Study on the changes of molecular clusters at the interface layer of diesel droplets in methanol atmosphere under high temperature and high-pressure environment

Abstract

During the fuel injection stage, the temperature and pressure in the cylinder will exceed the fuel critical point and reach the supercritical environment, which has a great influence on the evolution of the interface layer during fuel evaporation. In this study, a molecular dynamics evaporation model was developed for diesel droplets in pure nitrogen and methanol nitrogen at high temperature and high pressure, the variation of molecular clusters in the interface layer of droplet during evaporation was analyzed, and the relationship between droplet evaporation and molecular clusters was revealed. The results show that the initial heating stage of droplets is accelerated and the quasi-static evaporation stage is slowed down by the atmosphere gas mixed with methanol. The concentration of nitrogen and methanol in the interface layer is about 55 % and 158 % respectively at 1100 K, which indicates that the interaction between diesel droplets and methanol is stronger. The change of the interface layer is strongly related to the supercritical transition of droplets. The change of the interface thickness indicates that the dominant mixing mode of droplets changes from evaporation to diffusion The number of molecular clusters increased by 37 %, but the total mass of molecular clusters was little affected by the addition of methanol in the atmosphere, a small number of small molecular clusters have little effect on the evaporation of droplets, while a large number of molecular clusters remain in the interface layer, the interaction between droplet molecules and clusters takes the dominant position after entering the interface layer, which slows down the diffusion of droplet molecules in the interface layer and slows down the evaporation process of fuel droplet.