Combined effects of heat losses and curvature in hydrogen–air flame propagation below the lean flammability limit

Abstract

Recent experiments have shown that, below the flammability limit, ultra-lean hydrogen flame fronts can fragment into isolated flame cells when propagating within narrow channels. To further elucidate the underlying mechanisms of this behavior, three-dimensional (3D) numerical simulations are performed, systematically varying the heat losses through the confining walls. Under adiabatic conditions, a continuous flame front embedded with small cellular structures is sustained. When heat losses are introduced, this front fragments, forming isolated flame cells. The characteristic size of these cells is found to be comparable to the dominant wavelength of the corrugated front obtained under adiabatic conditions. The results indicate that heat losses have a pronounced impact on the flame curvature between the wall plates, which transitions from a highly asymmetric configuration to a nearly circular shape as the heat losses increase, while the curvature in the mid-channel plane is barely affected. The size and propagation velocity of the isolated flame cells exhibit a moderate dependence on the magnitude of the heat losses. Sufficiently large heat losses induce a bifurcation of the isolated fronts from a circular to a double-cell flame shape

Novelty and significance statement

This work presents fully three-dimensional numerical simulations to investigate the mechanisms governing the fragmentation of ultra-lean hydrogen–air flame fronts in narrow channels under the influence of varying wall heat losses. The main novelty is the quantification of the heat-loss effect on flame curvature, both across and along the confining plates. A direct correspondence is established between the characteristic size of the isolated cells and the dominant wavelength of flame corrugation under adiabatic conditions, revealing how increasing heat losses promote the transition from a non-symmetric flame to stable, symmetric isolated flame cells below the lean flammability limit of the adiabatic planar deflagration. This study explains that heat losses, typically associated with quenching, can instead stabilize discrete flame cells by reshaping the flame’s three-dimensional curvature and propagation dynamics. These findings clarify the mechanisms behind stable ultra-lean hydrogen combustion, with direct implications for hydrogen safety and the design of efficient, low-emission micro-combustion systems.