We were
discussing a new topic, in the subject of fluid mechanics and hydraulics
machine, i.e. an introduction to hydraulic
machine, various types of hydraulic
turbines and
some important terminologies associated with a hydraulic turbine such as Gross head, Net head and
efficiencies of a hydraulic turbine.

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We have also seen the fundamental of Pelton wheel
or Pelton hydraulic turbine, basics of radial flow
reaction turbines and
difference between inward radial flow reaction turbine and outward radial flow reaction turbine in our recent posts.

Now we
will focus here to understand the basics of Francis turbine with the help of
this post.

So let us
start here with the basics of Francis turbine, let us take one cup of tea and
see it in detail.

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**Francis turbine **

As name
suggest, this is a type of reaction turbine which is developed by an American
engineer, Sir J.B. Francis.

Francis
turbine is basically an inward flow reaction turbine with radial discharge at
its outlet. We have already discussed the basics of inward radial flow reaction turbine in our last post.

In modern
Francis turbine, the water will enter the runner of the turbine in the radial
direction at outlet and will leave in the axial direction at the inlet of the
runner. Therefore, the modern Francis turbine will be termed as mixed flow turbine.

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**Important Facts about Francis
Turbine**

Francis
turbine is one type of reaction turbine and hence only a part of the available head
will be converted in to the velocity head before water enters the runner.

The pressure
head goes on decreasing as the water flows over the runner blades.

The static
pressure at the runner exit may be less than the atmospheric pressure and as such,
water fills all the passages of the runner blades.

Francis
turbine is normally suitable for medium heads i.e. 45 m to 100 m and requires a
relatively large quantity of water.

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**Important relations for the Francis
turbines**

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*Ratio of width of the wheel to its diameter *

Ratio of
width of the wheel to its diameter will be given by following formula as
mentioned here

**n = B**

_{1}/D_{1}
Value of n
will vary in the range of 0.1 to 0.4.

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*Flow ratio*

The flow
ratio will be given by following formula as mentioned here

**Flow ratio = V**

_{f1}/ Square root (2gh)
Value of flow
ratio will vary in the range of 0.15 to 0.3.

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*Speed ratio*

*Speed ratio*

The speed
ratio will be given by following formula as mentioned here

**Speed ratio = u**

_{1}/ Square root (2gh)
Value of speed
ratio will vary in the range of 0.6 to 0.9.

Do you
have any suggestions? Please write in comment box. Further we will
find out, in our next post, axial flow reaction turbine.

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**Reference: **

Fluid mechanics,
By R. K. Bansal

Image
courtesy: Google

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