Asymptotic Analysis of the Structure of Nonpremixed Methane Air Flames Using Reduced Chemistry

Abstract

‐The asymptotic structure of counterflow, nonpremixed methane–air flames are analyzed using a reduced four step chemical–kinetic mechanism. 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 outer structure of the flame is the classical Burke–Schumann structure governed by the overall one–step reaction CH₄ + 20₂ 1; CO₂ + 2H₂0. The inner structure consists of an inner layer and an oxidation layer. Analysisof theinner layer is similar to that performed previously for premixed, methane–air flames and includes the influence of a number of additional reactions which were neglected in a previous analysis of the structure of diffusion flames. Attempts are made to provide a nearly complete description of the structure of the oxidation layer, by removing a number of assumptions introduced in previous analyses. Solutions are shown in the limit ω 1; ∝ and ω 1; 0, where w is the ratio of the thickness of the fuel consumption layer to the radical nonequilibrium layer. Results are obtained for the quantity X_q which is proportional to the strain rate at extinction, for various levels of dilution of the reactant streams. The values of X_q predicted in the limits ω 1; 0 and ω bracket the value of this quantity calculated from previous detailed numerical integrations. Although, the activation energy characterizing the rate of the elementary reactions is not large. the value of an effective overall activation energy describing the variation of Xf with dilutionof the reactant stream, deduced from this analysis is large and agrees reasonably well with the overall rate parameters deduced from experimental measurements of critical conditions of flame extinction. This observation provides a link between the present analysis and previous large activation energy asymptotic analysis.