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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 and also we have seen the fundamental of Pelton wheel or Pelton hydraulic turbine in our recent posts. 

Now we will focus here to understand the basics of Radial flow reaction turbines and we will also find out here the various parts of a radial flow reaction turbine with the help of this post. 

So let us start here with the basics of Radial flow reaction turbines 

Radial flow reaction turbines 

Do you have any idea about Radial flow reaction turbine? Why we have used word radial or reaction here? 

Let us first understand the meaning of radial here and then we will see the meaning of reaction in radial flow reaction turbines. 

Water will flow in radial direction in radial flow reaction turbines. Water may flow radially from outwards to inwards or from inwards to outwards. 

If water flows through the runner from outwards to inwards, turbines will be termed as inward radial flow turbine. 

If water flows through the runner from inwards to outwards, turbines will be termed as outward radial flow turbine. 

Now we will see here the meaning of reaction turbine. Reaction turbine means the water, at the inlet of turbine, will have kinetic energy and pressure energy. When water will flow through the runner, a part of pressure energy will be changing into kinetic energy. 

Water flowing through the runner will be under pressure. Runner will be completely enclosed in an air tight casing. Casing and runner will always be full of water. 

Main parts of a radial flow reaction turbine

There are following important parts of a radial flow reaction turbine.
  1. Casing
  2. Guide Mechanism
  3. Runner
  4. Draft-Tube


As we have already discussed that in radial flow reaction turbine, casing and runner will always be full of water. Water will enter in to the casing from the penstock. Casing of a radial flow reaction turbine is displayed here in following figure. 

Casing of a radial flow reaction turbine will completely surrounds the runner of the turbine. Casing of a radial flow reaction turbine, as displayed above in figure, will be in spiral shape so that water may enter the runner at constant velocity throughout the circumference of the runner. 

Area of cross-section of the casing will be decreasing gradually. Material of casting of casing will be concrete, cast steel or plate steel. 

Guide Mechanism 

Guide mechanism will be basically a stationary circular wheel all around the turbine runner. There will be stationary guide vanes fixed on the guide mechanism and these guide vanes will allow the water to strike the vanes fixed on the turbine runner without shock at inlet. 

Width between two adjacent vanes of guide mechanism could be varied with the help of a mechanism in order to alter the amount of water striking the runner. 


Runner of a radial flow reaction turbine is basically a circular wheel on which a series of radial curved vanes will be fixed. Surface of these radial curved vanes will be made smooth in order to minimize the hydraulic losses. 

These radial curved vanes will be shaped in such a way that water may enter and leave the runner without shock. 

Runner will be fixed with the shaft with the key assembly. Material of casting of runner will be cast steel, cast iron and stainless steel. 


The pressure at the exit of the radial flow reaction turbine runner will be usually less than the atmospheric pressure and hence water at exit could not be directly discharged to the tail race. 

Therefore a tube or pipe of gradually increasing area will be used in order to discharge the water from the exit of turbine runner to the tail race. 

This tube or pipe of increasing area will be termed as draft tube. 

Do you have any suggestions? Please write in comment box.  

Further we will find out, in our next post, difference between inward and outward radial flow reaction turbine.  


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

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