Abstract: 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.

Key words: pumped storage unit, S-characteristic zone, flow characteristics, energy loss, numerical simulation