Water flows into the Francis turbine runner in a horizontal direction from the edge to the centre of the runner (in the radial manner), and then out of the runner in the downward direction. After entering into the runner, the water flow will cause the rotation of the runner through the blades along the axial core direction, and at the same time, will also push the runner to rotate in the downward direction due to the forces upon the blades. That is to say when the water flows work effectively through the blades both in radial and axial directions. That is the reason why it is also called a mixed-flow turbine.
Francis turbine is one of the most widely used hydraulic turbines in the world because of its high efficiency and wide water head range. The main components include spiral case, stay ring, distributor, top cover, runner, main shaft, guide bearing, bottom ring, draft tube, etc.
Francis turbine is applicable in the range of water head from 20 meters to 700 meters. It is simple in structure, stable and highly efficient, but usually, it is used in the middle range of water head (from 50 meters to 400 meters). The output of a single unit ranges from tens of kilowatts to hundreds of thousands of kilowatts. At present, the maximum output has reached 1 million kW.