How Does A Francis Turbine Work? Definition, Working, Parts, Efficiency

In this article, I will explain the Francis turbine and its components. I will also explain the Francis turbine working principle with the help of a diagram. Francis turbine is a type of reaction turbine

What is a Francis Turbine?

Francis turbine is a mixed flow turbine. In a Francis turbine, the water enters radially to the impeller blades while exits axially. It is a combination of a reaction turbine and an impulse turbine. Mostly it uses in large or medium hydropower plants. These types of turbines can be used for heads from a maximum of 300 meters to a minimum of 2 meters.

The significant advantage of the Francis turbines is that these turbines work the same whether they arrange vertically or horizontally. These are the most commonly used turbines in hydroelectric plants.

Francis turbine

These turbines are a combination of reaction and impulse turbines. And the blades of these turbines rotate by both the impulse and reaction impact of the water flowing through the blades. This increases the efficiency of the turbine and produces more electricity in the hydroelectric plant.

In the Francis turbine, water enters radially. It means that it flows into the turbine perpendicularly to the axis of rotation. When the water enters the turbine, the water always flows inward to the center. In the case of Francis turbines, the water exits axially parallel to the axis of rotation. James B. Francis invented the first Francis turbine that was the 1st turbine for radial flow. 

Francis Turbine Working Principle:

The Francis turbine has a straightforward working principle. 

Francis

First of all, the water hits the edges of the settled blades. These blades quickly transform the accessible water head into an energy head. The settled blade releases a vortex of water, which allows the water to drain directly. Then the water hits the stationary blades. The stationary blade fixes in its position but rotates around its own pivot point.

The guide blade chooses the best position for the water to hits the sprinter. It cuts the edges and controls the speed at which the water gets into the Sprinter. The cutting edge controls the water stream rate into the sprinter. The Francis Turbine Sprinter has curved blades.

The structure of these blades is designed in such a way that water can penetrate radially into the Sprinter. If there is a problem with the current Francis turbine, the primary method is abandoned, but the Francis turbine’s simple old water goes in and out in a radial direction. This adjustment of the pivot axis from sprinter to outspread creates a circular force in the sprinter. Then the sprint begins to rotate and get output when the generator shaft combines with the sprint. With the help of the shaft, the controller is replaced by a generator that is used within a particular service life.

How does a Francis Turbine Work?

A Francis turbine works in the following way:

Francis turbine working
              Fig: Francis turbine working animation
  • During the working of the Francis turbine, water enters into the turbine volute casing and this water passes through the guide blades and stationary blades.
  • The volute casing keeps in reducing diameter to maintain the flow pressure.
  • The stationary blades remain fixed at their position, which eliminates the water vortices caused by flow through the volute casing and efforts to make the water flow deflected by the adjustable guide blades more linear.
  • The guide blades’ angle determines the angle of attack of the water on the impeller blades and ensures the performance of the turbine. The impeller blades are fixed and cannot be changed or modified their angles. Therefore, this is all about the guide blades that control the turbine performance.
  • The efficiency and performance of the turbine depend on the impeller blade’s design. In the case of the Francis turbine, the impeller blades divide into two sections. The bottom section is shaped like a small bucket so that this section uses the water’s impulse action to turn the turbine. The upper section of the blade utilizes the reaction action of moving water. 
  • Due to the presence of both impulse and reaction forces on the impeller blades, the blade uses the water kinetic and pressure energy to turn the impeller in the most effective way.
  • The water exiting the impeller will lack kinetic energy and pressure energy. Therefore, the draft tube uses to relieve pressure and move it towards the tailrace. However, the pressure can’t be recovered that the extents that you may avoid air from entering into the impeller casing and causes cavitation.

