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The asymptotic structure of stoichiometric methaneair flames

1987; Elsevier BV; Volume: 68; Issue: 2 Linguagem: Inglês

10.1016/0010-2180(87)90057-5

1556-2921

Nils Peters, Forman A. Williams,

Atmospheric chemistry and aerosols

A C1-chain mechanism for methane flames is systematically reduced through steady-state and partial-equilibrium assumptions to the three-step mechanism I CH4+O2→CO+H2+H2O, II CO+H2O⇄CO2+H2, III O2+2H2→2H2O, with rates that still contain the kinetic information of the elementary mechanism. A basic asymptotic structure for premixed stoichiometric methane flames is derived from this mechanism for pressures and temperatures sufficiently large that the Damköhler numbers of the second and third reactions are expected to be large. It turns out that the first reaction occurs in a thin fuel-consumption layer embedded between a chemically inert upstream layer and a broader (but still asymptotically thin) downstream layer where H2 and CO are oxidized. At the leading edge of the oxidation layer there is a nonequilibrium layer of the second reaction which tends to equilibrium downstream. The fuel-consumption layer is thin because the ratio of the rate coefficients of the reactions O2+H→klOH+O, CH4+H→kllCH3+H2 (which describe the competition of oxygen and fuel for the H atom) is small. The small parameter δ = k1[O2]o(k11[CH4]u) scales the fuel concentration within this layer; the subscripts o and u on the concentrations identify conditions at the layer and in the unburnt mixture, respectively. Since reaction 11 is fast, the fuel depletes the radicals, causing the upstream layer to be radical-free and therefore chemically inert. By using the basic structure to provide the orders of magnitude of concentrations of all intermediates, the influences of additional reactions are estimated and added to the kinetic scheme. Numerical calculations of flame velocities at various pressures and preheat temperatures are performed, and the range of validity of the assumptions is discussed. The asymptotic structure identified here relies entirely on competition between rate coefficients; the relevant activation energies are not large. Nevertheless, effective overall activation energies can be identified from the results. These activation energies are calculated and employed in exploring the relationship between the present asymptotic structure and activation-energy asymptotics.