Francis turbine is a famous type of turbine. It is a combination of both a reaction turbine and an impulse turbine. They are most commonly employed in hydroelectric power plants to produce electricity. This article explains the Francis turbine working principle, components, and applications.
What is a Francis Turbine?
Francis turbine is a mixed flow turbine. In a Francis turbine, the water enters radially to the runner blades while exits axially. It is a combination of a reaction turbine and an impulse turbine.
Francis turbines are most commonly used in large or medium hydropower plants to produce electricity. 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.
The runner blades of Francis turbines are designed in such a way that they 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.
Francis turbines are most commonly used in hydroelectric power plants because of their efficiency and adaptability to different flow and head conditions. These turbines have the capability to work efficiently in low, medium, and high-head conditions, making them ideal for different hydroelectric projects.
Francis Turbine Working Principle
The Francis turbine has a straightforward working principle. It is a combination of impulse and reaction turbines. Francis turbine contains a runner, runner blades, stationary vanes, penstock, and a casing.
- During the working of the Francis turbine, water enters the turbine volute casing and this water passes through the guide blades and stationary blades.
- The volute casing has a decreasing 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 runner blades and ensures the performance of the turbine. The runner blades are fixed that cannot be changed or modified their angles. Therefore, this is all about the guide vanes that control the turbine performance.
- The efficiency and performance of the turbine depend on the runner blade’s design. In the case of the Francis turbine, the runner blades divide into two sections.
- The bottom section of the runner blades 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 runner blades, the blade uses the water kinetic and pressure energy to turn the runner in the most effective way.
- When the water strikes the runner’s blades, it forces the runner to rotate, which further drives a generator to generate electricity.
- After striking through the runner blades, the water leaves the turbine via a draft tube and is transformed into the canal or other water source.
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The water exiting the runner blades of a Francis turbine has reduced kinetic and pressure energy. To maximize energy recovery, a draft tube is used to help convert the remaining kinetic energy into pressure energy as the water flows toward the tailrace. However, the draft tube can’t fully recover the pressure to a level that completely prevents air from entering the runner housing. This can lead to cavitation
Components of Francis Turbine
A Francis turbine has the following components:
- Spiral Casing
- Guide Vanes
- Stay vanes
- Draft Tube
- Runner Blades
- Penstock
- Runner
1) Spiral Casing
The spiral casing serves as the inlet for water entering the turbine from the dam or reservoir. This high-pressure water is directed toward the turbine blades, which are arranged in a circular pattern.
For efficient energy transfer, it is essential that the water flows along a circular path when striking the runner blades. However, as the water follows this circular trajectory, it tends to lose pressure.
To maintain consistent pressure and ensure that the momentum or velocity of the water striking the runner blades remains uniform, the diameter of the spiral casing slowly decreases. This design feature helps preserve the pressure and optimize the performance of the Francis turbine.
2) Guide Vanes
Guide vanes are the major components of the Francis turbine. 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.
The guide blade regulates the flow of water entering the runner blades to control the turbine performance according to the 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 runner buckets, making the turbine more efficient.
4) Draft Tube
The draft tube or drainpipe links the outlet of the runner to the tailrace. When water flows out of the runner 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 one of the essential components of the Francis turbine. The designs of the runner blades determine the turbine’s performance. These blades are divided 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 (i.e., dam or reservoir) to the turbine. Penstock makes of cast steel or cement.
7) Runner
The runner is also a major component 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 runner blade. In the case of Francis turbines, the runner blades are divided into two parts. The lower part of the runner 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 runner of the turbine starts rotating.
To get maximum efficiency by a Francis turbine, the water must be equally distributed to all the runner blades. The water flow rate controls by a volute casing that rotates around the turbine.
The casing supplies water to the runner 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.
Efficiency Formulas of Francis Turbine
The Francis turbine has the following types of efficiencies:
- Hydraulic efficiency
- Overall efficiency
- Volumetric efficiency
- Mechanical efficiency
1) Hydraulic Efficiency
The ratio between the energy generated by the runner of the turbine and the power delivered by the fluid at the suction or input side of the turbine is known as hydraulic efficiency.
The hydraulic efficiency of the Francis turbine can be calculated by the following formula:
2) Overall Efficiency
The ratio between the turbine shaft output power and the water power delivered at the turbine inlet is called Overall efficiency.
The following formula uses to calculate the overall efficiency:
3) Volumetric efficiency
- The ratio of the water volume passing by the runner and the total water volume delivered to the turbine is called volumetric efficiency.
- Some water may leak out from the space between the housing and the runner without flow via the runner.
4) Mechanical Efficiency
The ratio between the actual turbine shaft’s power and the power produced by the runner is known as mechanical efficiency.
