Experimental study on microscopic characteristics of transcritical spray for high density environment

As the power density of diesel engines increases, the in-cylinder spray process appears supercritical phase transition, but the transition boundary and judgment criteria are unclear. This study explores the supercritical phase transition of spray using a constant volume combustion chamber and a high-speed optical system, considering ambient temperatures of 700 K to 1600 K and densities of 12 kg/m³ to 22.8 kg/m³. The results show that under subcritical conditions, the droplets at the end of the spray remain spherical during the evaporation process due to the existence of surface tension, and eventually evaporate. Under supercritical conditions, the droplets begin to be spherical during the evaporation process. As time goes on, the wake appears and falls off, oscillates and deforms, and finally diffuses and disappears, which is regarded as the occurrence of supercritical phase transition. The temperature limit of supercritical phase transition decreases with the increase of ambient density, and the density limit decreases with the increase of ambient temperature. This is because as the ambient density or temperature increases, the heat absorption required to destroy the gas–liquid interface (reducing the surface tension, thickening the interface, and reducing the average free molecular path) decreases. The supercritical phase transition time is shortened with the increase of ambient temperature or ambient density. The reason is that the ambient density and temperature increase under supercritical conditions, the heat and mass transfer is accelerated, and the temperature rise of the droplet surface is accelerated. As the droplet size increases, the supercritical phase transition boundary remains unchanged, and the supercritical transition time is prolonged, which is due to the decrease of the surface volume ratio and the slow heating of the droplet.