Abstract: The characteristics of the water ring in a pump turbine are closely related to its stable operation under phase control conditions and the cooling of its clearance flow channel. Based on the discharge path of cooling water through the water ring drain pipe, this paper develops a full-passage 3-D model of the flow in a pump turbine including the local gaps, and examines the unsteady 3-D flows through VOF gas-liquid two-phase flow modeling. We focus on the drain valve’s two conditions of normal operation and failure, and have achieved the numerical simulations of pressure variations that agree well with the field measurements at a hydropower station. The results reveal the internal flow characteristics of the pump turbine during its phasing operation. The main runner channels are occupied fully by air, and its flow tracing shows formation of significant vortices; in the passage of bladeless areas, gas-liquid mixture flows are observed, and the liquid near the inner wall of the fixed guide vanes rotates at a higspeed leaving circular traces; the lower ring cavity features an upward helical flow of gas-liquid mixture. In the mode of the drain valve open, the thicker water ring in the bladeless area can be discharged into the tail pipe under its control to ensure a dynamic balance between water ring drainage and cooling water supply. If the drain valve fails, cooling water in the lower labyrinth will flow into the bladeless area, unable to enter the tail pipe, so that the water ring in this area grows in thickness and rubs the rotor due to its over-thickness and the water-filled clearance between the lower ring and the bottom ring of the rotor.

Key words: pump turbine, phase modulation condition, water ring, water ring drainage valve, fault conditions