The Francis Turbine is the most popular turbine of the reaction turbine. They are the most reliable hydroelectric plants. Hydropower plants often use Francis turbines. This represents approximately 60% of the world’s installed hydropower capacity, mainly because it can operate efficiently under a wide range of operating conditions. This turbine has less efficiency compared to a Kaplan turbine.
Working of Francis Turbine
In power plants, high-pressure water enters the turbine with swelling (tension). As the water passes through the pipe, this movement reduces the water pressure. However, the tide will remain constant. After passing the volute, the water pours at the desired angle onto the guide vanes and the paddle blades. As the water passes through the runner’s curved blades, the water shifts slightly to one side. This results in the loss of water in some rotational movements. The water also deflects in the axial direction so that the pull tube remains in the tail seat ring.
The tube reduces the speed of the water outlet to extract the maximum amount of energy from the incoming water. The process of rotating the water through the runner blades produces a force that pushes the blades in the opposite direction as the water is deflected. This reaction force (as we know it from Newton’s third law) causes energy to flow from the water to the turbine shaft and keep rotating. Since the turbine moves due to the reaction force, the Francis turbine is known as a reaction turbine.
The Francis turbine is the most popular turbine. These turbines are the most stable workhorses of hydroelectric power plants. Francis turbines provide approximately 60% of the world’s hydropower capacity, mainly because they can operate efficiently under a wide range of operating conditions.
Components of Francis Turbines
The Francis turbine consists of the following main components:
1) Spiral Casing
The chamber around the turbine impeller is called a volute casing or spiral casing. Along its length, there are many regular openings for the working fluid to affect the runner blades. These nozzles convert the pressure force of the fluid into the force that moves before the fluid reaches the blades. Although there are many openings for the fluid inlet to the blades, it still maintains its velocity because the cross-sectional area of the chamber decreases uniformly along the perimeter.
2) Guide Vanes
The main function of guide vanes is to change the pressure force of the fluid into the force of movement. It is also used to direct the flow to the runner blades with the design angle.
3) Impeller Blade
The impeller blade is the heart of every turbine. These are the centers of the impact of the fluid, and the force affecting the impact of the turbine shaft rotation creates torque. Great attention should be paid to the design of the entry and exit angles of the blade because these are the main parameters that affect power generation.
4) Draft Tube
This is a most important component of the Francis turbine. A draft tube is a tube that connects to the outlet of the channel and the back of the seat ring through which the turbine water drains. Its main function is to reduce the water discharge rate to reduce the energy loss moving towards the outlet. This allows the turbine to move over the tailwater without significantly reducing the available height.
5) Cavitation
Cavitation is a major problem in the Francis turbine, which negatively affects their performance and can cause damage. Cavitation is the phenomenon of corrosion on the metal surface of turbine components due to cavitation. Reverse high-speed turbines operate under low and medium loads and operate at different pressures, which are subject to cavitation.
Cavitation in hydraulic machines can affect their performance and cause serious damage. These injuries can be summarized as follows:
- Erosion of materials in turbine components
- The angle of the blade is deformed.
- Loss of performance due to deformation or landslide.
The efficiency of the Francis turbine
Turbine performance and efficiency depend on the design of the impeller blades. The impeller blade divides into two parts in Francis turbinate lower part is like a small cube, and the turbine turns due to the impact of the water. The upper part of the blade uses the reaction force of running water inside it. The two forces will work together to transform the runner.
The water must reach all the blades evenly for these turbines to function effectively, and a turbine screw controls the flow rate. This type of packaging is called spiral packaging. The chamber sends water through a series of stationary valves and blades to the moving blades of the turbine rotor. Properly designed, Francis turbines can absorb between 90 and 95 percent of the energy in the water.
