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.
We have also
seen the fundamental of Pelton wheel
or Pelton hydraulic turbine, basics of radial flow
reaction turbines,
difference between inward radial flow reaction turbine and outward radial flow reaction turbine and Francis turbine in our recent posts.
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.
- Propeller turbine
- 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.
- Scroll Casing
- Guide vanes mechanism
- Hub with vanes or runner of the turbine
- 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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