The inner structure of methaneair flames

The inner structure of a methaneair premixed flame is analyzed using a reduced four-step chemical kinetic mechanism ICO + H2O ⇌ CO2 + H2 IICH4 + 2H + H2O → CO + 4H2 IIIH + H + M → H2 + M IVO2 + 3H2 ⇌ 2H2O + 2H The rates of these four steps are related to the rates of elementary reactions appearing in the C1-chain mechanism for oxidation of methane. The inner layer is thin with reactions I–IV occurring in this layer, and is embedded between a chemically inert upstream layer and a broader (but asymptotically thin) downstream layer where reactions II, III, and IV occur and H2 and CO are oxidized. The analysis reported here extends a previous analysis by Peters and Williams of the structure of premixed methaneair flames, where a reduced three-step chemical kinetic mechanism was used. In the equations describing the structure of the inner layer, a parameter ω appears, which represents the ratio of the thickness of the fuel consumption layer to the thickness of the radical consumption layer of the previous analysis by Peters and Williams. Analytical solutions for the burning velocity eigenvalue L are obtained in the limit ω → 0 and ω → ∞, and by use of numerical integration, an approximation for L is obtained as a function of ω, which includes limiting expressions for ω → 0 and ω → ∞. The expression for L contains a number of parameters, which represent the influence of a number of elementary chemical reactions. In particular, a parameter defined as μ in this analysis is found to have a significant influence on the value of L, and consequently on the burning velocity, and the influence of this quantity increases with increasing pressure. The parameter μ represents the influence of the backward steps of the reactions CH4 + H ⇌ CH3 + H2 and CH4 + OH ⇌ CH3 + H2O. Using the results of the analysis, the burning velocity was calculated for a stoichiometric methaneair flame for values of the pressure p between 1 atm and 80 atm. At p = 1 atm, the calculated burning velocity was 35 cm/s in reasonable agreement with experimental results. The burning velocity decreased with increasing pressure, again in agreement with experimental measurements.