The role of conductive heat losses on the formation of isolated flame cells in Hele-Shaw chambers

Nonlinear dynamics of flame fronts with large-scale stabilizing effects

“…The distribution is now bimodal, particularly for G = −4.7s −1 , where the persistent patterns are clearly larger than the most probable size. The presence of double-headed persistent patterns has been reported in the literature for thermodiffusively unstable flames [47] and localized flames [48]. Here, we demonstrate that these gravity and heat-loss effects are favourably taken into account in the G parameter for continuous flames, even in the absence of thermodiffusive instability.…”

Unexpected Propagation of Ultra-Lean Hydrogen Flames in Narrow Gaps

“…From the experiments, one can conclude that one-and two-headed flames mainly appear when propagating up and downwards, respectively. However, we found both structures for gravity-free conditions in our simplified numerical model [14], a result that leaves the influence of gravity on the shape of these isolated traveling flames as an open question.…”

“…For methane and DME (Le ≥ 1), the flame does not withstand the heat losses that increase as the gap size h is reduced and extinguishes from a continuous reactive front. This result then identifies mass diffusivity as the mechanism that counteracts heat losses [14] and thermodiffusive instabilities triggered by the high diffusivity of hydrogen become the survival mechanism that enables local flame quenching under nonadiabatic conditions and gives birth to flame cells within which the temperature is high enough to sustain combustion [32]. The instantaneous high concentration gradient across the front triggers the fast diffusion of hydrogen from the unburned region toward the surroundings of the flame, increasing the local availability of H 2 and keeping the gas above the crossover temperature ∼1000 K, the temperature below which the chemical reaction cannot proceed [33][34][35].…”

“…However, premixed flames are inherently unstable. The viscosity and thermal expansion gradient across the flame front, the competition between heat conduction and mass diffusion in the fluid, the effect of gravity, the interaction with acoustic waves [12,13], and the heat losses [14] fold and stretch the flame altering some of its dynamic and morphological properties [15].…”

“…From the experimental results, it is unclear both how the flame cells are formed and why hydrogen flames withstand more adverse conditions than heavier hydrocarbon fuels [12]. To investigate the causes that lead to the new propagation regimes identified experimentally, we modeled the propagation of the H 2 -air flame using an in-house finite-element code [14]. Since the flow is laminar and the flame thickness δ T ∼ ð0.5-1Þ mm is comparable to the gap size h, the propagation problem of the flame can be treated as quasi-two-dimensional, considerably simplifying the simulations.…”