Investigation into the Energy Performance of a Pump-Turbine Under High-Load Conditions: Energy Loss and Output Power Decline

Abstract

Pump-turbines often experience performance deterioration under high-load conditions beyond their best efficiency point, while the underlying flow mechanisms remain insufficiently understood. In this study, we investigate the relationship between internal flow structures and energy performance in a pump-turbine operating at a rated head of 202 m over a range of guide vane openings. Energy losses are evaluated using an average kinetic energy-based method and compared with an entropy production approach. A threshold-independent rigid vorticity method is adopted for vortex identification, and a streamline-based coordinate system is introduced for spatial quantification of energy loss and blade loading. The results show that hydraulic losses are mainly concentrated in the draft tube (66–75%) and runner (25–30%) under high-load conditions. A coupled vortex system formed by separation vortices and horseshoe vortices governs localized dissipation in the runner. In the draft tube, a columnar vortex rope generates strong shear layers that dominate energy loss in the cone and elbow regions. At high flow rates, negative incidence induces pressure-side separation, forming negative torque regions that reduce net runner torque and lead to output power deterioration. These findings highlight the dominant role of coupled vortex structures and pressure redistribution in performance degradation under high-load operation.