Operation of Turbines: Pelton, Francis and Axial Flow Turbines

Running head: OPERATION OF TURBINES
OPERATION OF TURBINES
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Author Note

1OPERATION OF TURBINES
Task 1:
1.1
The Pelton wheel is basically a water turbine characterized by impulse-type. It came by the
name of its inventor Lester Allan Pelton. The energy is extracted by the Pelton wheel by the
impulse created from the moving water which is opposed by the dead weight of the
traditional overshot of the water wheel. The paddle geometry of the Pelton is designed in
such a way that at the time when the rim is running at half speed of jet water, the water
leaving the wheel has the least speed and hence most of the energy of the water is extracted
from the water. Thus the Pelton wheel is very much efficient turbine. The nozzles provide
forceful, super-speed streams of water acting against the series of buckets which are spoon
shaped and are mounted over the rim of drive wheel in the outer side. This particular series of
blades are also known as runner. As soon as the water jet is input to the blades then the
velocity of the water changes which is followed by the contours of the blades (Chouhan,
Kisheorey and Shah 2017). The water impulse energy is exerted on the bucket-wheel system
which revolves the wheel and then a ‘U-turn’ is made and the water exists at lower velocity.
The efficiency of the wheel turbine is given by,
η= 4 u ( Vi – u )
V i2 .
Hence, the efficiency is zero when u = 0 and efficiency is maximum when u = Vi. As soon as
the real Pelton wheel goes very close to the maximum efficiency then the water passing
through with minimum residual velocity.
The Pelton wheel has several limitations like large operating heads are required for these
turbines. Also, for generating given power output under a head which is small, the flow rate
in the turbines must be higher and hence the jet diameter must be large. The total Pelton

2OPERATION OF TURBINES
wheel setup is large and the set-up is bulky. The operating heads variation is very much
difficult for controlling and hence the turbine efficiency is very much reduced with time.
Now, Francis turbine is known to be the first hydraulic turbine which has the radial inflow.
This type of turbine is reaction turbine and main pressure drop happens in the turbine which
is unlike the impulse turbine where the pressure drop in the entry point and hence during the
operation time the turbine process is filled with the flow of water (Trivedi, Cervantes and
Dahlhaug 2016). The efficiency of the Francis turbine is measured by the blade efficiency.
The blade efficiency of the turbine is given by the equation,
ηb = e
e+ V f 2
2
2
Alternatively, ηb = 2 V f 1
2 (cot α1 ( cot α1+ cot β1 ) )
V f 2
2 +2 V f 1
2 (cot α1 ( cot α1+cot β1 ) )
Here, e = per unit mass energy transfer to the rotor
V f 2=flow velocity, α 1 and β1 are the blade angles.
The limitation or the disadvantages of the Francis turbine are it has very high cost, the turbine
design is very much complex although the operation of the turbine is simple. The moving
parts of the turbine is large and hence the design becomes complex. The repair and the
maintenance procedure of the turbine is very much difficult and hence it is very much
expensive. The Francis turbine is only applicable to the medium level of heads.
In case of the axial flow type turbine the working water or fluid flow is in parallel with the
shaft. This is different from the previous turbines as those are radial turbines where water
runs around a shaft. The axial turbine has the similar construction and working is like an axial
compressor but operation is exactly reverse that is conversion of energy of the fluid flow to