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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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