Numerical simulation of nitrogen oxide formation in lean premixed turbulent H2/O2/N2 flames

Lean premixed hydrogen flames are thermodiffusively unstable and burn in cellular structures. Within these cellular structures the flame is locally enriched by preferential diffusion of hydrogen, leading to local hotspots that burn more intensely than an idealized flat steady flame at comparable inlet conditions. We investigate the impact of this local enrichment on the formation of nitrogen oxides. We consider a two dimensional configuration in which lean premixed hydrogen–air flames interact with a weakly turbulent velocity field for a range of equivalence ratios. The simulations show that although peak temperatures remain well below 1800K (where thermal NOx traditionally is thought to become significant), these localized hot spots lead to significant production of nitric oxides, and the relative enhancement becomes increasingly significant with lower fuel equivalence ratios. A detailed examination of the reaction chemistry in these unsteady flames shows that at richer conditions the predominant path taken to convert nitrogen gas to nitric oxide is via NNH. For leaner flames a path through nitrous oxide becomes increasingly important.