Numerical Investigation of a Centrifugal Compressor for Supercritical CO₂ as a Working Fluid

The supercritical carbon dioxide (S-CO₂) Brayton cycle is considered as a strong candidate for power conversion systems. This includes concentrated solar power, coal power, bottoming cycle to fuel cells, and the next generation nuclear systems. In the previous studies, it was identified that the compressor consumes very small compressing work as operating condition approaches to the critical point. Thus, smaller amount of input work contributes to the enhancement of overall cycle efficiency. To achieve an efficient S-CO₂ cycle, one of the major technical challenges exists in the compressor design. At KAIST, a research team is conducting a S-CO₂ compressor tests to obtain fundamental data for advanced compressor design and to measure the performance of the compressor near the critical point. The measurements reveal the S-CO₂ fluid to have properties of gases and liquids at the same time, but in regards to compressibility and density variation, its behavior is much closer to the liquid rather than gas near the critical point. In this paper, a CFD analysis of S-CO₂ centrifugal compressor with the full geometry including diffuser and volute is presented. The numerical results are compared to the experimental data from KAIST SCO₂ Pressurizing Experiment facility. A 3D grid was generated starting from the model of the compressor full geometry provided by the manufacturer. Furthermore, a property table of CO₂ was generated by an in-house code and implemented to the CFD code. Then the performance characteristic of S-CO₂ compressor is investigated in terms of compressor efficiency and pressure ratio. Additional flow variables inside the compressor such as velocity, pressure and viscosity are also investigated to help understanding the main reason behind the relatively higher compressor efficiency near the critical point compared to other flow conditions far from this region. In general acceptable results in comparison to the experiment are obtained (order of error from 0.5 to 7% for the compressor efficiency). Hence, the current CFD results should be able to provide additional and detailed information to be used for design enhancements of the compressor for S-CO₂ Brayton power cycle.