Zhang, Kaiqi; Ma, Xiao; Luo, Kai H; Han, Zhenyu; Li, Yanfei; Shuai, Shijin; (2025) Phase transition of ammonia-methanol binary droplets from subcritical to supercritical conditions: A molecular dynamics study. Proceedings of the Combustion Institute , 41 , Article 105886. 10.1016/j.proci.2025.105886.

Text Luo 2025 PCI ammonia-methanol accepted.pdf - Accepted Version Access restricted to UCL open access staff until 9 October 2026. Download (797kB)

Abstract

Ammonia can easily dissolve in methanol and be co-injected as a blended fuel into the combustion chamber to enhance combustion performance. However, the phase transition behavior of ammonia–methanol fuel blends under engine-relevant conditions remains poorly understood. To gain insight into the interactions between these two polar fuel components, this study employed molecular dynamics (MD) simulations to explore the phase transition of ammonia-methanol binary droplets from subcritical to supercritical conditions, focusing on the effects of the ambient pressure and initial ammonia fraction. The results show that the binary droplet exhibits an “ammonia preferential evaporation” characteristic under subcritical and supercritical conditions, with increasing ambient pressure narrowing the evaporation rate difference between the two components. A higher initial ammonia fraction inhibits droplet thermal expansion and interface temperature rise at the initial heating stage. Moreover, the transcritical mixing behavior of binary droplets is analyzed in detail. Specifically, the interface Knudsen number is evaluated and found to be strongly pressure-dependent but weakly affected by initial ammonia fraction. Voronoi tessellation analysis further highlights the distinct differences in vapor–liquid local density evolution of fuel components during transcritical mixing. The different phase transition regimes of fuel droplets under supercritical conditions are identified, and the effects of initial ammonia fraction are quantified. The findings indicate that the initial ammonia fraction has a non-monotonic effect on the transcritical mixing rate of binary droplets. At 160 atm, increasing the ammonia fraction from 0 % to 20 % could increase the transition time and delay the transcritical mixing. However, further increases may shorten the classical evaporation regime and promote diffusive mixing. The hydrogen bonding interactions between ammonia and methanol are also analyzed to provide deeper micro-insight into the phase transition. The mole fraction of hydrogen bonds formed between two polar components shows a good linear decrease trend with time under supercritical conditions.

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