Zhang, Kaiqi; Ma, Xiao; Luo, Kai H; Li, Yanfei; Shuai, Shijin; (2025) The unique phase transition behavior of ammonia droplets in varying water vapor environments under subcritical and supercritical conditions. Proceedings of the Combustion Institute , 41 , Article 105902. 10.1016/j.proci.2025.105902.

Text Luo 2025 PCI ammonia accepted.pdf - Accepted Version Access restricted to UCL open access staff until 15 October 2026. Download (627kB)

Abstract

Ammonia, as a zero-carbon fuel, has attracted great attention in recent years. Previous studies on the phase transition of liquid ammonia have primarily focused on subcritical ammonia sprays, while fundamental studies of ammonia droplets under engine-relevant conditions remain scarce. Moreover, the mechanism of water vapor produced by combustion on the phase transition behavior of ammonia droplets remains unclear. To address the shortcomings, the present study systematically explores the phase transition process of ammonia droplets in varying high-temperature water vapor environments from subcritical to supercritical conditions via molecular dynamics simulations for the first time, revealing the distinct effects of water dissolution. This study analyzes the interface thermodynamic state and transcritical mixing behavior of ammonia droplets. An improved criterion based on fast prediction of the mixture's critical temperature is employed to quantify the effects of water vapor concentration on droplet phase transition regimes under supercritical conditions. The results indicate that increasing water vapor concentration could accelerate droplet evaporation under subcritical conditions. Higher water vapor concentration and ambient pressure increase the interface temperature. Under supercritical conditions, the elevated water vapor concentration could shorten the two-phase evaporation regime and promotes the supercritical transition of ammonia droplets, due to the stronger thermal effect of water dissolution. Furthermore, this study analyzes the hydrogen bonding interactions between ammonia and water, revealing the significant differences in hydrogen bond distribution under subcritical and supercritical conditions. A non-monotonic effect of ambient pressure on hydrogen bond density is captured. Notably, this study finds a distinctive droplet breakup phenomenon (like micro-explosion) induced by water vapor at the end of subcritical evaporation, closely resembles the findings in the latest ammonia droplet microgravity combustion experiment. The physical mechanism and the effects of ambient conditions on droplet breakup degree are discussed in this study.

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