CFD Study on S-CO2 Compressor Boundary Layer Separation During Surge and Stall

Abstract The supercritical carbon dioxide (S-CO2) Brayton cycle represents a variant of the Brayton cycle that utilizes supercritical carbon dioxide as the working fluid. In this system, the compressor operates at inlet conditions close to the critical point of carbon dioxide; 7.38 MPa and 304 K. Near the critical point, S-CO2 has large density and specific heat capacity and small compressibility, which minimizes compression work. Accordingly, from the perspective of system size and efficiency, the S-CO2 cycle represents a promising candidate, and various application areas are currently under investigation. One of the fundamental components of the S-CO2 cycle that differs significantly from the traditional air or gas Brayton cycle is the compressor. In the S-CO2 cycle, the compressor operates closest to the critical point, which means that the compressor spins at high speeds under high pressure and high-density conditions, and the properties of the fluid passing through the compressor change rapidly. This is a key consideration in the part-load operation of the S-CO2 cycle, as the operational characteristics of the compressor play a significant factor in the analysis of instability caused by boundary layer separation at the compressor blades. The Korea Advanced Institute of Science and Technology (KAIST) has conducted a study on surge and stall conditions by testing a radial compressor in an Autonomous Brayton Cycle (ABC) test loop. In this study, computational fluid dynamics (CFD) was applied to simulate the experiments to investigate the boundary layer separation in the compressor under surge and stall conditions. The rotor used in the CFD analysis was based on a CAD model of an actual tested centrifugal compressor. The simulations were performed using Ansys CFX with S-CO2 properties taken directly from the NIST REFPROP database to ensure accuracy in terms of physical properties.