Balanced investigation of impeller geometric effects on energy conversion and hydraulic loss in a pump as turbine under pump and turbine modes

Abstract

Pump as turbine (PAT) is a key technology for industrial residual pressure-energy recovery. However, achieving satisfactory hydraulic performance under both pump and turbine operating modes remains challenging, primarily because the internal flow structure and energy conversion mechanism are altered during reverse operation, and the influence of geometric parameters on dual-mode performance remains unclear. In this study, the entropy production theory and vorticity transport analysis were combined to investigate the effects of geometric parameters on dual-mode hydraulic performance and flow mechanisms, elucidating the intrinsic relationship between vorticity generation and energy dissipation. The results indicate that the influence of blade geometric parameters on dual-mode performance is markedly asymmetric, which can be attributed to the reversal of the flow direction leading to changes in the velocity triangles. The blade wrap angle exhibits a strong positive correlation with efficiency in pump mode, whereas a negative correlation is observed in turbine mode. In pump mode, turbulent entropy production dominates, whereas wall entropy production increases in turbine mode. Vorticity transport analysis reveals that the Coriolis force term and the relative vorticity stretching term are the dominant mechanisms of vorticity generation, and their spatial distribution is consistent with that of the regions of high entropy production, indicating a strong correlation between vortex dynamics and irreversible energy dissipation. Velocity triangle analysis further demonstrates that the attack angle is the key parameter determining impact losses and separation losses. The parameter coordination design strategy proposed in this study provides a theoretical basis and quantitative reference for achieving efficient dual-mode operation of PAT.