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

Abstract

Abstract. Research on submegawatt supercritical CO2 (sCO2) power blocks featuring single inward flow radial (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 high axial thrust because of large pressure differentials across the blade flow passage and the impeller backface. Consequently, significant bearing forces ranging between 2 and 8 kN have been reported even in kilowatt-scale IFR turbines. Furthermore, high-pressure fluid trapped between the backface 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 impellers for power scales varying between 100 kW and 5 MW. Comparisons with unscalloped impellers reveal a substantial decrease in axial thrust varying between 35% and 57% at 100 kW scale and 37–65% for the 5 MW case. Scalloping reduces leakage flow by 10–35% for 100 kW and 20–38% for the 5 MW case, compared to the unscalloped counterpart. The benefits of scalloped impellers are somewhat impaired due to a reduction in total-to-static efficiency by 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.