The role of low/high- temperature chemistry in computationally reproducing flame stabilization modes of hydrogen-fueled supersonic combustion

Numerical simulations of two typical flame stabilization modes in a cavity-assisted supersonic combustor were performed using improved delay detached eddy simulation and three hydrogen oxidation mechanisms with different levels of fidelity. The simulation results with Burke's detailed mechanism agree well with the experimental measurements in terms of flame morphology and wall pressure, in both jet-wake and cavity flame modes. The comparative study shows that, lacking necessary intermediate species, Eklund's reduced mechanism and Marinov's global mechanism incorrectly yield jet wake stabilization mode under low inflow stagnation temperature T₀. Through computational singular perturbation analysis, a sequential radical triggering mechanism was identified for flame stabilization, wherein the reaction R1: H + O₂ = O + OH dominates in fuel jet wake forming OH and O radicals, the reaction R2: H₂ + O = H + OH controls the reaction between H₂ and O forming the OH radical pool, and then the heat release completes via R3: H₂ + OH = H + H₂O. However, their activation differs in the two stabilization modes. The role of transport is key in the cavity flame mode, where the colder stream inhibits auto-ignition in the jet wake, activating low-temperature chemistry, and delaying R2 in the cavity region. Thus, the presence of H₂O₂ and HO₂ species was found to be essential for accurately reproducing the flame stabilization in the cavity flame stabilization mode, whereas their effect is marginal in jet wake mode. In fact, the jet-wake flame stabilization is characterized by auto-ignition under high inflow stagnation temperatures, with the chain-branching reaction R2 activating in the fuel jet-wake, causing an explosive dynamic therein. These findings suggest the H₂O₂ and HO₂ species and associated low-temperature reactions are necessary for the accurate prediction of the flame stabilization mode under low T₀, whereas their absence does not affect the prediction of the flame mode under high T₀, in which case all three chemical mechanisms give reasonably good agreements in flame characteristics and engine overall performances. © 2024 The Combustion Institute.