Francis turbine, create by James B. Francis, is a water turbine named after its developer. This turbine uses the potential and kinetic energy of water and converts into mechanical energy through rotation of the axel. This turbine uses the axial and radial flow of water to convert energy as water enters the turbine through radial flow and leaves the turbine axially. Due to which it is also called mixed flow turbine. A simple schematic diagram of Francis turbine is shown in figure 1.
Figure 1: The schematic diagram of Francis turbine
This is a reaction turbine which works because of inward flow of water. The most important portion of this turbine is its runner which contains large number closely attached complex shaped blades. When water hit the blades radially it exerts pressure on both sides of the blades but due to the curve on the blade the force on one side of the blades remains higher than the force of other side of the blade. The resultant force is referred as the lift force. When water leave the blade axially it generate the impulse force due to the slightly outward shape of the blades at the bottom (bucket shaped end). The blade of a Francis turbine is shown in the figure 2.
Figure 2: Blade in the runner of Francis turbine
So the total force on the blades is
Total Force = Lift force + Impulse force
It means this turbine is not totally reaction turbine because a portion of force is generated through impulse force. The area spiral casing (as shown in figure 1) is not uniform. It is due to the fact that the speed of water decreases gradually and decreasing area of the spiral case can keep the speed of water uniform. This turbine is mostly used for the generation of electricity through water heads. Almost 60% of world’s electricity from hydro power plant is generated through this turbine because it can work effectively in a broad range of operating conditions. Figure 3 is showing how this turbine is used in the hydro power plant.
Figure 3: The installation of Francis turbine in hydro power plant
The turbine can work in the water heads ranging from 40 meters to 600 meters. The velocity of water between 10 to 700 meters square per second is suitable to produce few kilowatts to 800 MW of electrical energy. The turbine rotates the axel which in turn rotates the coils in magnetic field to produce electricity. There are two types of vanes, guide vanes and stay vanes, in the turbine. Stay vanes prevent the spiraling of water so that water reaches the runner smoothly. Guide vanes regulate the flow of water because the demand of electricity varies. Low flow rate of water produces less energy. It is important to let water out quickly. The outlet of this turbine has large area which keeps on increasing to avoid cavitations of water inside the turbine otherwise high flow of water can rapture the spiral cage and damage the whole turbine.