The evaporation characteristics and molecular cluster variation of diesel droplets in supercritical environment

In the fuel injection phase of a diesel engine, the temperature and pressure within the cylinder surpass the fuel's critical point, entering a supercritical state. This condition significantly affects the fuel's evaporation and atomization processes within the cylinder. This study conducted an evaporation experiment on diesel droplets under supercritical conditions using a high-temperature, constant-volume droplet evaporation apparatus. A molecular dynamics model was developed to simulate the evaporation of diesel droplets in a nitrogen environment, composed of a mixture of two components: 85.1 mol. % n-dodecane and 14.9 mol. % isooctane. The study focused on the evaporation process in a sub-supercritical environment, analyzing the changes in molecular clusters near the interface during droplet evaporation to elucidate the relationship between droplet evaporation and these molecular clusters. The findings indicate that the lifetime of the droplets decreases with rising ambient temperature and pressure, with the effects being more pronounced in a subcritical environment. The enrichment of nitrogen molecules is 40% in the subcritical environment and 164% in the supercritical environment, suggesting that the enrichment phenomenon of nitrogen molecules is more pronounced under supercritical conditions. At supercritical temperatures, the ambient temperature's influence on the droplet evaporation process is primarily through accelerating the initial heating phase of the droplet. An increase in ambient temperature and pressure leads to a higher number of molecular clusters. The study proposes five levels of clusters based on the varying number of molecules they contain, with different levels transforming into one another. For each increment in the cluster level, the molecular forces within the clusters increase by approximately 0.025 kcal/mol Å. In subcritical environments, a distinct interface exists between the droplets and atmospheric gas, resulting in fewer molecular clusters. Conversely, in supercritical environments, this clear interface is replaced by an interface layer, which facilitates the formation of numerous, larger molecular clusters.