Investigation on the characteristics and formation mechanism of backflow in S-shaped region of a reversible pump-turbine

When reversible pump turbines go through the S-shape region, there is some significant backflow regions in the vaneless space, accompanied by severe pressure fluctuation. However, the internal correlation and mutual influence mechanism among pressure fluctuation, turbulent vortex structure, and backflow are still unclear enough in current research. In this work, the backflow characteristics and formation mechanism of pump turbine in the S-shape region are figured out by using computational fluid dynamics methods. The results indicate that the Leading-Edge-Passage-Vortex caused by adverse pressure gradient is dominant in the initial stage of the backflow. In this stage, the interaction between the inflow fluid and the backflow fluid is the main reason causing the high amplitude and wideband pressure fluctuation components in the vaneless space. In the stage of backflow development, the adverse pressure gradient and centrifugal force play dominant roles in different backflow regions, respectively. The rotating stall phenomenon appears in the runner and a significant low-frequency component 0.6fn and 0.3fn∼0.45fn appeared, respectively, in the vaneless space and runner. In the fully developed stage, the backflow is dominated by the centrifugal force of the runner. The backflow region is concentrated in the main flow area of the runner inlet, while the inflow region is pushed out to the upper crown and lower ring. These findings can provide the theoretical supports for improving the stability of pump turbine operating in off-design condition.