Leading edge hub vortex-induced flow instability and energy loss in a tubular-flow fish-friendly pump

A kind of flow distortion is found in the design and application of the so-called tubular-flow fish-friendly pump, which possesses a highly twisted blade leading edge for reducing fish damage. It is named as leading edge hub vortex (LEHV) and has a closer relationship with flow instability and energy loss compared to the well-known tip leakage vortex (TLV). Numerical and experimental investigations are implemented in a typical tubular-flow fish-friendly pump, in terms of the evolution of LEHV in size and strength, as well as the vortex-induced low pressure and energy loss. The evolution of LEHV is divided into four stages, namely the inception, radial development, alternation of rotation direction, and vortex detachment. The vortex dynamics exhibit distinct periodicity with varying intensity and position due to flow fluctuation in the rotating pump. Further results on vortex-induced instability reveal that the volume with low pressure near the hub is much larger than that near the tip under multiple flow conditions, indicating that the cavitation inception is more likely to occur near the hub, which seems different from the common sense that cavitation appears initially at the blade tip. Moreover, it is found that the LEHV plays a more significant role in causing energy loss compared to TLV at low flow conditions, and the wake vortex formed by the interaction between LEHV and wake flow causes considerable energy loss near the impeller exit. This study will provide scientific guidance and optimization strategies for the design of a fish-friendly pump to improve hydraulic performance and stability.