbulent burning velocity, resulting from the synergy of turbulence and thermo-diffusive effects.

Figures (8)

• Figure 1: Schematic of the computational domain for LES-FSD, where the blue arrows denote the swirling inflow direction.
• Figure 2: Radial profiles of averaged velocity $\widetilde{u}_x$ at $x=-20$ mm near the exit of mixing tube obtained from LES with 2 million cells (dotted line) and 4 million cells (solid line).
• Figure 3: Flame surfaces obtained via LES-FSD at different times in case (a) G1 and (b) G3. The red isosurfaces correspond to $\widetilde{c}=$0.81 and 0.77 in cases G1 and G3, respectively. The blue and red arrows indicate the swirling flow and flame propagation directions, respectively.
• Figure 4: Temporal variations of BLF depth in cases (a) G1, (b) G2, and (c) G3, where the green dotted line represents the mean BLF peak depth.
• Figure 5: Flow velocity $\widetilde{u}_x$ (black solid line), local flame displacement speed $\left\langle s_d \right\rangle_A$ (red solid line), and BLF depth (blue dotted line) at the flame base in cases (a) G1, (b) G2, and (c) G3.