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 H2. This local enrichment leads to hot spots in which the flame burns with considerably more intensity than the corresponding laminar flame. In this paper we investigate the impact of this local enrichment on the formation of emissions of oxides of nitrogen. We consider a two dimensional configuration in which premixed H2-air flames interact with a weakly turbulent velocity field for a range of equivalence ratios. The simulations show that although the peak temperatures in the hot spots remain well below 1800K where thermo NOx becomes significant, the hot spots are the location of the dominant production of emissions. Furthermore, the turbulent flames all show increased emissions compared to the flat laminar flames. with the relative increase in emissions becoming more significant at lower equivalence ratios. A more detailed examination of the reaction chemistry shows that at richer conditions the predominant path for the initiation of emissions is N2 → NNH. As the flame becomes leaner N2 → N2O becomes increasing important.