The interplay of transport, kinetics, and thermal interactions in the stability of premixed hydrogen/air flames near surfaces

The stability of premixed hydrogen/air flames near surfaces is studied using numerical bifurcation theory and detailed kinetics. Two-parameter continuation diagrams are constructed to determine the effects of inlet composition and pressure on flame stability. Sensitivity analysis is used to elucidate the roles of transport mechanisms, chemical heat, and chain-branching in ignition and extinction. We have found that flame instabilities can be caused by chemical autocatalysis (kinetic runaway) or thermal autocatalysis (thermal runaway) when three multiple solutions coexist, and by both mechanisms when five multiple solutions coexist. Low pressures favor the kinetic feedback mechanism and high pressures the thermal one. Loss of H atoms out of the reaction zone by diffusion or by heterogeneous reactions can strongly affect flame stability at low pressures, and diffusion and convection of H2O2 affects flame stability at high pressures. Energy conduction and convection can influence ignition and extinction, especially at low pressures. At high pressures flame instabilities are mainly driven by reaction heat generation and are strongly affected by transport effects and the enhanced third body efficiency of H2O in the reaction H + O2 + M → HO2 + M.