The structure and extinction of nonpremixed methane/nitrous oxide and ethane/nitrous oxide flames

Knowledge of combustion of hydrocarbon fuels with nitrogen-containing oxidizers is a first step in understanding key aspects of combustion of hypergolic and gun propellants. Here an experimental and kinetic-modeling study is carried out to elucidate aspects of nonpremixed combustion of methane (CH4) and nitrous oxide (N2O), and ethane (C2H6) and N2O. Experiments are conducted, at a pressure of 1 atm, on flames stabilized between two opposing streams. One stream is a mixture of oxygen (O2), nitrogen (N2), and N2O, and the other a mixture of CH4 and N2 or C2H6 and N2. Critical conditions for extinction are measured. Kinetic-modeling studies are performed with the San Diego Mechanism. Experimental data and results of kinetic-modeling show that N2O inhibits the flame by promoting extinction. Analysis of the flame structure shows that H radicals are produced in the overall chain-branching step 3H2 + O2 ⇌ 2H2O + 2H, in which molecular hydrogen is consumed. Hydrogen is also consumed in the overall step N2O + H2 ⇌ N2 + H2O where stable products are formed. Inhibition of the flames by N2O is attributed to competition between these two overall steps.