Analysis of Flow Characteristics in Hump Region of Pump Turbine in Pump Mode

Abstract

The hump region is inevitable when the pump turbine operates in the pump mode, which poses a threat to the safe and stable operation of the unit. The prototype pump turbine is selected as the research object and the numerical simulation method is used to analyze the flow characteristics inside the runner and guide vanes. The results show that the relative flow angle irregular fluctuations at the blade leading edge result in the formation of a positive attack angle under low discharge conditions, leading to the occurrence of flow separation. The development of separation vortex causes rotational stall inside the flow channel, blocking the flow passage. In severe cases, backflow vortex forms in the blade outlet leads to water flow back into the runner. The stall vortex also appears in the double row cascade, which not only blocked the water, but also caused the water flow that had already passed through the guide vanes and stay vanes to form a backflow in the other channels. The rotational stall inside the runner and guide vanes deteriorates the flow field, resulting in a significantly increase in hydraulic losses, which is the main reason for the head decrease in the hump region.