The dimensionless Jakob number (Ja) as a ratio of sensible heat to latent heat of vaporization in a phase change process was chosen as the characteristic parameter for flash evaporation process. The evolution of flash efficiency versus Ja was studied on a newly-designed experimental system for high-temperature and high-pressure spray flash evaporation under different operating conditions. The experimental results showed that flash efficiency was higher and flash evaporation was more violent at larger Ja. The flash efficiency increased with the increase of initial liquid temperature but decreased with the increase of evaporation pressure

which was closely related to the physical meaning of Ja. Based on the fact that both flash efficiency and Ja were increasing functions of the degree of superheat

a combination of high initial temperature and evaporation pressure was beneficial for flash evaporation under the same degree of superheat. An empirical equation between flash efficiency and Ja was developed from the experimental data. The findings were considered universal in the field of flash evaporations at high-temperature and high-pressure and thus could be a useful reference for industrial application.