Numerical study of the effects of pressure and air-dilution on NO formation in laminar counterflow diffusion flames of methane in high temperature air

Flame structure, extinction characteristics, and nitric oxide (NO) formation in diffusion flames of methane and diluted air have been investigated numerically by adopting counterflow as a model. Effect of pressure on NO emission is emphasized in high-temperature and diluted-air conditions. With undiluted air, extinction strain rate and flame temperature at a specified strain rate increase with pressure. In this case, thermal NO is dominant over prompt NO and pressure rise enhances chain branching/recombination reactions and increases flame temperature and mole fractions of H, O, and OH. Thereby, the emission index of NO increases appreciably with pressure. Air-dilution narrows extinction limit and lowers flame temperature appreciably, resulting in low production rate of NO. The crossover pressure, above which recombination reactions are dominant over chain branching reactions, depends on the extent of air-dilution, e.g., with air diluted to X_O2 = 0.09, it is only about 3 atm. In this case, extinction strain rate increases with pressure below the crossover pressure and then decreases above it. This is because recombination reaction becomes dominant at high pressures over the crossover pressure. At high pressures, with highly diluted air, prompt NO is comparable with thermal NO because thermal NO mechanism is highly suppressed. The emission index of NO increases appreciably with pressure for undiluted air case, while it increases at low pressures and then decreases at high pressures over the crossover pressure in diluted air. In view of emission index, both thermal NO and prompt NO mechanisms are relatively suppressed for diluted case compared to either undiluted case or diluted low-pressure case. Pressure rise enhances the effect of air-dilution on NO reduction through the change of chemical kinetics into recombination reaction.