Numerical simulation of ammonia-simulated cracking mixture combustion characteristics and regulation of chemical reaction pathways in a swirl combustor

The mixing and combustion characteristics of a swirl combustor in air turbo ramjet engine with expand cycle were systematically characterized using ammonia-simulated cracking gas as fuel. The characteristics of ammonia-based fuel was compared to methane to evaluate the feasibility as an alternative for ATR engines while maintaining combustion performance. Analysis of combustion efficiency, NOx emissions, and unburned fuel emissions under varying ammonia cracking ratios was characterized by mutual constraint and trade-off. Chemical reactor network modeling and chemical kinetics analysis were utilized to identify key elementary reactions influencing NOx formation, elucidate the relationship between combustion efficiency and pollutant emissions, and suggest optimization strategies based on reaction pathway regulation. The results showed that incomplete combustion of ammonia at low cracking ratios led to reduced combustion efficiency and increased unburned fuel emissions. At a 40% cracking ratio, combustion efficiency surpassed 95% and that of methane, with relatively lowest NOx and unburned fuel emissions. However, combustion efficiency declined beyond the cracking ratio of 60%, accompanied by flame front moving upstream and elevated NOx and unburned fuel emissions. Sensitivity analysis of NO concentration at the combustor outlet indicated that introducing NH2 radicals could enhance combustion efficiency and reduce NOx emissions, albeit with a slight increase in unburned fuel emissions, while NH and NNH radicals had adverse effects. Under high cracking ratios, minor addition of H radical could both further inhibit NOx formation. These findings suggest combustion regulation strategies according to distinct combustion characteristics of mixture under various cracking ratios. This study establishes a theoretical framework to enable zero-carbon fuel utilization in ATR engines while maintaining combustion efficiency. Additionally, it offers practical engineering insights for enhancing the performance of swirl combustors utilizing ammonia-derived cracking gas.