A direct numerical simulation study on NO and N[formula omitted]O formation in turbulent premixed ammonia/hydrogen/nitrogen-air flames

Abstract

The formation of nitric oxide (NO) and nitrous oxide (N2O) in turbulent NH3/H2/N2-air premixed flames is investigated utilizing three-dimensional Direct Numerical Simulation (DNS) with detailed chemical kinetics in order to gain insight on the role of equivalence ratio and pressure. To this end, results from five large-scale DNS in temporal jet and shear layer configurations with equivalence ratios of 0.45, 0.9 and 1.1 (at 1 bar) and pressures of 1 and 10 bar (at an equivalence ratio of 0.45) are presented. Statistics of the temporal evolution of global formation rates of NO and N2O are analyzed. Overall, an increase of NO production (compared to heat release) is observed, except for the lean high-pressure case, which sustains lower NO but increased N2O net production rates. Time-instants of peak NO and N2O are analyzed in more detail, including progress variable and curvature conditioned statistics that reveal the impact of local flame geometry on NO production pathways and key chemical reactions involving N2O. It is found that the production of NO generally displays a high dependence on the local flame geometry, while the formation of N2O is less sensitive to curvature, except at high pressure. These observations are discussed in light of differences in thermo-diffusive effects and the topologically-driven increased availability of certain key radicals, e.g., atomic hydrogen in the consumption of N2O.