Pathway dynamics to double-cell premixed flames in lean hydrogen-air mixtures

The propagation of premixed flames over lean hydrogen-air mixtures have been found to be bistable in recent studies, considering slender channels with non-negligible conductive heat losses. In particular, two stable configurations conformed by either a circular or a double-cell flame front arise for the same combination of controlling parameters (fuel mixture, channel size and thermal conductivity). Nevertheless, this multiplicity of solutions lacks a satisfactory explanation to predict the formation of either one structure or the other during a particular ignition transient. In this work, detailed analyses are performed over the unsteady evolution of a set of numerical simulations. Specifically, the initial temperature profiles (distribution and peak value) and subsequent expansion of the flow field prescribe the early growth of the flame front leading to different curvatures and sizes of the kernel that control the evolution into each of the canonical structures. This study explores the underlying physics controlling the final-stage bistable behavior. In particular, the local convective effects and interactions between fronts have been identified as the key causes to produce each of the two stably propagating flames. Frequently found division of kernels cannot ensure the formation of double cells, which require additional reorientation dynamics and merging events due to asymmetric seeding of flame fragments or to interaction with neighboring structures.