Francis Turbine and its Working.

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Francis Turbine 

A Francis turbine is a type of water turbine. Francis turbine was developed by an American engineer James Bichens Francis around 1855.  In the Francis flow enters the turbine is radial directions flowing ti its axis, but after striking and interacting with the turbine blades its along the direction of that axis. Due to this reason it is also known as mixed flow turbine. In order for it to operate efficiently, water must reach at all blades equally and flow a controlled by the casing which curl by the turbine in a spiral shape . this shape is called simply the spiral or volute casing. the casing feed water through a set of valves and fixed blades of the turbine rotor.

The blades of a Francis turbine rotor are carefully shaped to extract the maximum amount of  energy to extract the maximum amount of energy from the water flowing through it. water should flow smoothly through the turbine for best efficiency. the force extracted by the water on the blades cause the turbine to spin and the rotation is converted into electrical energy by the help of generator.

Francis turbine is the most comgmon water turbine and it is use today.In this turbine for operate the water head 40 to 600 m  and are used primarily for electric power production. When well designed, the Francis turbine can be captured 90 to 95% of the energy in the water. While much of the energy captured is through reaction to the pressure of the water ,there is also an important element of  impulse transfer to the turbine blades to in consequence of the kinetic energy of the motion of the water. the motion is generated by the spiral casing and the gate that feed the water into the turbine.

Application of Francis turbine
  • Francis turbine is the most widely used turbine in hydro power plant to generate electricity.
  • It is also used in irrigating water pumping sets to pump water from ground for irrigation.
  • Electricity production can be estimated with the help of flow rate and head.
  • It is efficient over a wide range of water head and flow rates.
  • It is most efficient hydro turbine we have till date.

Main components of Francis turbine

There are the various components of the Francis turbine are as follows-

1. Spiral Casing : 
The design of spiral casing makes the uniformly decreasing cross section area, along the circumference. Its decreasing cross section area make sure that it have a uniform velocity of the water striking the runner blades , it have opening for the water flow into the runner blades from the very starting of the casing , so flow  rate would decrease at it travels along the casing. So we reduce its cross section area along its circumferences to make pressure uniform , thus the uniform momentum or velocity striking the runner blades.

2. Stay vans and Guide vanes 
Stay Vanes and guide vanes guide the water to the runner blades. 
Stay vanes remain stationary at their positions and reduce the swirling due to radial flow, as it enter the radial flow. water are passing through the stay vans.
Guide vanes can change the angle thus can control the angle of attack of  water to the runner blades making them more efficient , it also regulate the flow rate of water into the runner blades thus controlling the power output of the turbine according to load on the turbine.

3.Draft Tube :
It is gradually expanding tube which discharge the water passing through the runner to the tailrace.
Its cross section area increase along along its length , as the water coming out of runner blades is at considerably at low pressure , so its expending the cross section area help to it recover the pressure as it flows towords tailrace.

4. Runner blades: 
Design of runner blades decides how well a turbine is going to perform. so runner blades of mixed flow turbine can we dived into two parts. One is upper part of the blades which is used the reaction force of the water flowing through it, and the other second is lower pats is in a shape of small buckets using the impulse action of the water flowing through it.

Working 

First the water is allowed to enter into spiral casing of the turbine. which is guide the water through the stay vanes and guide vanes. The spiral case is kept here in decreasing diameter so that the flow pressure will maintain.

The stay vanes being stationary at their place removes the swirls from the water, which is generated due to flow through the  spiral casing and tries to make the flow of water more linear to be deflect by the adjustable guide vanes.
The angel of guide vanes determines the angle of striking of the water at the runner blade thus make sure of the out put  of the turbine. the runner blades are the stationary and can not pith or change their angle. In short their guide vanes controls the power output of a turbine.
The performance and efficiency of the Francis turbine are dependent on the design of the runner blades.
The upper part of blades uses the reaction forces of water flowing through it.and the lower half is uses the impulse action of water to rotate the turbine. Thus runner blades make use of both pressure energy and kinetic energy of water and rotate the runner most efficiently. Thus the kinetic energy of water is converted into mechanical energy and again it is converted in electrical energy with the help of generators.
The water  which is coming out of the runner blades would lack both the kinetic energy and the pressure energy, so we use the draft tube to recover the pressure as it advanced towords tailrase, but we still , we can not recover the pressure to that extent that we can stop air to enter to runner housing thus causing cavitation.
































































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