Evolution mechanism of mixing layer during droplet supercritical phase transition

In high power density diesel engines, the phase transition mixing mechanism under supercritical conditions, especially the one within the mixing layer, is of crucial importance for studying the heat and mass transfer characteristics of supercritical fluids, but it has not yet been fully understood. Thus, a transient supercritical phase transition model of a single droplet is developed in this study. Using an n-heptane/nitrogen binary system, the effects of ambient pressure (8–15 MPa) and temperature (900–1100 K) on mixing behavior under supercritical single-phase conditions are investigated. The results indicate that increased ambient pressure inhibits the diffusion of supercritical fluid, leading to a thicker mixing layer and prolonged droplet phase-transition time, thereby hindering fuel/air mixing. Elevated pressure significantly reduces the diffusion coefficient within the mixing layer. Although the thermal conductivity increases, the heat transport dominated by mass transfer exceeds the conductive heat transfer, which slows down the droplet heating and extends transition time. In contrast, higher ambient temperature enhances supercritical fluid diffusion, resulting in a thinner mixing layer and shorter phase-transition duration. This is attributed to the concurrent increase in both thermal conductivity and diffusion coefficient. Under coupled heat and mass transfer promotion, the diffusion in the mixing layer accelerates, the temperature of the droplets rises more rapidly, and the rate of the supercritical phase-transition significantly increases. Based on above findings, “moderate boosting-coordinated temperature increase” may be an optimized path for achieving faster and more uniform combustion, which provides important theoretical guidance for the development and performance improvement of highly intensified diesel engines.