RATE-RATIO ASYMPTOTIC ANALYSIS OF INHIBITION OF NONPREMIXED METHANE-AIR FLAMES BY CF3BR

Rate-ratio asymptotic analysis is performed using a reduced four-step chemical-kinetic mechanism to elucidate the influence of CF3Br on the structure of nonpremixed methane-air flames. The inhibitor CF3Br is ad ded to the oxidizer stream of the nonpremixed flame. The primary focus of the analysis is to obtain the critical conditions of extinction. T he asymptotic flame structure is constructed using the results of nume rical calculations performed using detailed chemical-kinetic mechanism made up of elementary reactions. The strain rate and the scalar dissi pation rate represent the characteristic residence times in the reacti on zone. For very small values of the strain rate and the scalar dissi pation rate, the fuel CH4 and the inhibitor CF3Br are presumed to be c onsumed in different regions of the flame. The nondimensional distance between these regions is of the order of unity. At conditions close t o extinction these regions merge and this merged reaction zone is anal yzed. In the merged reaction zone chemical reactions are presumed to t ake place in three layers which are called the fuel-consumption layer, the oxidation layer and the CF3Br-consumption layer. The fuel is cons umed in the fuel-consumption layer and the inhibitor in the CF3Br-cons umption layer. The elementary reaction H + Br-2 --> HBr + Br plays a c entral role in the inhibition of the chemical reactions taking place i n the flame. This reaction increases the rates by which radicals recom bine in the oxidation layer. The results of the asymptotic analysis sh ow that at fired values of the scalar dissipation rate, with increasin g concentrations of CF3Br in the oxidizer stream the maximum temperatu re in the reaction zone increases, and the amount of oxygen leaking th rough the reaction zone increases. Also the scalar dissipation rates a t extinction decrease with increasing concentrations of CF3Br in the o xidizer stream. The qualitative aspects of the changes in the values o f the scalar dissipation rate at extinction with increasing concentrat ions of CF3Br agree well with numerical results obtained using a detai led chemical-kinetic mechanism and with experimental data. (C) 1998 by The Combustion Institute.