Numerical study on non-equilibrium condensation and flashing mechanisms in rapid expansion process of transcritical CO2

When transcritical CO2 expands at a high speed, the pressure and temperature drop sharply, and a non-equilibrium phase change occurs. The non-equilibrium condensation phase change of CO2 would occur in natural gas supersonic separation equipment and supercritical CO2 centrifugal compressor. Besides, the non-equilibrium flashing phase change of transcritical CO2 occurs in the ejector primary nozzle in the ejector expansion refrigeration system. In order to solve the problem that the physical properties of transcritical CO2 change sharply during the expansion process and it is difficult to simulate the non-equilibrium phase change, a new non-equilibrium phase change CFD model was constructed to study the transcritical CO2 non-equilibrium condensation and flashing phase change process and expansion mechanisms in the supersonic converging-diverging nozzle. The model coupled the temperature-driven evaporation-condensation phase change mechanism and the pressure-driven cavitation-condensation phase change mechanism, and the accuracy of the model was verified by the experimental results in literature. The results showed that the pressure-driven condensation mass transfer had a major influence on condensation phase change. The pressure-driven condensation mass transfer mainly existed in the nozzle throat and internal flow zone, and the temperature driven condensation mass transfer mainly existed on the wall of the nozzle diverging section. The condensation mass transfer rate increased with the increase of inlet pressure and the decrease of inlet temperature, so that the non-equilibrium degree of condensation and the quality in the nozzle were reduced, and the position of speed of sound was delayed accordingly in the nozzle diverging section. In addition, the temperature-driven evaporation mass transfer dominated the flashing phase change, the evaporation phase change mainly occurred near the nozzle throat, the cavitation phase change mainly occurred in the nozzle diverging section, and the two-phase CO2 reached speed of sound in the nozzle diverging section. With the increase of inlet pressure and the decrease of inlet temperature, the non-equilibrium degree of flashing increased and the quality in the nozzle decreased. This study was helpful to clarify the non-equilibrium flashing and condensation phase change mechanism in the rapid expansion of transcritical CO2, and it provided a method for the analysis and optimization design of transcritical CO2 expansion equipment. © 2022 Chemical Industry Press. All rights reserved.