Rate-ratio asymptotic analysis of inhibition of nonpremixed methane-air flames by CF 3Br

Abstract

Rate-ratio asymptotic analysis is performed using a reduced four-step chemical-kinetic mechanism to elucidate the influence of CF 3Br on the structure of nonpremixed methane-air flames. The inhibitor CF 3Br is added to the oxidizer stream of the nonpremixed flame. The primary focus of the analysis is to obtain the critical conditions of extinction. The asymptotic flame structure is constructed using the results of numerical calculations performed using a detailed chemical-kinetic mechanism made up of elementary reactions. The strain rate and the scalar dissipation rate represent the characteristic residence times in the reaction zone. For very small values of the strain rate and the scalar dissipation rate, the fuel CH 4 and the inhibitor CF 3Br are presumed to be consumed in different regions of the flame. The nondimensional distance between these regions is of the order of unity. At conditions close to extinction these regions merge and this merged reaction zone is analyzed. In the merged reaction zone chemical reactions are presumed to take place in three layers which are called the fuel-consumption layer, the oxidation layer and the CF 3Br-consumption layer. The fuel is consumed in the fuel-consumption layer and the inhibitor in the CF 3Br-consumption layer. The elementary reaction H + Br 2 → HBr + Br plays a central role in the inhibition of the chemical reactions taking place in the flame. This reaction increases the rates by which radicals recombine in the oxidation layer. The results of the asymptotic analysis show that at fixed values of the scalar dissipation rate. with increasing concentrations of CF 3Br in the oxidizer stream the maximum temperature in the reaction zone increases, and the amount of oxygen leaking through the reaction zone increases. Also the scalar dissipation rates at extinction decrease with increasing concentrations of CF 3Br in the oxidizer stream. The qualitative aspects of the changes in the values of the scalar dissipation rate at extinction with increasing concentrations of CF 3Br agree well with numerical results obtained using a detailed chemical-kinetic mechanism and with experimental data.