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How does Francis turbine work ?

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Francis turbines are the most preferred hydraulic turbines. They are the most reliable workhorse of hydroelectric power stations. It contributes about 60 percentage of the global hydropower capacity, mainly because it can work efficiently under a wide range of operating conditions. This video is aimed at giving a conceptual overview of working of Francis turbine.
Webpage version of the video gives more elaborated information on its working.

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Water head and flow rate are the most vital input parameters that govern performance of a hydraulic turbine. But these parameters are subjected to seasonal variation in a hydroelectric power station. Francis turbine is capable of delivering high efficiency even if there is a huge variation in these flow parameters. Following are the head and flow rate under which Francis turbine is preferred to operate.

• Head = 45-400 m
• Flow rate = 10-700 m^3/s

In this article we will understand working of Francis turbine and will also realize why it is capable to work under varying flow conditions.

Runner – At the heart of the system

Most important part of Francis turbine is its runner. It is fitted with a collection of complex shaped blades as shown in Fig.1

Fig.1 Runner - The most vital part of Francis turbine

In runner water enters radially, and leaves axially. During the course of flow, water glides over runner blades as shown in figure below.

Fig.2 Water flow through Francis turbine runner

Blades of Francis turbine are specially shaped. One such blade is shown in Fig.2. It is clear from the figure that shape of blade cross-section is of thin airfoils. So when water flows over it, a low pressure will be induced on one side, and high pressure on the other side. This will result in a lift force.

Fig.3 Airfoil cross section shape of Francis blades & production of reaction force

You can also note one more peculiar thing about the blade. It is having a bucket kind of shape towards the outlet. So water will hit, and produce an impulse force before leaving the runner. Both impulse force and lift force will make the runner rotate.

Fig.4 Francis turbine derive energy from combined action of reaction and impulse force

So Francis turbine is not a pure reaction turbine, a portion of force comes from impulse action also. Thus as water flows over runner blades both its kinetic and pressure energy will come down. Since flow is entering radially and leaves axially, they are also called ‘mixed flow turbine’. Runner is connected to generator, via a shaft, for electricity production.

Use of Spiral Casing

Runner is fitted, inside a spiral casing. Flow is entered via an inlet nozzle. Flow rate of water will get reduced along length of casing, since water is drawn into the runner. But decreasing area of spiral casing will make sure that, flow is entered to runner region almost at uniform velocity.

Fig.5 Spiral casing makes sure that flow is entered uniformly along the periphery of runnner

Stay vanes and guide vanes are fitted at entrance of runner. The basic purpose of them is to convert one part of pressure energy into kinetic energy.

Fig.6 Stay vanes and guide vanes used in Francis turbine

Flow which is coming from the casing, meets stay vanes, they are fixed. Stay vanes steers the flow towards the runner section. Thus it reduces swirl of inlet flow.

Governing of Francis Turbine

Demand for power may vary over time. The guide vane mechanism is used to control water flow rate and makes sure that power production is synchronized with power demand.

Fig.7 First figure shows guide vanes in closed position; In 2nd figure guide vanes in open position

Apart from controlling flow rate guide vanes also control flow angle to inlet portion of runner blade. Thus guide vanes make sure that inlet flow angle is at optimum angle of attack for maximum power extraction from fluid.

Living with Cavitation

Most often local pressure at exit side of runner goes below vapor pressure of water. This will result in formation water bubbles and eventually damage to turbine blade material.This phenomenon is known as caviation. It is impossible to prevent cavitation completely. So a carefully designed draft tube is fitted at exit side to discharge the fluid out. Draft tube will transform velocity head to static head due to its increasing area and will reduce effect of cavitation.

Fig.8 Conversion of velocity head to static head with help of drafttube

Pelton Turbine - Working & Design Aspects

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Pelton turbines/wheels are suitable for power extraction, when the water energy is available at high head and low flow rate. In this video we will go through working principle and design aspects of Pelton turbine.
Following article gives detailed description of the video lecture.

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Pelton Turbine – The Basic Working Principle

Working principle of Pelton turbine is simple. When a high speed water jet injected through a nozzle hits buckets of Pelton wheel; it induces an impulsive force. This force makes the turbine rotate. The rotating shaft runs a generator and produces electricity.

Fig.1 Pelton wheel derives rotation from impulse force produced by the water jet

In short, Pelton turbine transforms kinetic energy of water jet to rotational energy.