For a better understanding, watch the below-given video:

Major Components of Francis Turbine

A Francis turbine has the following major components:

Francis turbine components

  1. Spiral Casing
  2. Guide Vanes
  3. Stay vanes
  4. Draft Tube
  5. Runner Blades
  6. Penstock 
  7. Runner or Impeller

1) Spiral Casing:

The cross-sectional area of ​​the spiral casing decreases uniformly along the circumference. This reduces the cross-sectional area. This makes the water’s speed hitting the impeller blades uniform, just as water flows from the top of the casing into the opening of the impeller blades. Therefore, the flow rate, along with the housing, decreases. Therefore, the cross-sectional area reduces so that the pressure is uniform, and water hits the impeller blade.

2) Guide Vanes or Blades:

The water flows through the fixed vanes and enters into the blades of the runner. The guide vanes can change their own angle so that the angle of attack of the water on the blade can be controlled and the runner blade can operate normally. Also, This guide blade regulates the flow of water entering the impeller blades to control the turbine performance according to the Francis turbine load.

3) Stay Vanes:

The stay vanes guide the water to the rotor blades. The stator blades remain stationary in this position, which reduces the water vortex caused by radial flow and penetrates the impeller buckets, making the turbine more efficient.

4) Draft Tube:

The draft tube or drainpipe links the outlet of the impeller to the tailrace. When water flows out of the impeller blade, it is low pressure. The increased cross-sectional area helps to regain pressure as it flows into the exhaust tailrace.

5) Runner Blades:

It is in the essential components of the Francis turbine. The designs of the impeller blades determine the turbine performance. The blade of the impeller of the Francis turbine distributes into two parts. The upper blade part uses the reaction force of the flowing water, and the lower blade part is bucket-shaped and flows under the impulse of the water. The runner rotates with the upper force.

6) Penstock:

The function of the penstock is to transfer water from the supply to the turbine. Penstock makes of cast steel or cement.

7) Runner:

The runner includes in the most major components of the Francis turbine. These turbines are useless without a runner. The blades are fixed at the runner. The Blades rotate as the runner rotates. 

What is the Efficiency of Francis Turbine?

The efficiency and power of a turbine depend on the shape of the impeller blade. In the case of Francis turbines, the impeller blades divide into two parts. The lower part of the impeller is designed in such a way that it utilizes the water impulse force to turn the turbine. In contrast, the upper portion of the blade utilizes the reaction action of moving water. Due to the combination of these two forces, the impeller of the turbine starts rotating.

To get maximum efficiency by a Francis turbine, the water must be equally distributed to all the impeller blades. The water flow rate controls by a volute casing that rotates around the turbine. The casing supplies water to the turbine impeller blades through a series of valves and stationary blades. When a Francis turbine designs appropriately according to the requirement, it can achieve 90 to 95% water energy.  

How does cavitation occur in a Francis Turbine?

In the case of a turbine, the outlet pressure is very low than outlet pressure. The cavitation in a Francis turbine produces in the following way.

  • Because the outlet has very low water pressure, and in some cases, this outlet pressure can also drop than the pressure of the vapor.
  • As the outlet goes lower than the vapor pressure, the air starts to enter inside the turbine.
  • Due to this air entry inside the turbine, the bubbles start to produce.
  • When these bubbles reach the high-pressure area, they explode near the surface of the impeller blades.
  • This process generates a continuous shock wave.
  • The generation of the shock waves damages the impeller blade, and this process is known as cavitation. Preventing cavitation is a very hard job.
  • This cavitation can also cause vibrations and damage the guide vanes.

How do you prevent the blade from cavitation?

After the occurrence of cavitation problems, it is a very complex job to fix them. However, the following are some steps that may prevent you from a cavitation problem:

  • Choose a draft tube having an appropriate design
  • Always purchase a turbine that has the highest degree of efficiency.
  • The surface of the impeller blades should be hard.
  • Use very hard surfaces, such as the use of high-grade stainless steel.

Advantages and Disadvantages of Francis Turbine

Advantages:

  1. These turbines are best for the radial flow of water.
  2. These turbines don’t have failure even at low water discharge.
  3. In these reaction turbines, the operating head can be controlled more quickly than other types of reaction turbines.
  4. The Pelton wheel turbine efficiency drops faster than the Francis turbine efficiency.
  5.  Compared to Pelton wheel turbines, Francis turbines have easier control over differences in operating height.
  6. The size of the runner is small.
  7.  The head change is easy to control.

