Unique phase change behaviors of ammonia droplets under varying ambient water vapor concentrations and pressures: A molecular dynamics simulation study

Abstract

This study employs molecular dynamics (MD) simulations to study the effect of polar water vapor on the phase change characteristics of ammonia droplets under varying ambient pressures. First, a new flexible potential model for ammonia was developed based on first-principles calculations. Then, the accuracy of this model in predicting the thermodynamic and transport properties of ammonia was extensively validated. Consequently, MD simulations using the new potential model were conducted to explore phase change behaviors of ammonia droplets under different ambient environments. The results reveal that both elevated ambient pressures and increased water vapor concentrations can promote the ammonia droplet evaporation. A unique phase change behavior of ammonia droplets in nitrogen/water environments was observed. Specifically, the polar water vapor dissolves and subsequently condenses within the ammonia droplet, thereby facilitating a transition from the ammonia-dominated evaporation to the water-dominated evaporation. Moreover, the dissolution and condensation become more intense at higher initial water vapor concentrations or pressures. Finally, the specific mechanisms by which water vapor enhances ammonia droplet evaporation were explored. During the dissolution and condensation process, water vapor releases latent heat and increases thermal conductivity, raising the droplet temperature. Additionally, water weakens the hydrogen bonding among ammonia molecules, thereby lowering the evaporation energy barrier. These findings provide essential insights into the phase change mechanisms of liquid ammonia and their dependence on ambient conditions.