Abstract: The flow features in the interior of in Francis turbine, especially in the draft tube are studied . The main aim is to find the cause of inducing the hydraulic instability in Francis turbine. The flow features in each flow passage are shown. In inducer components, pressure fluctuations of nose of spiral case are anomalous and pressure distributions of spiral case outlet are asymmetric on different openings at the same unit speed. But circumferential pressure distributions are symmetrical at stay vane outlet and guide vane outlet. Under the small opening and low unit speed, due to minor guide vane outlet angle, the impinging streams at the head of blade are severe. And there is the flow pattern of cross-flow from hub to shroud at blade inlet. Interblade vortex can also be visible. And there are big back flow regions and cross-flow at blade outlet near crown. When the opening becomes bigger, the flow conditions at runner outlet near crown and inlet are improved. In draft tube, with small opening, there are also serious back flows in the center of conical tube and main flow is located at regions near the wall. Due to the effect of elbow tube, the two vortex regions are formed in conical and elbow tube. It will augment the intensity of vortex in conical tube and back flow below the runner cone. And then the runner outlet flow will be disturbed. The mainstream flows through the left side of pier and the flow state on the right side of pier is disordered. With optimal opening, the flow state at runner outlet and in draft tube is good. And the flow flieds on both sides of pier is identical. And with bigger opening, the flow flieds at runner outlet are improved and the main stream is located at the center of conical tube. Some local vortexes appear on account of the turn effect of elbow tube. They impede the upstream flow. And the flow flied at the right side of pier is good and the flow state on the left side of pier is disordered. The effect of pier is also distinct. This results provide theoretical basis for the selection of measurement position of pressure pulusation.

Key words: Draft tube vortex rope, Francis turbine, Numerical simulation, Steady flow