ASYMPTOTIC ANALYSIS OF THE STRUCTURE OF MODERATELY RICH METHANE-AIR FLAMES

The asymptotic structure of laminar, moderately rich, premixed methane flames is analyzed using a reduced chemical-kinetic mechanism compris ing four global reactions. This reduced mechanism is different from th ose employed in previous asymptotic analyses of stoichiometric and lea n flames, because a steady-state approximation is not introduced for C H3. The aim of the present analysis is to develop an asymptotic model for rich flames, which can predict the rapid decrease of the burning v elocity with increasing equivalence ratio phi. In the analysis, the fl ame structure is presumed to consist of three zones: a preheat zone wi th a normalized thickness of the order of unity, a thin reaction zone, and a postflame zone. The preheat zone is presumed to be chemically i nert, and in the postflame zone the products are in chemical equilibri um and the temperature is equal to the adiabatic flame temperature T-b . In the reaction zone the chemical reactions are presumed to take pla ce in two layers: the inner layer and the oxidation layer. The rate co nstants of these reactions are evaluated at T-o, which is the characte ristic temperature at the inner layer. In the inner layer the dominant reactions taking place are those between the fuel and radicals, and b etween CH3 and the radicals. An important difference between the struc ture of the inner layer of rich flames and that of lean flames analyze d previously is the enhanced influence of the chain-breaking reaction CH3 + H + (M) --> CH4 + (M) in rich flames. Here M represents any thir d body. This reaction decreases the concentration of H radicals, which in turn decreases the values of the burning velocity. In the oxidatio n layer of rich flames, the reactive-diffusive balance of O-2 is consi dered. This differs from the structure of the oxidation layer of lean flames where the reactive-diffusive balance of H-2 and CO was of prima ry interest. The burning velocities calculated using the results of th e asymptotic analysis agree reasonably well with the burning velocitie s calculated numerically using chemical-kinetic mechanisms made up of elementary reactions. The values of the characteristic temperature at the inner layer T-o are found to increase with increasing values of th e equivalence ratio and to approach T-b at phi = 1.36. When T-o is ver y close to T-b, the asymptotic analysis developed here is no longer va lid and an alternative asymptotic analysis must be developed for even larger equivalence ratios. (C) 1998 by The Combustion Institute.