Axial Force Balance of Supercritical CO2 Radial Inflow Turbine Impeller Through Backface Cavity Design

The supercritical CO2-based power cycle is very promising for its potentially higher efficiency and compactness compared to steam-based power cycle. Turbine is the critical component in the supercritical CO2-based cycle which delivers the power. Compared to the gas turbine or steam turbine of similar power output, the size of the supercritical CO2 radial turbine is much smaller and the axial force on the impeller is much larger. The load on the thrust bearing could be too heavy for long-term safe operation. Therefore, it is necessary to balance the axial force on the impeller through aerodynamic design to reduce the load on the thrust bearing. The impeller backface design with radial pump-out vanes proves to be an effective design to reduce the axial force on the impeller of radial turbomachinery, which is widely used in the pump industry. This work investigates the impeller backface cavity flow of a supercritical CO2 radial turbine and the application of the pump-out vanes to the impeller through computational fluid dynamics simulations. Design variations of the pump-out vane are presented and their performance variations are discussed from the view of viscous compressible fluid, instead of the commonly assumed inviscid incompressible fluid in the pump industry.