Simulation of the emission of nitrogen and carbon oxides during the turbulent combustion of a partially mixed methane–air mixture

A two-dimensional simulation of the combustion of a lean methane–air mixture in a rectangular combustion chamber with a reverse step is carried out. The calculations are performed based on the previously developed, modified quasi-laminar approach, the distinctive feature of which is the setting of nonstationary profiles of the velocity components and fuel concentration (in the case of the incomplete mixing of fuel and air) at the entrance to the computational domain, and the reduced kinetic mechanism of methane oxidation, taking into account thermal and $\rm N_2\rm O$ mechanisms of $\rm NO$ formation. Both the kinetic mechanism and the technique as a whole are validated on the experimental data. The calculations show that in the case of a lean mixture's combustion, the incomplete mixing of fuel with air leads not only to an increase in the emission of nitrogen oxide, which was observed in experiments earlier but also to an increase in the emission of carbon monoxide. Moreover, the dependences of the $\rm NO$ and $\rm CO$ emissions on the intensity of the fuel concentration pulsations have a nonlinear and increasing character. The effect of incomplete mixing on the emission of carbon monoxide in the considered chamber turns out to be as significant as on the emission of nitrogen oxide.