To investigate the flow behaviors and energy loss mechanisms of a pumped storage unit operating in the S-characteristic zone

this study focuses on a model pump-turbine with a rated head of 451 m. Unsteady flows across the full flow passage are simulated numerically for the operating conditions in turbine

runaway

and braking modes; Distribution patterns and dominant mechanisms of energy losses are revealed by integrating an analysis of internal flow and the entropy production calculated using an improved near-wall loss function. The results indicate that near-wall entropy production accounts for 21.7% to 26.8% of the total

obviously a significant contribution; the major production

however

comes from the mainstream. The runner section contributes most of the total production

or in the 45% - 51% range in the three typical modes

establishing it as the core region for energy dissipation and hydraulic excitation in S-zone operation. Key flow structures responsible for flow instability and increased energy loss include runner inlet backflows

blade channel vortices

and draft tube vortices. This study lays a basis for future studies of stable S-zone operation and hydraulic optimization of pumped storage units.