Performance of supercritical carbon dioxide (sCO₂) centrifugal compressors in the Brayton cycle considering non-equilibrium condensation and exergy efficiency

The Brayton cycle, with supercritical carbon dioxide (sCO₂) centrifugal compressors at its core, has received widespread attention for its high efficiency and energy savings. Therefore, the in-depth exploration of the internal flow field behaviour of sCO₂ centrifugal compressors becomes especially critical. In the present study, we develop a mathematical model to evaluate the performance of a sCO₂ compressor considering non-equilibrium condensation in transonic flows. It is validated that the model can well predict the sCO₂ phase change and is a reliable vehicle for the performance analysis in this study. Further, by incorporating the cubic spline curve extrapolation method, the model can handle the complex thermodynamic properties of CO₂ under a metastable state. Then, this study presents a series of analyses for the effects under different conditions, such as different inlet temperatures and rotational speeds, on the flow field characteristics and performance of the compressor. It's further found that with an increase in inlet temperature, the condensation intensity is suppressed and the condensation position moves backwards. At the 50% blade span, the liquid phase fraction decreases from 5.15% at 306 K to 1.32% at 325 K. Finally, from an energy perspective, this study implements a quantitative analysis of the exergy destruction and exergy efficiency of the compressor. The results show that the exergy efficiency under the wet steam model is quite different when the inlet temperature changes. With the decrease of rotating speeds, the exergy efficiency will decrease. The exergy efficiency reaches 81.35% at 20,000 rpm, which is 51.02% higher than that at 10,000 rpm.