Steady State CFD Investigation of a Radial Compressor Operating With Supercritical CO2

Abstract

The supercritical closed Brayton cycle concept is widely recognized as an attractive new option for energy conversion because of the very high-efficiency, reachable at moderate turbine inlet temperature, and the very compact general assembly. Carbon dioxide is chosen as the working fluid because it allows for its compression to occur close to the critical point at suitable temperatures, and high pressure. Compression work is thus small, if compared for instance to air compression. The concept was first studied in the sixties, and recent interest spreading in the scientific and technical community led to the realization of a small-scale proof-of-concept prototype operating at Sandia’s National Laboratories. Moreover, the CSP SunShot project was recently funded by the U.S. National Renewable Energy Laboratory, and it is aimed at the realization of a multi-megawatt concentrating solar power plant, whereby the power block will be a supercritical CO2 Brayton cycle turbine. Other pre-commercial activities are ongoing. This paper focuses on the study of the fluid dynamics of turbomachinery operating with fluids characterized by a complex thermodynamic behavior. The goal is to develop a complete methodology to help the aerodynamic design of scaled-up turbomachinery for supercritical CO2 gas turbine power plants. Starting from a previous analysis of the impeller of the radial compressor of the Sandia proof-of-concept test bench, the new detailed computational domain includes the tip clearance and the vaned diffuser, and has been obtained using an in-house meshing tool suited for turbomachinery geometries. The steady state interface between the impeller and the diffuser is treated with a mixing-plane. In order to correctly calculate the thermophysical properties of the fluid, affected by strong variations close to the critical point, the solver is coupled with an extensive library for the computation of properties of pure fluids and mixtures. An accurate multiparameter equation of state is selected and a look-up table approach is used to speed up the fluid properties evaluation. The results are finally compared with experimental data, and demonstrate the potential of the tool.