The runner always delivered more power than the actual power of the turbine shaft. The shaft power is low because of the wind drag losses, bearing mechanical losses, and other friction losses.
How does cavitation occur in a Francis Turbine?
In the case of a turbine, the outlet pressure is very low than the 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 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 runner blades.
- This process generates a continuous shock wave.
- The generation of the shock waves damages the runner 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.
Advantages and Disadvantages of Francis Turbine
Francis turbines have the following advantages and disadvantages:
Advantages of Francis Turbines
- Francis turbines are best for the radial flow of water.
- These turbines don’t have failure even at low water discharge.
- In these turbines, the operating head can be controlled more quickly than other types of reaction turbines.
- The Pelton wheel turbine efficiency drops faster than the Francis turbine efficiency.
- Compared to Pelton wheel turbines, Francis turbines have easier control over differences in operating height.
- The electricity production using Francis turbines helps to reduce greenhouse gas emissions and dependence on fossil fuels.
- The head change of this turbine is easy to control.
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The draft tube of these turbines assists recover some of the kinetic energy from the water existing the runner, increasing overall efficiency.
Disadvantages of Francis Turbines
- The efficiency of these turbines is less as compared to the Kaplan turbine.
- These turbines have a large size than Kaplan turbines.
- Mostly, these turbines require high head conditions.
- Francis turbines may be susceptible to cavitation.
- It has a high initial cost.
- It has a low rotational speed as compared to the Kaplan turbine.
- They may not be as responsive to quick load variation as other turbine types, such as Kaplan turbines.
Applications of the Francis Turbines
- Francis turbines are most usually employed turbine in hydro-power plants to produce electricity.
- It can be employed in industries where a significant amount of water flow is used or generated as a byproduct. The turbines are used to extract the power from the water and transform it into electricity.
- They are also used in irrigation applications to pump water from the ground.
- 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.
Difference between Francis Turbine and Pelton Wheel Turbine
| Criteria | Francis Turbine | Pelton Turbine |
|---|---|---|
| Type | It is a reaction turbine that converts the dynamic pressure head of the fluids into mechanical work. | The Pelton wheel turbine has a low specific speed |
| Efficiency | Its effectiveness is 90% | Its effectiveness is 85% |
| Uses | It uses for medium heads | It uses for high heads |
| Specific Speed | The Francis turbine has a high specific speed | The Pelton wheel turbine has low specific speed |
| Types of Energy | It transforms potential energy into mechanical energy | It transforms kinetic energy into mechanical energy |
| Speed | The speed of the Francis turbine varies from 75rpm to 1000rpm | The speed rate in a Pelton turbine varies from 65rpm to 800rpm |
| Power Production | It produces power of about 800MW | It produces power of about 400MW |
| Fluid flow direction | The flowing fluid by the blades has a combination of both axial and radial flow. | The fluid flow direction is perpendicular to the impeller. |
| Impeller | The runner diameter alters from 0.9m to 10.6m | The runner diameter alters from 0.6m to 0.8m |
| Head | Francis turbines use for medium heads (40m-600m) | Pelton turbines use for high heads (250m-1000m) |
FAQ Section
What is the major application of the Francis turbine?
Francis turbine is usually employed in hydroelectric power plants to produce electricity.
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 is released from the fluid, and the rest is removed by the turbine volute casing. 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 for?
A Francis turbine uses in 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 to produce electricity
Who Invented the Francis Turbine?
James Francis invented the first Francis turbine in 1849. This was the 1st turbine for radial flow.
What makes a Francis turbine different from a Kaplan turbine?
The Kaplan turbine boasts higher efficiency compared to the Francis turbine. In a Kaplan turbine, water flows in an axial direction for both entry and exit, whereas, in a Francis turbine, water enters radially and exits axially. With a smaller cross-section and reduced rotational speed, the Kaplan turbine is distinct from its counterpart.
How do you prevent the blade from cavitation?
- Choose a draft tube having an appropriate design
- Always purchase a turbine that has the highest degree of efficiency.
- The surface of the runner blades should be hard.
- Use very hard surfaces, such as the use of high-grade stainless steel.
What are the components of Francis turbine?
- Spiral casing
- Penstock
- Guide vanes
- Runner blades
- Stay vanes
- Draft tube
- Runner
- Bearings
- Shaft
Is the Francis turbine an impulse turbine?
The Francis turbine combines features of both impulse and reaction turbines, utilizing both the impulse and reaction forces of water as it flows past the blades to generate enhanced power output.
Which type of turbine is a Francis turbine?
Francis turbine is one of the most famous types of reaction turbine.