Energy loss mechanism due to tip leakage flow of axial flow pump as turbine under various operating conditions

Owing to its ability to handle large flows, an axial flow pump as turbine (PAT) can generate considerable amounts of electricity in small-scale hydropower plants. However, a PAT's efficiency can be hindered by tip leakage flow (TLF), namely, flow through the clearance between the impeller blade tip and shroud. Accordingly, this study investigates the influences of TLF on the PAT's energy performance through numerical simulations in which the entropy production method has been adopted. TLF and the associated tip leakage vortex (TLV) are found to both decrease the hydraulic efficiency and increase the flow rate; the shaft power output is also affected, especially near the machine's best efficiency point. The effect of TLF on the pressure distribution along the blade depends on the flow conditions, and the form of the TLV directly generated by TLF is affected by the flow incidence angle. The vorticity transport equation reveals that the vortex stretching term plays a dominant role in the spatial evolution of the TLV and has the greatest impact on the pressure distribution. Finally, different operating conditions lead to different energy loss mechanisms: turbulent dissipation is the main cause of energy loss, and high flow conditions are marked by an increase in TLF-dependent wall shear stress dissipation.