Modeling of Supercritical $CO2$ Flow Through Short Tube Orifices

The transcritical cycle of carbon dioxide $(CO2)$ is a promising alternative approach to heat pumps and automobile air conditioners. As an expansion device, the short tube orifice in a transcritical $CO2$ system usually receives supercritical fluid at the entrance and discharges a two-phase mixture at the exit. In this work, a two-fluid model (TFM) is developed for modeling the flow characteristics of supercritical $CO2$ through the short tube orifice. The deviations between the TFM predictions and the measured mass flow rates are within $±20%$⁠. Meanwhile, the TFM predicts reasonable pressure, temperature, and velocity distributions along the tube length. The small values of interphase temperature difference and velocity slip indicate that the nonequilibrium characteristics of the two-phase flow of $CO2$ in the short tube orifice are not significant. Consequently, the homogeneous equilibrium model reduced from the TFM gives a good prediction of the mass flow rate as well.