Molecular dynamics study of methanol phase change characteristics at subcritical and supercritical conditions

This study uses molecular dynamics simulations to investigate the phase transition behavior of methanol under subcritical and supercritical conditions. A gas–liquid–gas model is constructed in a nitrogen atmosphere to simulate the phase change process of methanol at various ambient temperatures and pressures. The evolution of density distribution, hydrogen bond number, methanol molecule number, and mean square displacement of methanol molecules, mixing layer thickness, and molecular cluster distribution under subcritical and supercritical conditions is estimated and compared. The results indicate that under subcritical conditions, methanol phase change is dominated by classical two-phase evaporation. However, under supercritical conditions, the phase change shifts from classical two-phase evaporation to diffusion-driven mixing. Both ambient pressure and temperature influence the fuel's phase change. Increasing ambient pressure promotes energy exchange between the gas and liquid phases; however, excessive pressure inhibits methanol diffusion, which is detrimental to droplet evaporation. Rising temperature weakens the hydrogen bonding between methanol molecules, lowering the evaporation energy barrier and thereby promoting the phase change process. Analysis of molecular clusters further reveals significant differences in evaporation patterns under subcritical and supercritical conditions: under subcritical conditions, methanol molecules at the interface primarily leave the droplet as individual molecules, making it difficult to form a wide gas–liquid interface mixing layer; whereas under supercritical conditions, a large number of methanol molecules diffuse into the interface region, where dispersed molecules aggregate to form clusters, which then leave the droplet as clusters, leading to a rapid increase in the thickness of the gas–liquid interface mixing layer.