Numerical investigation of the effects of inlet conditions and wall heat transfer on subcritical/transcritical CO2 nozzle performance

In ejector-based refrigeration cycles and CO2 heat pump cycles, inlet flow state and wall heat transfer play an important role in improving nozzle performance. A CO2 two-phase nozzle model was firstly established. Then, the numerical model is validated by experimental data and void fraction correlation, and the effect of condensation coefficient and gas equation of state was analyzed. In addition, the CO2 flow behavior and the thermodynamic performance of the nozzle under different inlet gas phase volume fraction (GVF) and wall conditions (temperature and materials) were studied. The results show that although the GVF can reduce the critical mass flow rate of the nozzle, its adverse impact on the thermodynamic performance of the nozzle is more significant (The GVF varied from 0 to 0.6, with the former decreasing by 10.2% and the latter increasing by 25.1%). Low temperature on the wall will exacerbate the non-equilibrium phase transition, leading to an increase in the critical mass flow rate of the nozzle. At this time, high thermal conductivity walls will also exacerbate exergy losses, by using low thermal conductivity walls can improve the consequences. In addition, the study also found that the inlet supercooling degree (ISCD) should be set between 0 and 7 K, and excessive ISCD would lead to an increase in outlet humidity and a rise in critical mass flow.