To address the unclear flow regime variations and energy loss mechanisms of pumped-storage units operating under variable loads with different guide vane openings in pump mode, this paper conducts a refined quantitative investigation into the internal flow field structures and energy dissipation mechanisms of a pump-turbine under three typical guide vane openings. The methodology is based on the SST k-ω turbulence model and entropy production theory. The results demonstrate that the guide vane is the most critical component inducing the macroscopic head loss of the unit, and its energy dissipation is quantitatively dominated by turbulent entropy production induced by mainstream vortex dynamics, while localized direct viscous dissipation intensifies sharply near high-shear throat boundaries under throttled regimes. In the low-flow-rate region, large-scale high-entropy production areas emerge across all openings due to the incidence angle mismatch. As the flow rate increases, these high-dissipation areas gradually shrink. Upon entering the high-flow-rate region, the high-entropy production area at the leading edge of the guide vanes under the small opening condition dramatically rebounds and expands, which is attributed to the severe flow blockage effect. In contrast, under the large opening condition, the high-entropy production regions are essentially eliminated owing to the superior flow passage capacity. This study systematically analyzes the spatial distribution and variation laws of irreversible energy losses across a wide operating range with multiple openings, revealing the intrinsic correlation between internal flow regimes and microscopic thermodynamic dissipation. The findings provide a crucial theoretical reference for the wide-load efficient operation scheduling and anti-stall hydraulic optimization design of pumped-storage units.

High-head reversible pump-turbines frequently suffer from hump-instability under pump-mode operation, and inherent performance trade-offs exist between pump-mode and turbine-mode characteristics for bidirectional operating requirements. The 6+6 long-short-blade runner is regarded as a promising candidate for mitigating pump-mode hump-instability. Taking a high-head pumped-storage pump-turbine as the research object, this study follows the above-mentioned engineering principle. A full-passage CFD model including volute, stay-ring, guide vanes, runner and draft-tube is established with the SST k-ω turbulence model. Numerical methods are validated against model-test external-characteristic data. Steady and unsteady simulations are carried out for the prototype 9-blade runner and the 6+6 long-short-blade runner. The results show that, under pump-mode operation, the 6+6 configuration shifts the hump-instability region toward lower mass-flow rates away from the unit’s primary operating range. Short blades divide flow passages, suppress large-scale channel vortices within the hump zone, and alleviate flow asymmetry in the vaneless space. Nevertheless, local impingement and secondary-flow losses occur at long-short-blade junctions, and short-blade leading edges bear potential cavitation risk. This configuration only shifts the instability range rather than completely eliminating hump-instability. A typical 50% partial-load turbine-mode condition is selected for constraint-based verification. The 6+6 runner reduces pressure-pulsation amplitudes at draft-tube monitoring points by 25.52% and 19.83%, which suppresses occasional large-amplitude pressure fluctuations and improves signal periodic consistency. The novelty of this work lies in the quantitative revelation of such trade-off characteristics under the pump-priority-design philosophy for a 655 m high-head unit. It demonstrates that the 6+6 long-short-blade scheme is not a universal optimization measure.