Role of hydrogen enrichment in ammonia forced ignition at elevated pressures

Abstract

Numerous studies have demonstrated that hydrogen enrichment can improve ammonia reactivity, leading to enhanced ignition and combustion performance. However, the role of hydrogen enrichment in forced ignition of ammonia, especially at elevated pressures, remains not fully understood. This study employed a localized energy deposition technique to initiate the forced ignition of ammonia/hydrogen mixtures. The role of hydrogen ratio and ignition energy in ignition and flame kernel initiation was numerically investigated, and the critical ignition conditions were identified by assessing the correlations between heat release and thermal diffusion. The results show that the forced ignition at low pressures involves four traditional stages, whereas only two stages are present at high pressures, i.e., ignition assisted flame kernel propagation and normal laminar flame propagation. The weakening stretching responses at high pressures cause the flame kernel to propagate outward without additional ignition energy. Then deposited ignition energy mainly heats ignition kernels and increases the local temperature, thereby reducing ignition delay time and accelerating ignition initiation. Hydrogen enrichment enhancing ignition performance is mainly due to the changed fuel property and reduced ignition delay time. Kinetic analysis suggests that this enhancement is primarily attributed to the increased sensitivity of H+O₂=O+OH and the substantial H generation from the reverse of NH₃+H=NH₂+H₂, both of which promote the chain branching of H+O₂=O+OH. Besides, successful ignition also depends on the competition between chemical heat release and thermal diffusion. Chemical heat release dominates within a timescale of ∼0.1 ms, while thermal diffusion prevails beyond the threshold. Hydrogen enrichment can significantly reduce minimum ignition energy, but this tendency becomes less pronounced when hydrogen ratio exceeds 20 %.