Numerical Investigation of the Axial Thrust Load of a Prototype Radial Turbine for Supercritical CO2 Cycles

Abstract Radial turbines enable flexible operation at high stage expansion ratios in supercritical CO2 based power cycles for waste heat recovery or thermal energy storage. However, due to the pressure difference between the front and back side of the wheel, they are exposed to high axial thrusts. The net axial thrust and the flow field of a recently developed radial turbine for the operation in energy storage systems using either dry air or supercritical CO2 as working fluid are investigated. The influence of the working fluid on the operating behavior of the stage is discussed by keeping the expansion ratio and the circumferential Mach number constant. To assess the performance of the covered impeller and to predict the static pressure distributions on the wheel surfaces, three-dimensional RANS simulations are conducted with an in-house density based CFD solver using a Spline-Based Table Look-Up Method (SBTL) for an efficient evaluation of the SpanWagner reference equation of state. In contrast to the majority of existing literature, a covered rotor design was chosen in order to minimize tip leakage losses and to yield an impeller, which is less susceptible to vibration-induced fatigue under flexible load conditions. Assessment of isentropic efficiency demonstrates reasonable performance over the desired operating range, even at part-load conditions. At the investigated design conditions, the compressibility factor ranges between 0.77 and 0.83. It was shown, that next to the labyrinth seals, the presence of the cover disc leads to a considerable reduction in net thrust.