Influence of Real Gas Properties on Loss in a Super Critical CO2 (sCO2) Centrifugal Compressor

Abstract

The performance of the SCO2 centrifugal compressor is the key component influencing the efficiency and stability of the whole Close-Bryton cycle system. It is important to understand the loss mechanism inside the compressor to guide the design and performance optimization of the compressor. However, the physical properties of SCO2 are strongly nonlinear near the critical point, and the internal flow of the compressor is highly coupled with its properties, which inevitably profoundly influences the loss generation in the device. In this paper, the loss mechanism of SCO2 compressor is investigated comprehensively based on numerical method compared with experimental data. CO2 real gas model embodied in RANS model are used for the study. Firstly, grid independence and influence of the resolution of real gas properties tables are discussed. The numerical simulation results are in good agreement with the experimental data of Sandia SCO2 compressor. Secondly, the distribution of the loss of the compressor are evaluated by means of local entropy generation at different mass flow rates. In particular, a direction comparison between the cases with real gas properties and the constant properties is carried out for manifesting of the influence by real gas properties. The results manifest that the loss of the case with constant properties is profoundly higher than that of the real gas properties, especially at small flow rate. Detailed flow field is compared to understand the influence of the properties. It is shown that the tip leakage flow is evidently decreased with the real gas properties and results in a much more uniform flow distribution throughout the impeller. The decrease in static temperature and pressure due to the acceleration of SCO2 at the tip of the main blade causes an increase in fluid density, reducing the tip leakage and secondary flow. Moreover, the influence on the flow in the impeller results in smaller incidence angle near hub side for the case with real gas properties, thus the separation on the suction surface which happens in the case with constant properties is alleviated. Therefore, the loss in both the impeller and diffuser is significantly reduced by the real gas properties.