Internal vortex flow energy dissipation and spatiotemporal pressure pulsation during partial load operation of pump-turbine under power generation conditions

When the pump-turbine operates under partial-load generating conditions, the deviation between flow rate and blade design state can induce runner blade channel separation, rotating stall in the bladeless region, draft tube vortex, and complex non-uniform flow structures, resulting in increased energy loss, enhanced pressure pulsation, and reduced unit stability. Therefore, revealing the coupling mechanism among internal vortex flow, energy dissipation, and pressure pulsation is essential for improving the wide-load operation capability of pumped storage units. In this study, a reversible pump-turbine of a pumped storage power station is investigated, and a three-dimensional unsteady full-flow passage model including the volute, guide vane, stay vane, runner, draft tube, and leakage channels is established. Two typical generating conditions, 85% Pr and 40% Pr, are selected for comparative analysis. Based on the Shear Stress Transport k–ω turbulence model, the internal flow structures, energy loss characteristics, and pressure pulsation responses under different load conditions are investigated. The coupling between flow instability and irreversible energy dissipation is further analyzed using the Q criterion, local energy loss decomposition, and entropy generation theory. Meanwhile, multivariate variational mode decomposition (MVMD) combined with time-series imaging methods, including Markov transition field (MTF), Gramian angular field (GAF), and recurrence plot (RP), is employed to characterize the dynamic evolution of pressure pulsations. The results show that under 85% Pr condition, the runner outlet flow matches well with the guide vane, the draft tube velocity field remains stable, and the vortex structures are relatively weak, leading to a uniform pressure distribution and radial hydraulic load. When the load decreases to 40% Pr, increased incidence angle at the runner inlet intensifies flow separation and recirculation between blades, strengthens rotor–stator interaction in the vaneless region, and induces large-scale eccentric vortex structures in the draft tube. Under low-load conditions, high turbulent kinetic energy regions mainly occur near the blade trailing edge, runner outlet, vaneless region, and draft tube vortex zone, resulting in significant local energy dissipation. Entropy generation analysis indicates that turbulent dissipation and wall friction are the dominant sources of irreversible losses, and the high entropy generation regions show strong spatial consistency with vortex structures and pressure fluctuation sources. Pressure pulsation analysis demonstrates that low-frequency components and broadband modulation become more prominent at 40% Pr. MVMD effectively extracts different frequency components, while MTF, GAF, and RP reveal state transitions, periodic reconstruction, and nonlinear evolution characteristics of pressure signals. A multi-scale analysis framework integrating flow structure evolution, energy dissipation mechanisms, and pressure pulsation responses is established to clarify how internal vortex flows induce stability degradation under partial-load operation. The findings provide theoretical guidance for wide-load operation optimization and stability improvement of pump-turbine units.