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Francis Turbine is a combination of both impulse and reaction turbine, where the blades rotate using both reaction and impulse force of water flowing through them producing electricity more efficiently. Francis turbine is used for the production of electricity in hydro power stations. Majorly there are 2 turbines flow patterns on which they work, namely radial and axial flow concepts. An American civil engineer by name, James B. Francis in Lowell, Massachusetts comes up with an idea of combining both impulse and reaction turbine where water enters the turbine radically and exits axially. The working principle, main components and its application is discussed in this article.
Fig 1: Components of Francis Turbine
Contents:
Main Components of Francis Turbine
The major components of Francis turbine are1. Spiral Casing
Spiral casing is the inlet medium of water to the turbine. The water flowing from the reservoir or dam is made to pass through this pipe with high pressure. The blades of the turbines are circularly placed, which mean the water striking the turbines blades should flow in the circular axis for efficient striking. So the spiral casing is used, but due to circular movement of the water, it looses its pressure. To maintain the same pressure the diameter of the casing is gradually reduced, so as to maintain the pressure uniform, thus uniform momentum or velocity striking the runner blades.2. Stay VanesÂ
Stay vanes and guide vanes guides the water to the runner blades. Stay vanes remain stationary at their position and reduces the swirling of water due to radial flow, as it enters the runner blades. Thus making turbine more efficient.
Fig 2 : Stay Vanes and Guide Vanes of Francis Turbine.
3. Guide Vanes
Guide vanes are not stationary, they change their angle as per the requirement to control the angle of striking of water to turbine blades to increase the efficiency. They also regulate the flow rate of water into the runner blades thus controlling the power output of a turbine according to the load on the turbine.4. Runner Blades
The performance and efficiency of the turbine is dependent on the design of the runner blades. In a Francis turbine, runner blades are divided into 2 parts. The lower half is made in the shape of small bucket so that it uses the impulse action of water to rotate the turbine. The upper part of the blades use the reaction force of water flowing through it. These two forces together makes the runner to rotate.5. Draft Tube
The pressure at the exit of the runner of Reaction Turbine is generally less than atmospheric pressure. The water at exit cannot be directly discharged to the tail race. A tube or pipe of gradually increasing area is used for discharging water from the exit of turbine to the tail race. This tube of increasing area is called Draft Tube. One end of the tube is connected to the outlet of runner while the other end is sub-merged below the level of water in the tail-race.How Does a Francis Turbine Work?
The water is allowed to enter the spiral casing of the turbine, which lead the water through the stay vanes and guide vanes. The spiral case is kept in decreasing diameter so as to maintain the flow pressure. The stay vanes being stationary at their place, removes the swirls from the water, which are generated due to flow through spiral casing and tries it to make the flow of water more linear to be deflected by adjustable guide vanes. The angle of guide vanes decides the angle of attack of water at the runner blades thus make sure the output of the turbine. The runner blades are stationary and can-not pitch or change their angle so it’s all about the guide vanes which controls the power output of a turbine. The performance and efficiency of the turbine is dependent on the design of the runner blades. In a Francis turbine, runner blades are divided into 2 parts. The lower half is made in the shape of small bucket so that it uses the impulse action of water to rotate the turbine. The upper part of the blades use the reaction force of water flowing through it. Thus runner blades make use of both pressure energy and kinetic energy of water and rotates the runner in most efficient way. The water coming out of runner blades would lack both the kinetic energy and pressure energy, so we use the draft tube to recover the pressure as it advances towards tail race, but still we cannot recover the pressure to that extent that we can stop air to enter into the runner housing thus causing cavitation.
Fig 3: External Francis Turbine attached to a dynamo.
Applications of Francis Turbine
- Francis turbine is the most widely used turbine in hydro-power plants to generate electricity.
- Mixed flow turbine is also used in irrigation water pumping sets to pump water from ground for irrigation.
- It is efficient over a wide range of water head and flow rate.
- It is most efficient hydro-turbine we have till date.
