Francis turbine, as the name suggests, came from James B. Francis, who has developed it. This turbine is basically combined with the radial flow and axial flow concepts.
We will learn here the details of this turbine, its working principle, parts, advantages, disadvantages, etc.
Like all other turbines, the main parameters for the performance of the turbine are,
- Water head
- Water flow rate
Francis turbine is highly efficient even in the flow variation due to seasonal changes.
What is Francis Turbine?
Francis Turbine Definition
Francis turbine is defined as the combination of impulse turbine and reaction turbine, and based on the impulse and reaction forces, the blade rotates and electricity is produced.
- It is used in hydro power plants.
- Widely used turbine.
- It is used in a wide range of flow and head.
- Normally it is used to produce electricity.
- In addition to electricity, it uses for pump storage.
- It operates best when water has filled the turbine completely.
The preferable value of head and flow rate for Francis turbine, are as follows,
- Head = 40 – 500 m
- Flow rate = 10-700m^3/s
Francis Turbine Diagram
Look at this simple diagram in a hydropower plant to have a basic idea of Francis turbine connection as well as different parts.
Main Parts of a Francis Turbine
A Francis Turbine consists of the 5 main parts those are:
- Spiral Casing
- Stay Vanes
- Wicket gates or Guide Vanes
- Runner Blades
- Draft Tube
The spiral casing looks like a spiral design and it creates an encased water path for the water pressure.
- Water in the dam or the reservoir has a very high head.
- This high-pressure water comes into the casing.
- The casing has a runner.
- Runner blades are circularly placed in the casing.
- Water strikes the blade with high velocity and the blade rotates.
- Due to the strike, the pressure is reduced.
- To maintain a uniform pressure, the area of the casing is gradually reduced, which is spiral design.
- Velocity as well as momentum remains uniform.
Stay vanes are used to guide the water to the blades of the runner.
- These are stationary at their position.
- It helps to reduce the swirling of the inlet water, as water enters radially into the runner.
- It helps to increase the efficiency of the Francis turbine.
- The stay vanes are used to convert the pressure energy into kinetic energy.
- It helps to direct the flow to the runner blades based on design angles.
Wicket Gates or Guide Vanes
Guide vanes are also called as wicket gates, are used to guide the water towards the runner.
- It is placed at an angle and that is appropriate for the design.
- It helps to control the flow of water
- Wicket gates are having a series of small openings.
- When the guide vanes or wicket gates are opened more, the flow of water into the turbine runner will be more.
- More water flow results in higher power output.
- The guide vane angle is set to get the highest performance.
Francis turbine blades are specially designed. Water strikes the blades of the runner, and the runner rotates.
- Runner absorbs the energy from the water.
- This energy is converted into rotational motion.
- Mani shaft connected to the runner rotates.
- Blades are designed properly so that turbine will be efficient.
- The runner blades are fixed pitch angle.
The runner blades are classified into two parts.
The lower half of the blade
It is the shape of a small bucket for impulse action, which helps to rotate the turbine.
The upper half of the blade
It is a thin airfoil type. Now, if water flows over the airfoil section, low pressure is created on one side and high pressure on the other side. Due to this, a lift force is created.
These two forces together help the runner to rotate.
The water after the rotation needs to be taken out and it is necessary to reduce the velocity as well. To fulfill these requirements, the draft tube is used.
- It reduces the velocity of water at the discharge.
- It is an expanding tube or conduit.
- It connects the tailrace to the runner exit.
- The cross-sectional area of the draft tube is increased gradually along its length.
- The reduction of pressure is increased or recovered by the expanding cross-sectional area.
- It minimizes the kinetic energy loss at the outlet.
Is Francis Turbine a Mixed Flow Turbine?
Let us see, whether the Francis turbine is a mixed flow turbine or not. Francis turbine has two kinds of force,
- reaction turbine,
- impulse action,
When water strikes over the runner blades, pressure energy, as well as kinetic energy, reduces. Water enters radially and exits axially. Since both radial and axial flow are associated, it is a mixed flow turbine.
Governing of Francis turbine
Power requirement varies based on the seasons or other requirements. Hence, governing is an important factor for turbines.
In Francis’s turbine, there are guide vanes that control the water flow rate and meet the requirements.
Guide vanes are used as the governing system of Francis turbines.
Cavitation in Francis Turbine
How Does Cavitation Occur?
The pressure at the inlet of the turbine is very high and it is low at the exit.
