Effect of pressure-balance pipes on the axial hydraulic thrust of a reversible pump-turbine under the runaway condition

Axial hydraulic thrust is a crucial factor contributing to the unstable operation of the pump-turbine especially under off-design conditions. Pressure-balance pipe can effectively achieve axial hydraulic thrust balance of the pump-turbine. In this study, the impact of diameter and location of the pressure-balance pipe on the axial hydraulic thrust under the runaway condition is investigated. The influence mechanism of pressure distribution and internal flow patterns on the axial hydraulic thrust is further explored. The results show that the location of the pipes have a relatively minor impact on the axial hydraulic thrust while the increase in the pipe diameter causes a rise of the axial hydraulic thrust. Compared with the outer pressure-balance pipe, the inner pressure pipe demonstrates a more significant effect on the balance of axial force of the pump-turbine runner. Additionally, low-frequency components at 0.6 times runner rotational frequency are observed, which indicate the occurrence of rotational stall in vaneless area. The variation in pipe diameter also affects the number of runner passages with vortices, as well as the quantity of vortices. With the increase in pipe diameter, backflow phenomena and negative pressure regions gradually emerge in the pressure-balance pipe, which result in more chaotic flow.