Investigation of supercritical transition and evaporation process of a hydrocarbon droplet under diesel engine-relevant conditions

•Evaporation process of a micron-droplet at supercritical conditions is investigated.•Droplet lifetime is more sensitive to ambient temperature at higher pressure.•Impacts of ambient pressure on evaporation process change with ambient temperature.•For micron-droplets, attainment of critical mixing state is independent of diameter.•Minimum ambient temperature needed for transition reduces as ambient density rises. Fuel spray supercritical transition in the diesel engine has been concerned widely, especially supercharged ones, however, whether the transition can significantly affect the fuel–air mixing under diesel engine-relevant conditions is an issue that still needs evaluating carefully. Thus, the transition and evaporation of individual n-heptane droplets in a nitrogen environment under various ambient conditions were investigated through a high-pressure evaporation model. The influence of ambient pressures on droplet lifetime (τlife) and evaporation rate constant (K) varies with ambient temperature. When Tr