Rotating stall mechanism and pressure pulsation evolution of a mixed-flow pump-turbine in pump mode for pumped-storage hydropower plants

Rotating stall in mixed-flow pump-turbines during pumping operation poses a serious challenge to the operational stability of pumped-storage power plants. However, the spanwise development of rotating stall and its coupling with entropy production redistribution and pressure pulsation evolution remain insufficiently clarified. This study addresses this gap by integrating unsteady flow field analysis, entropy production evaluation, and pressure pulsation analysis based on power spectral density (PSD) and continuous wavelet transform (CWT) in a guide vane mixed-flow pump-turbine. The results show that stall structures first appear near the trailing-edge separation region on the suction side of shroud-side blades and then expand radially and circumferentially as the flow rate decreases. High entropy production regions are spatially consistent with high-vorticity stall structures, indicating that stall vortex migration, blade trailing-edge separation, and impeller inlet shear layers dominate irreversible energy loss. Pressure pulsations evolve from shaft-frequency dominance to low-frequency rotating stall dominance and finally to multiscale coupling with rotor-stator interaction. The main contribution of this study is to link impeller spanwise stall development, entropy production redistribution, and pressure pulsation mode transition, providing a physical basis for stall assessment based on pressure pulsations in pump mode.