What is a Francis Turbine?

A Francis turbine is a mixture of the impulse turbine and reaction turbine. These most commonly used in medium to large hydroelectric power plants. These turbines can be used for heads from a minimum of 2 meters to a maximum of 300 meters. These turbines are also advantageous because they are just as easy to operate when installed horizontally as when installed vertically.

Francis turbines are the most widely used turbines in hydropower plants.

The water flowing through the Francis turbine loses pressure but maintains roughly the same speed and is therefore called a reaction turbine.

The water is radiated onto these turbines. This means that the water flows into the turbine perpendicular to the axis of rotation. When entering the turbine, the water always flows inwards, towards the center. When flowing through the turbine, the water flows axially parallel to the axis of rotation. Francis turbine was a 1st turbine that designed for a radial flow.

How a Francis Turbine Work?

First, the water is introduced into the spiral casing of the Francis turbine and directed through the stationary blades and guide blades. The spiral casing remains small in diameter in order to maintain the flow pressure.

The fixed blades are fixed in their positions to eliminate the water vortices caused by the passage of the spiral sleeve and try to flow the water more linearly so that it is deflected by the adjustable guide vanes.

The angle of attack of the guide vanes determines the angle of attack of the water to the impeller blades, thereby ensuring the operation of the turbine. The blades are stationary and cannot be tilted or tipped. In short, the guide vanes control the power of the turbine.

The performance and efficiency of Francis turbines vary according to the design of the impeller vanes.

In case of a Francis turbine, the blade of the runner divides into two parts. The lower part is shaped like a small bucket and the impact of the water causes the turbine to rotate. At the top of the blade, this force is used to resist the water moving by it. Therefore, the blades use the pressure and kinetic energy of the water to turn the impeller in the most effective technique.

The water exiting the vanes of the impeller has no pressure or kinetic energy, so a suction pipe uses to relieve pressure in the exhaust pipe, but enough to keep air from entering the impeller housing. It cannot restore and causes cavitation.

Main Parts of a Francis Turbine

1) Volute casing:

It has decreasing diameter. This casing uses to transforms the speed of the water into the pressure energy. This component of the Francis turbine plays a most important role for increasing the pressure of the water.

2) Stay Vanes

This directs water toward the impeller blades. Leave the blade in place to reduce water eddies from radial flow and entry into the impeller blades. Therefore, this turbine has high efficiency.

3) Guide Blades

These blades are also wicket gates. The main function or purpose of the guide blade is to direct the water into the corridor. The water flow must be at an angle suitable for the construction.

4) Runner or impeller blade:

It takes energy from the water and converts it into a rotary motion of the shaft.

The design of the impeller blades determines the operating efficiency of the turbine.

The corridor palette is divided into two parts. The lower part is shaped like a small bucket and the impact of the water causes the turbine to rotate.

The upper part of the blade utilizes the reaction force of the water flowing through the blade.

5) Draft Tube

It is a type of expansion pipe that is used to drain water through the channel on the side of the suction pipe.

The main function of this tube is to reduce the flow rate of the water in the drain.

The water emerging from the rotor blades has a relatively low pressure, so that its cross-sectional area increases over the length. Therefore, its expanded cross-sectional area helps relieve pressure as it flows through the exhaust pipe.

Advantages and Disadvantages

Advantages

  • A Francis turbine is best for radial flow fluid
  • The efficiency of these turbines decreases slowly
  • Its impeller size is very small
  • The changes in the pressure head are very easy to control

Disadvantages

  • These have low efficiency than Kaplan turbines
  • It requires high head