Rate-ratio asymptotic analysis of the structure and mechanisms of extinction of nonpremixed CH4/N2–O2/N2O/N2 flames

Abstract

Rate-ratio asymptotic analysis is carried out to elucidate the influence of nitrous oxide on the structure and critical conditions for extinction of nonpremixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream is made up of a mixture of methane and nitrogen. The other stream is a mixture of oxygen, nitrous oxide, and nitrogen. A reduced mechanism of five global steps is employed in the analysis. Chemical reactions are presumed to take place in a thin reaction zone that is established in the vicinity of the stagnation plane. On either side of this thin reaction zone, the flow field is inert. These inert regions are called outer zones. Methane and nitrous oxide are completely consumed in the reaction zone, while oxygen is presumed to leak through the reaction zone. In the reaction zone, chemical reactions are presumed to take place in two layers—the inner layer and the oxidation layer. In the inner layer fuel (methane) is consumed and the intermediate species hydrogen and carbon monoxide are formed. These intermediate species are oxidized in the oxidation layer to water vapor and carbon dioxide. Radicals are produced in the oxidation layer from chain-branching reactions that consume hydrogen. Asymptotic analysis gives the scalar dissipation rate at extinction. Critical conditions for extinction predicted by the analysis agree well with previous experimental data. Nitrous oxide is found to have an inhibiting effect on the flame, promoting extinction. The inhibiting effect is attributed to the competition between the net reaction of nitrous oxide with hydrogen to form water vapor and nitrogen and the chain-branching reaction between oxygen and hydrogen that produces radicals.