Effect of blade number on rotating stall mechanisms and energy dissipation in mixed-flow pump

Mixed-flow pumps are widely used as energy conversion machines but are susceptible to rotating stall under low-flow conditions, which leads to performance degradation and operational instability. In this study, mixed-flow pumps equipped with 3-, 4-, and 5-blade impellers are investigated numerically based on RANS and URANS formulations, employing the SST k–ω and SST–SAS turbulence models to resolve steady and unsteady rotating stall flows. The numerical approach is validated against external performance tests of the 4-blade pump. Rotating stall characteristics are analyzed in terms of three-dimensional flow structures, blade-tip velocity fields, vortex evolution, entropy production, spatiotemporal stall development, and pressure pulsations. The results show that the 3-blade impeller does not develop distinct rotating stall, whereas pronounced rotating stall occurs in the 4- and 5-blade impellers, manifested by saddle-shaped head–flow characteristics. Stall inception is highly sensitive to flow-rate reduction and is governed by blade-tip flow deviation and the interaction between tip leakage flow and trailing-edge stall vortices. Increasing blade number intensifies tip leakage interaction, resulting in more complex vortex structures and an earlier onset of rotating stall. Entropy production analysis reveals that turbulent dissipation dominates irreversible energy losses, with high-entropy regions concentrated around stall vortices and blade-tip wake structures. Under deep rotating stall, strong unsteady vortex evolution and dominant low-frequency pressure fluctuations are observed, particularly for the 5-blade impeller. These findings provide physical insights into blade-number-dependent stall mechanisms and energy dissipation characteristics in mixed-flow pumps.