Effects of oxygen-enrichment on the flame and emission characteristics of ammonia combustion in a swirl combustor

Oxygen-enriched combustion (OEC) is an efficient method to optimize the combustion performance of ammonia from the oxidizer side. However, the intrinsic mechanism through oxygen enrichment enhances NH₃ combustion remain poorly understood. In this study, the combustion and blow-off characteristics of oxygen-enriched ammonia swirl flames were experimentally investigated. Three-dimensional simulations were conducted to further elucidate the mechanisms of oxygen enrichment and its impact on emission characteristics. Notably, a high-spatial-resolution depiction of the blow-off transition was constructed via OH-PLIF. The results indicate that increasing the oxygen fraction significantly enhances the stability and combustion intensity of NH₃ swirl flames. Specifically, the equivalence ratio ranges of the lean-to-rich blow-off limits as well as each macrostructure of oxygen-enriched NH₃ flames are broadened. The lean blow-off limit of NH₃ flame exceeds that of methane flames at 25 % O₂. Moreover, the transition from a steady state to an unsteady state of the oxygen-enriched flame is substantially prolonged, and a precursor that can provide an earlier warning of blow-off is identified. Oxygen-enriched combustion can increase the combustion temperature and reduce the temperature gradient near the wall, thereby mitigating the instability caused by wall heat loss. Furthermore, the concentrations of OH and NH in oxygen-enriched flames are significantly elevated. Oxygen-enriched combustion was also found to exacerbate NO emissions, which dominate NO_x, but effectively reduce NH₃ and H₂ emissions under stoichiometric and rich conditions, thereby greatly enhancing combustion efficiency.