Lei Wang

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.

It is crucially important to understand the dynamics of flash boiling bubbles of liquid fuels in order to achieve optimal fuel spray and mixing performance in realistic combustion engines. In this study, a modified pseudo-potential lattice Boltzmann method was adopted to capture the dynamics of flash boiling bubbles of liquid ammonia. The critical conditions for the flash boiling state of liquid ammonia were determined using isothermal depressurization techniques. Meanwhile, the evolutions of bubble clusters were investigated, allowing for bubble deformation, coalescence, and collapse in different flash boiling stages. The results show that the evolution of flash boiling bubble clusters presents two main stages, i.e., the early rapid expansion stage and the later slow expansion stage, and both stages are highly sensitive to temperature variations. Low-temperature environments can intensify the competition between bubble coalescence and collapse events, thereby changing the density, pressure, and velocity distribution and delaying the transition of bubbles to the eventual equilibrium state. In addition, the evolution of bubble clusters conforms to the Ostwald ripening mechanism, where large bubbles absorb smaller bubbles, promoting the formation and propagation of pressure waves in liquid ammonia. These pressure waves not only disturb the surrounding fluid to form vortices but also cause significant deformation of adjacent bubbles, thereby affecting the overall efficiency of flash boiling processes.