Governing in Pelton Wheel

Demand of power may fluctuate over time. A governing mechanism which controls position of the spear head meets this requirement. With lowering power demand the spear head at water inlet nozzle is moved in. So that water flow rate is reduced. If power demand increases spear head is moved out this will increase the flow rate. Following figure illustrates this mechanism. The first position of the spear head produces a low flow rate, while the second position produces a high flow rate.

Fig.2 Water flow rate control in Pelton wheel by monitoring position of spear head

So in Pelton turbine synchronization between power demand and power supply is met by controlling the water flow rate. The same technique is used in other types hydroelectric turbines. If the power supply is more than the demand, then the turbine will run over speed otherwise in under speed. But such a governing mechanism in turn will balance the power supply and demand and will make sure that the turbine rotates at a constant specified RPM. This speed should also conforms to the power supply frequency. So this mechanism acts as a speed governing mechanism of Pelton wheel.

Number of Buckets in Pelton Wheel

One of the most important parameter of Pelton turbine design is number of buckets on the disk. If number of buckets is inadequate, this will result in loss in water jet. That means when one bucket departs from the water jet next bucket may not get engaged with the jet. This will result in loss in water jet for a small time duration, thus sudden drop in turbine efficiency. Following figure illustrates what happens when the number of buckets are lowered. With lowering number of buckets at some point of operation, complete water jet might be lost (3^rd figure). So there should be an appropriate number of buckets, which will make sure that no water is lost (1^st figure).

Fig.3 Effect of number of buckets on water-bucket interaction

Pelton Bucket - Design & Features

Most vital component of Pelton wheel is its bucket. Buckets are casted as single solid piece, in order to avoid fatigue failure. You can note that force acting on the turbine bucket is not constant with time. If you follow one particular bucket, it will have high force for a small time duration (at the time of jet impingement) after that a larger idle period where no jet interaction takes place. So the force acting on the bucket is also not constant. It varies with the time but it is having a cyclic nature. If bucket were made using pieces by welding attachment such cyclic fore will easily lead to premature fatigue failure.

Fig.4 Different views of Pelton bucket

Water jet is split into 2 equal components with help of a splitter. The special shape of bucket makes the jet turn almost 180 degree. This produces an impulsive force on bucket. Force so produced can easily be derived from Newton’s 2nd law of motion. Blade outlet angle close to 180 degree is usually used in order to maximize impulsive force.
A cut is provided on bottom portion of buckets. This makes sure that water jet will not get interfered by other incoming buckets.

Pelton – An Impulse Turbine

Since the water jet is always open to atmosphere, inlet and exit pressure of water jet will be same and will be same as atmospheric pressure. However absolute velocity of fluid will have huge drop from inlet to exit of bucket. This kinetic energy drop is the maximum energy the bucket can absorb.
So it is clear that Pelton turbine gains mechanical energy purely due to change in kinetic energy of jet, not due to pressure energy change. Which means Pelton turbine is a pure impulse machine.

Fig.5 Pressure and velocity variation across Pelton bucket

Impulse force produced by water jet is high when jet is having high velocity. Water stored at high altitude can easily produce high jet velocity. This is the reason why Pelton turbine is most suitable for operation, when water is stored at high altitude. You can easily understand why there is a nozzle fitted at water jet injection portion. Nozzle will increase velocity of jet further, thus will aid in effective production of impulse force.

Extracting Maximum Power from Water Jet

Pelton turbine design is always aimed at extracting maximum power from water jet, or maximizing efficiency. Power extracted by the bucket, P is product of jet impulse force and bucket velocity.

So power extraction is maximum when product of impulsive force and bucket velocity is maximum. Let's consider 2 different operating conditions.

• Buckets are Held Stationary

If Pelton wheel buckets are held stationary, there will be a huge impulse force produced. But power extraction will be zero since buckets are not moving.


• Bucket Speed Same as Jest Speed

If buckets are moving with same speed of jet, water jet won't be able to hit the bucket. This will lead to zero impulse force. Again power extraction will be zero.

Fig.6 Relative magnitude of bucket and jet velocity is important in power extraction from fluid

In short, power extraction is zero both at zero bucket speed and when bucket speed is same as jet speed. So with respect to jet to bucket speed ratio, power extraction will vary with as shown below.

Fig.7 This graph shows how power extraction from fluid varies with respect to bucket to jet velocity ratio

It is clear from the above graph that optimum power extraction happens in between. It can be shown using Euler's turbo machinery equation that maximum power extraction happens when bucket speed is half the jet velocity. So it is always desirable to operate Pelton wheel at this condition. Pelton turbines can give efficiency as high as 90 %, at optimum working conditions.