how does francis turbine work _ _ learn engineering

22/04/2015 HowdoesFrancisturbinework?~LearnEngineering

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How does Francis turbine work ?| |

Francis turbines are the most preferred hydraulic turbines. They are the most reliable workhorse of hydroelectric powerstations. It contributes about 60 percentage of the global hydropower capacity, mainly because it can work efficientlyunder a wide range of operating conditions. This video is aimed at giving a conceptual overview of working of Francisturbine.

Webpage version of the video gives more elaborated information on its working.

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

Head = 45400 mFlow rate = 10700 m^3/s

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

Runner At the heart of the systemMost important part of Francis turbine is its runner. It is fitted with a collection of complex shaped blades as shown inFig.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 infigure below.

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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 ofblade crosssection is of thin airfoils. So when water flows over it, a low pressure will be induced on one side, and highpressure 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. Sowater will hit, and produce an impulse force before leaving the runner. Both impulse force and lift force will make therunner 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 flowsover 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 CasingRunner is fitted, inside a spiral casing. Flow is entered via an inlet nozzle. Flow rate of water will get reduced alonglength of casing, since water is drawn into the runner. But decreasing area of spiral casing will make sure that, flow isentered 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 pressureenergy into kinetic energy.

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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 runnersection. Thus it reduces swirl of inlet flow.

Governing of Francis TurbineDemand for power may vary over time. The guide vane mechanism is used to control water flow rate and makes sure thatpower 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 vanesmake sure that inlet flow angle is at optimum angle of attack for maximum power extraction from fluid.

Living with CavitationMost often local pressure at exit side of runner goes below vapor pressure of water. This will result in formation waterbubbles and eventually damage to turbine blade material.This phenomenon is known as caviation. It is impossible toprevent cavitation completely. So a carefully designed draft tube is fitted at exit side to discharge the fluid out. Drafttube 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

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