Disadvantages:

  1. The efficiency of these turbines is less as compared to the Kaplan turbine.
  2. These turbines have a large size than Kaplan turbines.
  3. Mostly, these turbines require high head conditions.
  4. It has a low rotational speed as compared to the Kaplan turbine.

Applications of the Francis Turbines:

  1. These are the most efficient hydraulic turbines.
  2. Large Francis turbines are exclusively designed for the field and operate as efficiently as possible (typically 90% or more).
  3. Francis-type devices have a large selection of heads from 20 to 700 m, and the output fluctuates between a few kilowatts and 200 megawatts.
  4. We can also use it as a pump. Because during periods of low energy demand, a water turbine (which works like a pump) fills the reservoir and reverses during peak demand to be used to generate electricity.
  5. Francis turbine designs for a variety of heads and flows. In combination with its high efficiency, it is the most widely used turbine in the world. 
  6. As the most efficient turbine is currently available, the vitality of pressurized water can often be harnessed for various purposes to achieve the power generation era’s goals.

What is the difference between a Francis and Pelton wheel turbine? 

Sr. NoCriteriaFrancis TurbinePelton Turbine
1 TypeIt is a reaction turbine that converts the dynamic pressure head of the fluids into mechanical work.It is an impulse turbine that converts the static head pressure into mechanical work.
2Efficiency Its effectiveness is 90%Its effectiveness is 85%
3UsesIt uses for medium headsIt uses for high heads
4Specific SpeedThe Francis turbine has a high specific speedThe Pelton wheel turbine has low specific speed
5Types of EnergyIt transforms potential energy into mechanical energyIt transforms kinetic energy into mechanical energy
6SpeedThe speed of the Francis turbine varies from 75rpm to 1000rpmThe speed rate in a Pelton turbine varies from 65rpm to 800rpm
7 Power ProductionIt produces power of about 800MWIt produces power of about 400MW
8Fluid flow directionThe flowing fluid by the blades has a combination of both axial and radial flow.The fluid flow direction is perpendicular to the impeller.
9ImpellerThe impeller diameter alters from 0.9m to 10.6mThe impeller diameter alters from 0.6m to 0.8m
10HeadFrancis turbines use for medium heads (40m-600m)Pelton turbines use for high heads (250m-1000m)

FAQ Section

Why is whirl velocity at the outlet of a Francis turbine zero?

Francis turbine is a reaction turbine in which the working fluid introduces into the turbine with enormous pressure, and the turbine blade removes energy from the working fluid. Due to changes in the fluid pressure at the turbine blades, some of the energy releases from the fluid, and the rest is removed by the turbine volute casing. The fluid acts on the rotating cup-shaped impeller and leaves at a low vortex and low velocity with low kinetic or potential energy at the outlet. The shape of the turbine outlet pipe slows the flow of fluid and helps to improve the pressure.

When examining an ideal velocity diagram, under ideal conditions, the whirl component of the outlet velocity is “zero”, and the flow is fully axial. Therefore, the whirl velocity at the outlet of a Francis turbine is zero.

Where is Francis turbine used?

A Francis turbine uses in the medium as well as large hydroelectric power plants for producing electricity. In addition, these turbines also use for the following applications:

  • It uses in irrigation systems for transferring water from the ground to irrigation.
  • It also uses for pumped storage
  • Mainly it uses for producing electricity

Who Invented the Francis Turbine?

James Francis invented the first Francis turbine in 1849. This was the 1st turbine for radial flow.

Francis turbines are most widely used all over the world. These turbines use at the hydroelectric powerplant for generating electricity. The head of these turbines is easy to control as compared to the Pelton wheel turbines. So, these turbines are best suitable for the radial flow of water.

For getting knowledge about turbines, you must read the below-given articles.

1) Different types of turbines

2) How does a Reaction turbine work?

3) How does a Kaplan Turbine work?

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