Impeller scallops are intentional cuts in the hub and shroud surfaces at the exit of centrifugal impellers that are intended to prevent high cycle fatigue in these areas. The goal of this analysis is to determine what effect the scallops have on aerodynamic performance, rotordynamic stability, rotating stall criteria, axial thrust, and mechanical shaft power. Furthermore, this study improves the understanding of the interaction with the secondary passage and determines the effect on the flow field entering the diffuser. A 3D viscous CFD model is generated of a single blade passage that couples the primary flow passage with the secondary shroud passage. This approach captures the complex interaction that occurs at the impeller exit due to the presence of the scallops. The predictions show a small penalty in thermodynamic efficiency with the presence of the scallops compared to a baseline case. The results also show a moderate increase in shaft horsepower calculated by integrating static pressure and shear stress acting on the impeller and shroud surfaces. Circumferential swirl in the shroud passage shows only modest increases with the addition of the scallops. Therefore, rotordynamic performance is not significantly affected nor is axial thrust acting on the impeller. The predictions demonstrate the effect the scallops have on the flow field entering the diffuser including variations in flow angle at the impeller exit.

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