Droplet evaporation and phase transition modes in supercritical environment by molecular dynamic simulation

Abstract

The phase change process of a single hydrocarbon droplet in a supercritical environment has different features compared with subcritical evaporation. In this study, droplets surrounded by supercritical nitrogen are investigated by molecular dynamic simulation. Dodecane droplets are considered in order to reproduce the phase change process of actual fuel, and the radius ranges from 10 nm to 25 nm under various subcritical, transcritical, and supercritical conditions. The emphasis is to describe the entire phase change process and distinguish its different stages. Under supercritical conditions, traditional D2-law theory is invalid and the radius of droplets increases dramatically accompanied by huge fluctuation after a special time. Based on analysis toward fluid properties crossing this time point, the phases classification for supercritical mixture is provided and three phase change stages are distinguished in order, which can be divided by the onset time of supercritical transition, the disappearance of surface tension, and one-phase diffusion, respectively. Additionally, gas/liquid-like features have been observed in the supercritical binary dodecane-nitrogen coexisting system and the defined pseudo-boiling mole fraction (Xpb) is introduced to clarify such deviation in properties of supercritical mixture. At last, by judging whether supercritical transition and undroplet phenomenon will happen, three phase transition modes are summarized for droplets under supercritical conditions.