Blade rotation angle on energy performance and tip leakage vortex in a mixed flow pump as turbine at pump mode

The energy performance and tip leakage vortex in a mixed flow pump as turbine (PAT) at pump mode is investigated by numerical method validated by experiment measurement. With the blade rotation angle α increasing, the head drops at pump mode, but it rises at turbine mode. Meanwhile, the best efficiency point shifts towards a larger flow rate at both modes. The tip leakage vortex (TLV) evolution is categorized into three stages, namely the splitting stage, the developing stage and the merging stage in an impeller rotation period at both α = −4° and α = 4°. An intensive oscillating characteristic of the TLV is observed at α = −4°, with an oscillation frequency of 8.2 fi (fi is the impeller rotation frequency). The analysis based on the vorticity transportation equation illuminates that the relative vortex stretching item related to velocity gradient, especially its component in axial direction, dominates the splitting, stretching and merging processes of the primary tip leakage vortex (PTLV). Furthermore, the impeller rotation has a crucial effect on the secondary tip leakage vortex (STLV) evolution, and further contributes to the formation and merging process of the vortexes. The TLV oscillation characteristic occurring at α = −4° generates a severe impact on pressure fluctuation in the impeller, which is almost 10 times that at α = 4°.