Effect of Blade Scalloping on Performance of Inward Radial Flow Supercritical CO2 Turbines

Research on sub megawatt supercritical CO2 (sCO2) power blocks featuring single stage inward radial flow (IFR) turbines has gained momentum on account of growing demand for waste heat recovery applications. A key challenge in designing IFR turbines for sCO2 applications stems from substantial axial thrust on account of large pressure differentials across the blade flow passage and the impeller back face. Consequently, significant bearings forces ranging between 2 to 8 kN have been reported even in kilowatt-scale IFR turbines. Furthermore, high-pressure fluid trapped between the back face of the impeller and the casing causes severe disk friction losses impacting turbine efficiency. The situation is further aggravated by higher shaft speeds (10–30 krpm) resulting in high centrifugal stresses. This work investigates the effect of scalloping on the aerodynamic and structural performance of IFR turbine rotors for power scales varying between 100 kW to 5 MW designed with optimal thermodynamic conditions. Comparisons with un-scalloped impellers reveal substantial decrease in axial thrust varying between 35–57% at 100 kW scale and 37–65% for the 5 MW case. Scalloping is found to reduce leakage flow from 10–35% for 100 kW and 20–38% for the 5 MW case compared to the un-scalloped counterpart. The benefits of scalloped impellers are somewhat impaired due to a reduction in total-to-static efficiency of 3–6%, observed across all power scales. Structural and modal analysis limits the scallop depth to curtail maximum stresses and torsional natural frequencies within prescribed limits.