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We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious 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

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. 

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. 

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. 

Important relations for the Francis turbines

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 = B1/D1
Value of n will vary in the range of 0.1 to 0.4. 

Flow ratio

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

Flow ratio = Vf1/ Square root (2gh)
Value of flow ratio will vary in the range of 0.15 to 0.3. 

Speed ratio

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

Speed ratio = u1/ 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


Fluid mechanics, By R. K. Bansal 
Image courtesy: Google  

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