Flow mechanism and back gap windage loss of a sCO2 radial inflow turbine with impeller scallops

The Internal flow mechanisms and windage loss in impeller back gap of a supercritical carbon dioxide (sCO₂) radial inflow turbine with scallops are comprehensively investigated in this paper. The study emphasizes the effects of scallop depth and leakage outlet pressure. The results indicate that scallop structures lead to a degradation in overall turbine performance. Under design conditions, a turbine with a scallop depth ratio of 0.5 exhibits a 3.7 % reduction in efficiency and a 3.4 % decrease in total power compared to no scallop configuration. Furthermore, as scallop depth increases, the skin friction coefficient decreases in the disk gap while it increases for the seal in the impeller back gap. Increasing leakage outlet pressure reduces the leakage flow rate and skin friction coefficient. Fitted models for skin friction coefficient are proposed respectively. The conclusions providing valuable insights for designing and optimizing sCO₂ radial inflow turbines with scallops.