Extinction of diffusion flames burning diluted methane and diluted propane in diluted air

A theoretical and experimental investigation of the extinction limits of counterflow diffusion flames burning methane and propane is outlined. A diffusion flame is stabilized between counterflowing streams of a fuel diluted with nitrogen and air diluted with nitrogen. Extinction limits for such flames were measured over a wide parametric range. Results for methane and propane were found to be in approximate agreement with previous measurements.The experimental results are interpreted by use of activation energy asymptotic theories developed previously. The gas-phase chemical reaction is approximated as a one step, irreversible process with a large value for the ratio of the activation energy characterizing the chemical reaction to the thermal energy in the flame. Equilibrium dissociation of products is neglected. The theoretical predictions are compared with experimental results, and the overall chemical kinetic rate parameters characterizing the gas-phase oxidation of methane and propane in a diffusion flame are deduced. The overall chemical kinetic rate parameters deduced by use of this procedure are valid only at flame temperatures where equilibrium dissociation is negligible. The scalar dissipation rate at extinction is predicted over a wide range.