Tip leakage effects on near-tip power transfer and energy dissipation in an axial pump-as-turbine

To quantify the near-tip power-distribution reconfiguration and energy-dissipation characteristics induced by tip leakage in an axial pump-as-turbine (PAT) operating in pump mode, experimentally validated numerical simulations were performed on an axial PAT model. Tip leakage flow, mechanical energy transport, power distribution, and entropy production were analyzed under different flow coefficients and clearance ratios. The results show that pronounced tip leakage vortices and recirculation occur under low-flow rate conditions, whereas local reverse flow develops near the blade leading edge under high-flow rate conditions. Increasing the tip clearance expands the spatial extent of the leakage flow and tip leakage vortex and significantly enhances the forward mechanical energy input within the clearance. However, the specific energy carried by the leakage flow gradually decreases and subsequently exhibits reduced sensitivity to further clearance enlargement, indicating that the increased mechanical energy input is mainly driven by leakage-magnitude expansion. Power decomposition shows that near-tip energy input remains dominated by pressure power, whereas viscous shear power is more sensitive to clearance variation; the outermost 2% of the blade span is the most affected region. Within the investigated clearance range, the modeled local entropy production rate becomes less sensitive to further clearance enlargement beyond the intermediate-clearance range, indicating a gradual saturation tendency rather than a universal threshold. An additional open-tip mixed-flow PAT operating in pump mode exhibits consistent clearance-dependent trends. These results demonstrate the coupled responses of leakage-flow expansion, cross-clearance mechanical energy transport, near-tip wall-power reconfiguration, and associated local irreversible loss.