Homogeneous nucleation and condensation characteristics of water vapor-hydrogen (H2O-H2) binary systems from molecular dynamics simulation

Abstract

The condensation of water in wet hydrogen occurs in various applications such as fuel cells and nuclear power plants. However, the microscopic process of water condensation in wet hydrogen is not well understood. In the present study, the molecular dynamics (MD) was used to investigate the impact of various conditions on the condensation of saturated water vapor from a microscope perspective. It was found that the liquefaction ratio of H₂O molecules increased from 72.33% to 83.10% as the initial pressure increased from 1 MPa to 1.5 MPa when the cooling temperature was fixed at 380 K, while it increased from 72.33% to 87.05% as the cooling temperature decreased from 380 K to 350 K when the initial pressure was fixed at 1 MPa. Furthermore, hydrogen gas was introduced into the system to study the impacts of different initial pressures and temperatures on the condensation of saturated water vapor in the mixed gas. It was observed that the number of H₂O molecules contained in the final cluster increased with increasing initial temperature. As the initial pressure increased, plenty of H₂ molecules were adding to the system, hindering the nucleation of H₂O molecules. Through the comparison of nucleation rates, it was found that the computation of the nucleation rate of water in wet hydrogen flow concurs well with the rate determined by classical nucleation theory (CNT) under this simulation condition. However, the nucleation model proposed by Kantrowitz is closer to the actual condensation process of H₂O in pure steam at high temperatures and pressures and the nucleation rate of CNT is 1-2 orders of magnitude higher than that of MD in this situation.