Analysis of flow characteristics and pressure pulsation under vortex condition in impeller of Francis turbine

The blade passage vortex (BPV) is one of the crucial factors contributing to the instability of hydraulic turbine operation under low-load conditions. This study conducts full-flow passage numerical simulations based on the Navier-Stokes equations and the SST k – ω turbulence model to analyze the flow characteristics and pressure pulsation under low opening conditions for a Francis turbine. The results demonstrate that BPV is positioned between two adjacent blades near the suction surface side of the blade in the form of a spiral vortex. The backflow in the upper crown of the blades is the key factor in the formation of the BPV. The pressure pulsation inside the runner is related to both BPV and RSI phenomena. The BPV intensifies the nonuniform distribution of forces on the blade surfaces, resulting a periodic fluctuation of impeller radial forces. This study reveals the mechanism of BPV under small opening conditions, thereby providing a foundation for the optimal design of the runner.