Abstract: Twophase flow generated from expansion of subcooled subcritical CO2 when flowing through a convergingdiverging nozzle was studied numerically using the mixture model of the Fluent with the sliding velocity neglected. Mass transfer rate between the two phases was calculated with the cavitation model under 61MPa and 29315K at the nozzle inlet. The results show that phase change occurs at the throat and the pressure drops steeply with the mass transfer rate reaching the maximum. The vapor volume fraction increases with decrease of pressure when the CO2 flows through the nozzle diverging section. When the pressure at nozzle exit gets to 165MPa， the volume fraction reaches 093. The comparison of calculated and experimental results show that the maximum relative error of pressure is less than 101% and that of temperature is less than 19%. Mechanism of the twophase flow expansion process of the subcooled CO2 through a convergingdiverging nozzle was well described by numerical simulation.

Key words: subcritical CO2, gasliquid twophase flow, convergingdiverging nozzle, CO2 ejector, mass transfer rate, cavitation model theory