Intrinsically unstable hydrogen-enriched premixed ammonia flames: Analysis and modeling of NO formation

Enriching a lean ammonia/air mixture with hydrogen can increase the reactivity of pure ammonia flames. However, this may lead to the onset of intrinsic flame instabilities, which in turn affect flame characteristics such as heat release rates, morphology, propagation and pollutant formation. In this study, we examine the propagation characteristics and NO production in lean premixed hydrogen-enriched ammonia/air flames (50% H2%–50% NH3 by volume) using direct numerical simulations with a detailed chemical kinetic mechanism. The individual effects of pressure and thermodiffusion (Soret effect) are assessed by comparing results with an atmospheric pressure flame. Statistics on the rate of NO production are presented and analyzed. NO production is decomposed into its main production/depletion pathways (thermal, HNO, DeNOx, isomerization and fuel), revealing that NO production is promoted, mainly through the fuel pathway, due to the presence of superadiabatic cells, typical of thermo-diffusively unstable flames. At atmospheric pressure, the Soret effect is observed to play only a marginal role in NO production. To further understand NO production, the NO rate is split into two contributions, respectively due to area increase and change in flame reactivity. At high pressure, while NO production is overall mitigated compared to the atmospheric pressure flame, the wrinkled flame is found to produce more NO than its one-dimensional counterpart. A data-driven approach is finally used to address the issue of flame structure dimensionality of hydrogen-enriched ammonia flames. An irreducible error analysis reveals that at least three progress variables are required to correctly reconstruct the flame structure.