Fluid Mechanics and Machinery

Francis Turbine-Main parts, Working Principle Diagram, Efficiency, Advantages, Disadvantages, Application

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Francis Turbine:

  • The Francis turbine is a reaction turbine or This is a type of water turbine that was developed by James B Francis.
  • It is an inward flow reaction turbine that combines radial and axial flow concepts.
  • The Francis turbine is the most common water turbine used in todays.
  • The Francis turbine operates a head range of 10 meters to several hundred meters and is primarily used for electrical power production.

It consists of the main 5 parts which are,

Francis Turbine

1. Spiral Casing:

Provides encased water path to contain the water pressure.

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 blades of the turbine should flow in the circular axis for efficient striking. So, the spiral casing is used, but due to the circular movement of the water, it loses its pressure.

To maintain the same pressure, the diameter of the casing is gradually reduced, so as to maintain the pressure uniformly, thus uniform momentum or velocity striking the runner blades.

2. Stay Vanes:

Stay 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 and as it enters the runner blades. Hence, makes the turbine more efficient. 

3. Guide Vanes:

Guide vanes are also known as wicket gates. The main function or usages of the guide vanes are to guide the water towards the runner. The water flow must be an angle and that is appropriate for the design. 

4. Runner Blades:

Absorbs the energy from the water and converts it to rotational motion of the main shaft. 

The runner blades design decides how effectively a turbine is going to perform.

The runner blades are divided into two parts. The lower half is made in the shape of a small bucket so that it uses the impulse action of water to rotate the turbine.

The upper part of the blades uses the reaction force of water flowing through it. These two forces together make the runner rotate.

5. Draft Tube:

The draft tube is an expanding tube which is used to discharge the water through the runner and next to the tailrace.

The main function of the draft tube is to reduce the water velocity at the time of discharge.

Its cross-section area increases along its length, as the water coming out of runner blades, is at considerably low pressure, so its expanding cross-section area helps it to recover the pressure as it flows towards tailrace. 

Working of Francis Turbine:

First, the water is allowed to enter into the spiral casing of the turbine, which guides the water through the stay vanes and guide vanes. The spiral case is kept here in decreasing diameter so that 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 the spiral casing and tries it to make the flow of water more linear to be deflected by adjustable guide vanes.

Francis Turbine

The angle of guide vanes determines the angle of strikes of water at the runner blades thus make sure the output of the turbine. The runner blades are a stationary and can-not pitch or change their angle. In short, the guide vane controls the power output of a turbine.

The performance and efficiency of the Francis turbine are dependent on the design of the runner blades. In a Francis turbine, the runner blades are divided into two parts. The lower half is made in the shape of a small bucket so that it uses the impulse action of water to rotate the turbine.

The other or you can say the upper part of the blades uses the reaction force of water flowing through it.  Thus, runner blades make use of both pressure energy and kinetic energy of water and rotate the runner in the most efficient way.

The water which is 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 tailrace, but still we cannot recover the pressure to that extent that we can stop air to enter into the runner housing thus causing cavitation.

Cavitation:

The difference in the pressure of water when the entrance to the turbine and exists to the turbine after striking the runner blades is too high, due to more amount of pressure difference, the air molecules which are relatively at high pressure then the water comes out, enters the turbine casing in the form of bubbles. These bubble keeps on exploding near the surface of the runner blades continuously causing a shock wave, which produces a kind of defect at runners’ surface called cavitation.

Thus, causing a serious problem for turbines efficiency.

So, to prevent blades from cavitation?

To use a really hard surface material like stainless steel or else we can also go with other material like surface Hardening of the runner blades, to prevent them from cavitation.

The Efficiency of Francis Turbine:

Hydraulic efficiency:

It is the ratio of work done on the wheel to the head of water (or energy) actually supplied to the turbine.

Mechanical efficiency:

It is a ratio of actual work available at the turbine to the energy imparted to the wheel.

Overall efficiency:

It is the measure of the performance of a turbine and the ratio of power produced by the turbine to the energy actually supplied to the turbine.

Advantages and Disadvantages of Francis Turbine:

Advantages:

  • No head failure occurs still at the low discharge of water.
  • Francis turbine variation in operating head can be more simply controlled.
  • Mechanical efficiency of Pelton wheel turbine decreases faster than Francis turbine.
  • The difference in operating head can be extra simply controlled in Francis turbine than in Pelton wheel turbine.
  • Runner size is small.
  • Changing the head is easily controlled.

Disadvantages:

  • The water which is not durt free can cause extremely rapid wear in a high head Francis turbine.
  • As a spiral casing is stranded, the runner is not simply available. Therefore dismantle is hard.
  • The repair and inspection are much harder reasonably.
  • Cavitation is an ever-present hazard.
  • Current losses are certain.

Application:

1. This is the most efficient hydraulic turbine.

2. Large Francis turbine is individually designed for the site to operate at the highest possible efficiency, typically over 90%.
Francis type units cover a wide head range, from 20 to 700 M and their output varies from a few kilowatts 200 megawatt.

3. In addition to electrical products and they may also be used for pumped storage;  Where is Reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand.

4. Francis turbine may be designed for a wide range of heads and flows. This, along with their high efficiency, has made them the most widely used turbine in the world.

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