Temperature-Gradient-Induced Multiregime Detonation Development in Ammonia–Hydrogen–Air Mixtures

Hydrogen enrichment is recognized for improving ammonia combustion performance, with high hydrogen blending ratios facilitating auto-ignition and detonation. However, the underlying mechanisms of auto-ignition and detonation characteristics of hydrogen-enriched ammonia remain incompletely understood. This study utilizes both one-dimensional and two-dimensional numerical simulations to examine auto-ignition and detonation characteristics in ammonia–air mixtures with varying hydrogen blending ratios. Hotspots with varied radii and wide-range temperature gradients are employed to initiate reaction front propagation, and the role of hydrogen enrichment in auto-ignition and detonation is examined. The findings reveal a non-monotonic response of reaction front propagation to temperature gradients, leading to multiregime detonation developments across a wide range of temperature gradients. Hydrogen enrichment increases the reaction front speed, facilitating detonation development under both shallow and steep temperature gradients. Under shallow gradients, hydrogen enrichment accelerates the initial reaction front and reduces the reaction progress, allowing more residence time for detonation transition. In contrast, reaction fronts under steep gradients decelerate within the hotspot unless sufficient reactivity triggers a direct detonation. Two-dimensional simulations further demonstrate multiregime detonation development with a cellular structure resulting from transverse wave disturbances. Compared to shallow gradients, steep gradients lead to detonation with higher pressures and larger cell sizes.