The Francis turbine is a combo of a reaction turbine and impulse turbines. It uses the impulse for and reaction force of the water flowing through the blades to spin and produce electricity more efficiently. These types of turbines are utilized to generate electricity in hydropower plants. These turbines have two operating concepts (axial and radial flow) on which they work. The Francis turbine was invented by an American civil engineer named James B. Francis. It is known as Francis turbine because of the name of its inventor. This article describes the principles of operation, key components, and applications of the Francis turbines.
The Francis turbines are most effective because of the impeller blades design, which rotate due to the reaction and impulsive forces of the water flowing by the blades. By using this type of turbine, the turbine utilizes both kinetic and potential energy to generate electricity, thereby eliminating the primary problem of head availability. For this reason, this turbine also called a mixed flow turbine.
How Does a Francis Turbine Work?
A Francis turbine is a combo of the impulse and reaction turbines. It has an impeller, blades, nozzle and casing. A Francis turbine works on the following way:
- Firstly, the water introduces from the penstock into the turbine nozzle.
- As the water passes through the nozzle, it transforms water potential energy into kinetic energy and sends the water toward the impeller.
- As the water hits the impeller blades. The impeller of the turbine is connected with an electric motor via a shaft. The impeller rotates according to the movement of the shaft.
- These blades are fixed and mounted on the impeller. These rotate with the rotation of the impeller.
- The stay blades remove the swirls from the water which produce due to water flow inside the casing so that water can flow more linearly.
- For maintaining the flow pressure, the volute casing has a reducing diameter.
- As the water strikes the blades of the impeller, these convert the kinetic energy into speed.
- The blades of the Francis turbine divide into two sections: the lower half section uses an impulse force of the water for the turbine rotation while top section of the turbine uses reaction force of the water.
- The blades of the impeller use both kinetic and potential energy of the water to the impeller most efficiently.
- As the water discharges from the impeller, it strikes the turbine and produces rotation of the turbine.
- The turbine is connected with an electric generator through a crankshaft
- As the turbine rotates, it also rotates the shaft which further rotates the generator coil. In this way, electricity produces by a Francis turbine.
Components of the Francis Turbine
A Francis turbine has the following components:
1) Volute Casing
The volute casing is a most major component of the Francis turbine. The water that flows from reservoirs flows by the volute casing with high pressure. The turbine blades are arranged in a circle. This means that the water that hits the turbine blades must flow along a circular axis in order to achieve an efficient impact. Therefore, spiral casing uses, but the pressure is lost due to the water circular motion. To maintain the same pressure, gradually decrease the casing diameter to keep the pressure steady and the moment or speed of the colliding blades steady.
2) Stay Blades
The guide blades and stay blades direct the water into the impeller blades. When water enters the impeller blade, it blocks the blade to reduce the water vortex caused by the radial flow. This makes the turbine more efficient.
3) Guide Blades
These ae not stationary components. These vary their angles as needed to control the angle of impact of the water on the turbine blades and increase productivity. The guide blade also regulates the flow of water to the impeller blades and controls the turbine power according to the turbine load.
4) Impeller blade
The turbine efficiency and power depend on the design of the impeller blade. In Francis turbines, the impeller blade is divided into two sections. The lower portion of the blade uses the impact of the water to turn the turbine. The top portion of the blade uses the reaction force of the water flowing through the blade. Together these two forces turn the impeller.
Francis Turbine Advantages and Disadvantages:
The Francis turbine has following advantages:
- These turbines have no head failure
- The mechanical efficiency of the Francis turbine reduces more slowly compared to Pelton wheel turbine.
- The working head of these turbines can control more easily compared to other turbines.
- It has small size of the impeller
- The change in the head can control easily.
The Francis turbine has the below given disadvantages:
- The water contains pollutants which may cause extremely rapid wear in a Francis turbine.
- These turbines have high price
- These have complex design
- There are many chances of water hammering
- The Francis turbine has high maintenance cost
- Its maintenance is very complicated
- It has cavitation problems