Abstract
The asymptotic structures of methane-air flames, for equivalence ratios from 0.5 to 1.4 and pressures from 1 to 70 atm, are analyzed on the basis of a reduced four-step chemical-kinetic mechanism that has previously predicted burning velocities with reasonable accuracy. The rates of these four steps are related to the rates of elementary reactions appearing in the C 1 -chain mechanism for oxidation of methane. In the analysis, the overall flame structure is subdivided into four zones—a preheat zone with thickness of order unity, an inner fuel-consumption layer with thickness of order δ, a H 2-oxidation layer with thickness of order ϵ, and a CO-oxidation layer with thickness of order ν. It is presumed here that δ ⪡ ϵ ⪡ ν < 1, contrary to previous investigations which treated δ ⪡ ν ⪡ ϵ ⪡ 1. The reason for introducing this modification is that recent estimates suggest that ϵ < ν, so that this opposite limit seems worthy of exploration. The inner layer is located between the preheat zone and the oxidation layers, and in this layer finite-rate reactions related to the consumption of the fuel are introduced by appropriate analysis essentially identical to that of Seshadri and Peters. In the H 2-oxidation layer, the variations of the concentrations of CO and O 2 are presumed to be negligible, leading to a new asymptotic analysis, and the H atoms are presumed to be in steady state. In the CO-oxidation layer, H 2 and H are both presumed to be in steady state, again requiring a new analysis. Analytical expressions are obtained for the burning velocity and for the characteristic temperature at which the hydrocarbon chemistry related to the fuel consumption occurs. The predictions are in good agreement with results of full numerical integrations and experiment for fuel-lean flames but give burning velocities somewhat too high for stoichiometric and rich flames.
Published Version
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