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Wednesday, 7 August 2019

AXIAL FLOW REACTION TURBINE

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 Axial flow reaction turbine with the help of this post. 

Axial flow reaction turbine 

Axial flow reaction turbine, as name suggest, consist two important terms i.e. axial flow and reaction turbine. We will first understand here the meaning of saying axial flow and then we will see the term reaction turbine. 

Once, we will come to know the above two important terms of axial flow reaction turbine, it will be quite easy to understand the fundamental of this type of turbine. 

If the water will flow in a direction parallel to the direction of rotation of the shaft, the turbine will be considered as axial flow turbine. 

If the head at the inlet of the turbine is the sum of pressure energy and kinetic energy and during the flow of water through the runner a part of pressure energy is converted in to kinetic energy, the turbine will be considered as reaction turbine. 

The shaft of the axial flow reaction turbine will be vertical and the bottom portion of the shaft will be made larger which will be termed as hub of the turbine. 

Vanes will be fixed over the surface of the hub and therefore hub of the turbine will be considered as the runner of the axial flow reaction turbine. 

Classification of Axial flow reaction turbine

On the basis of installation of vanes over the surface of hub of turbine, there are basically two types of axial flow reaction turbine.
  1. Propeller turbine
  2. Kaplan turbine

Propeller turbine

In case of propeller turbine, vanes fixed over the surface of hub are not adjustable.

Kaplan turbine

In case of Kaplan turbine, vanes fixed over the surface of hub are adjustable. Kaplan turbine is suitable where a large quantity of water at low head is available. 

Following figure, displayed here, indicates the runner of a Kaplan turbine. Kaplan turbine will have one hub over which adjustable vanes will be fixed. 
There are following main parts of a Kaplan turbine as displayed here in following figure.
  1. Scroll Casing
  2. Guide vanes mechanism
  3. Hub with vanes or runner of the turbine
  4. Draft tube 

Water from penstock will enter the scroll casing and then it will move to the guide vanes. From the guide vanes, the water will turn through 90 degree and will flow axially through the runner as shown in figure.
Discharge through the runner will be determined with the help of following equation or formula.

Where,
D0 = Outer diameter of the runner
Db = Diameter of the hub
Vf1 = Velocity of flow at inlet

Let us see some important relations in respect of Kaplan turbine

1. Peripheral velocity at inlet and outlet will be equal and will be given by following equation or formula.
u1 = u2 = πD0N/60
Where, D0 is the outer diameter of the runner 

2. Velocity of flow at inlet and outlet will be equal i.e. Vf1 = Vf2

3. Area of flow at inlet and area of flow at outlet will be equal and will be given by following equation as mentioned here
A = (π/4) x [D02-Db2]
Do you have any suggestions? Please write in comment box.  Further we will find out, in our next post, the basics of Draft tube

Reference: 

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

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