- Due to low water pressure at the outlet, sometimes, this pressure may be reduced to vapor pressure also.
- Due to vapor pressure, air enters into the turbine casing.
- Bubbles are formed.
- These bubbles remain to explode near the runner blade surface, as it reaches to the high-pressure zone.
- Create a shock wave continuously.
- Damages the blades, and this is called cavitation. It will be very difficult to prevent cavitation.
- It incurs vibrations and damages on the guide vane and runner blades.
How to Prevent Cavitation of Blades?
It is really hard to prevent cavitation, once it starts. There are two things which can help, in this regard,
- The proper design of the draft tube can help to prevent the cavitation.
- Using a very hard surface, for example, high-grade stainless steel
- Surface Hardening can be done for the runner blades.
- Always design the turbines at the highest efficiency level
Francis Turbine Working Principle
How does the Francis turbine work? The working principle of a Francis turbine is elaborated, as below
The water is entered into the spiral casing of the turbine, with very high pressure. The spiral casing has a decreasing diameter so that water pressure will be maintained at the uniform.
Water is guided through the stay vanes and guide vanes and removed the swirls, make the water flow more linear.
The guide vane angles determine the angle of strikes of water flow at the runner blades. This angle of strike meets the output power requirements.
In the runner blades, both impulse and lift or reaction rotates the turbine blades. Both pressure energy and kinetic energy of water help to rotate the runner of the turbine.
Finally, water comes out of the runner blades and pressure energy and kinetic energy are reduced. A draft tube is used with expanding the area and recover the pressure.
The performance and efficiency of the Francis turbine varies based on the design of the draft tube, design of runner blades, etc.,
Efficiencies of Francis Turbine
Hydraulic efficiency is defined as the ratio of work done on the wheel to the water head supplied to the turbine.
Mechanical efficiency is defined as the ratio of the actual work available at the turbine to the energy imparted by the water to the turbine wheel.
Overall efficiency means the measure of the performance of a turbine and it is defined as the amount of power produced by the turbine to the unit energy supplied to the turbine, which means the ratio of power produced to the energy supplied.
In the Francis turbine, the blade angle is fixed and designed considering the best efficiency.
But when the demand is reduced, water strikes at different angles which causes a swirl in the water flow or backflow or flow separation, and efficiency reduces drastically.
There are three different scenarios of efficiency at three different loads,
The turbine is at a high load
- Normally the turbine will be stable with small pressure fluctuations.
- But if the pressure difference is more, at a very high load, after entering the turbine, water tends to have a swirl, which can create low pressure, even below the vapor pressure.
- Cavitation may happen.
- Entire hydraulic plants will have a problem.
The turbine is at Best Efficiency Point
- At this point, water flow happens based on the ideal blades as well as runner or casing etc. design.
- Smooth water flows in runner blades.
- Stable water flows in the draft tube.
The turbine is at Part load
- Water may leave the runner with a swirl.
- It can create backflow to the runner.
- At part load, efficiency reduces.
Advantages of Francis Turbine
These are many advantages of the Francis turbine:
- In the different operating head, the variation of the Francis turbine can be controlled easily.
- This control of variation is easy than the Pelton wheel turbine.
- In the case of a low discharge of water, there will not be any head failure.
- The mechanical efficiency of the Francis turbine decreases slowly comparative to the Pelton wheel turbine.
- Small runner size.
- Change in efficiency is very small with time.
- Very little change in efficiency with time
Disadvantages of Francis Turbine
There are few disadvantages of Francis turbines, as well,
- The problem in the case of dismantling requirements. In the case of Francis’s turbine, the casing is stranded and it is very hard to dismantle the runner.
- Water can have dirt and can create wear and tear.
- Difficulty in repairing.
- Cavitation problem.
- The problem in the inspection.
- Current losses.
- In the Francis turbine, the blade angle is fixed and designed considering the best efficiency. But when the demand is reduced, water strikes at different angles which causes swirl in the water flow or backflow or flow separation, and efficiency reduces drastically.
Applications of Francis Turbine
Francis Turbine used in many fields like:
- Most efficient hydraulic turbine.
- A large Francis turbine gives about 90% efficiency, which helps to generate more power as well as meets the power demands.
- It uses a wide head range, from 40m to 500m.
- Power output varies from a few kW to 800 MW.
- It uses electrical power generation.
- It also uses for pumped storage.
- High efficiency
Francis turbine is explained in detail, hope, its basic is clear. Any doubt about this turbine! Write to us!
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