The asymptotic structure of inhibited nonpremixed methane-air flames

As asymptotic analysis is performed to determined the influence of CF3Br on the structure and extinction of nonpremixed methane-air flames. The inhibitor CF3Br is added to the air-stream of the diffusion flame. A reduced four-step mechanism is used to describe the chemistry taking place in the reaction zone. The outer structure of the flame, in the limit where the rates of the chemical reactions are infinitely fast, is found to consist of two infinitely thin nonpremixed reaction zones which are separated by a chemically inert region. Previous analysis has shown that these reaction zones must merge at conditions close to extinction. Therefore only the asymptotic structure of the merged flame is analyzed here. In the inner structure of the merged flame chemical reactions are presumed to take place in three distinct layers which are identified as the “fuel-consumption-layer”, the “oxidation layer” and the “CF3Br-consumption layer”. The asymptotic analysis predicts the value of the scalar dissipation rate at extinction to decrease and the corresponding value of the maximum flame to increase, with increasing concentration of CF3Br in the oxidizing stream. However the inhibiting effect of CF3Br predicted by the asymptotic model is found to be weaker than that measured previously. The differences between the predictions of the asymptotic model and the measurements are attributed to the neglect of inhibition chemistry in the oxidation layer.