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In this paper, the gas–liquid flow is comprehensively analyzed under different inlet gas void fractions, and the effect of tip leakage vortex (TLV) on the gas flow pattern in multiphase pumps is revealed. The results show that the gas flow pattern in an impeller is closely related to the centrifugal force, low-pressure region, and vortex motion. Most gas is present near the hub and suction surface of the blade as well as in the TLV. The two- and three-dimensional spatiotemporal evolution of the gas is presented, and the gas motion during the inception, development, and dissipation of TLV is revealed. It is reflected that the gas volume fraction is the highest at the TLV core and gradually weakens along the radial direction with the vortex core at the center. Additionally, the TLV energy dissipation is closely related to the gas and pressure difference, and strong energy dissipation occurs in the jet-wake flow.

The axial hydraulic thrust has great influence on the safety and stability of a pump turbine. A common way to balance hydraulic thrust is to install a pressure-balance pipe, and the change in pipe diameter is one of the important factors affecting axial hydraulic thrust. In this paper, the influence of the diameter changes in a pressure-balance pipe on axial hydraulic thrust of a pump turbine, plus the seal clearance flow, is studied and analyzed under three work conditions, i.e., 100%, 75%, and 50% loads. It is found that under 100% and 75% load conditions, the axial hydraulic thrust increases vertically with the increase in pipe diameter; whereas, under 50% load condition, the axial hydraulic thrust increases first and then decreases with the increase in pipe diameter. The results aim to give guidelines for the choice of pressure-balance pipe diameters and to control the axial hydraulic thrust of a pumped-storage power station, so that the hydraulic excitation force can be better matched with the hydraulic mechanism.

The S region on the full characteristic curve of pump-turbine is one of the important key features that affect its operation stability. In S region, one specific rotational speed value ( n11) corresponds to three specific flow rate values ( Q11), which causes the unit to fluctuate between different operation conditions. In this paper, focusing on the special region of pump-turbine, based on the combination of model experiment and numerical simulation, the S-region dynamic model of pump-turbine is established. The internal flow mechanism of pump-turbine is analyzed in detail from the perspective of Flow Energy Dissipation (FED). The results show that the high FED component is different at different operating points, but the flow energy maintain balance in the pump-turbine unit. High FED area displayed in near-wall or inter-channel of different components under different flow conditions. Besides, the generation of S region is related to the dynamic balance of energy especially in the no-load region. The analysis results of this paper provide theoretical basis for scientific and stable operation of pump-turbine.