Unveiling the bi-stable character of stealthy hydrogen-air flames

TL;DR

This paper reveals that ultra-lean hydrogen-air flames can stably exist in two distinct shapes and speeds in the same conditions, due to physical mechanisms and ignition dynamics, impacting hydrogen combustion system design.

Contribution

It provides the first experimental evidence of coexisting stable flame structures in horizontal channels and explains their emergence through numerical simulations.

Findings

Two stable flame structures coexist under identical conditions.

Symmetry-breaking during ignition causes different stable configurations.

Implications for hydrogen system safety and control.

Abstract

Ultra-lean hydrogen-air flames propagating in narrow gaps, under the influence of cold walls and high preferential diffusion, can form two distinct isolated structures. They exhibit either circular or double-cell shapes and propagate at different speeds, with the latter roughly doubling in size and traveling speed to the former. Hydrogen mass diffusivity, convective effects and conductive heat losses are the physical mechanisms that explain the alterations in morphology and propagation speed. In previous experiments, Veiga et al. Phys. Rev. Lett. 124, 174501 (2020) found these clearly distinguished flame structures for different combinations of equivalence ratio, channel gap and the effect of gravity on the dynamics of upwards and downwards propagating flames in a vertical chamber. Present observations in horizontal channels show the simultaneous appearance of these two stable structures, which arise under identical experimental conditions and conform the first evidence that multiplicity of stable solutions coexists in real devices. To explain the observations, we performed numerical simulations using the simplified model, which show that symmetry-breaking details during the ignition transient explain the concurrent emergence of the two stable configurations. This discovery urges the need to implement additional engineering tools to account for the possibility of formation and propagation of isolated flame kernels at different speeds in hydrogen-fueled systems.