Quantitative analysis of energy dissipation mechanisms and vortex evolution in an axial flow pump-as-turbine based on energy loss mechanism and mode decomposition

Unsteady flow phenomena, especially in the blade tip region, play a critical role in the energy loss and performance degradation of axial-flow pumps operating as turbines. In this study, turbulence-related energy loss mechanisms in the axial-flow pump operating as turbine are systematically investigated using numerical simulations. Entropy production theory combined with the energy balance equation is employed to quantify the contributions of different energy loss terms. The results show that turbulence-related mechanisms dominate the energy loss in most components of the AFPAT, where the turbulent kinetic energy diffusion term and the turbulence production term together account for more than 60% of the overall dissipation. In contrast, the inlet section exhibits relatively weak turbulence activity and is mainly influenced by wall-related losses. High-loss regions are found to be strongly associated with complex vortex structures in the blade tip clearance. Time-resolved analysis further reveals a pronounced phase-dependent modulation of turbulence-related energy loss within each blade-passing cycle. To elucidate the underlying dynamic mechanisms, dynamic mode decomposition is applied to the Reynolds shear stress field. A low-frequency mode synchronized with the shaft rotation (2.09 Hz) is identified as the dominant energy-modulating mode, exhibiting a strong temporal correlation coefficient of 0.96 with the energy loss evolution. These findings deepen the understanding of unsteady turbulence-induced energy losses in axial-flow pumps operating as turbines and provide a theoretical basis for blade tip clearance optimization and stable low-head energy recovery.