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Introduction to Hydraulic (Water) Turbine -Working Of Impulse and Reaction Turbine

Introduction to Hydraulic (Water) Turbine 


Turbine is a machine wherein rotary motion is obtained by centrifugal forces, which result from
a change in the direction of high velocity fluid jet that issues from a nozzle.

Water turbine is a prime mover, which uses water as the working substance to generate power.
A water turbine uses the potential and kinetic energy of water and converts it into usable mechanical
energy. The fluid energy is available in the natural or artificial high level water reservoirs,
which are created by constructing dams at appropriate places in the flow path of rivers. When water
from the reservoir is taken to the turbine, transfer of energy takes place in the blade passages of the unit.

Hydraulic turbines in the form of water wheels have been used since ages; presently their application
lies in the field of electric power generation. The mechanical energy made available at the turbine shaft is used to run an electric generator, which is directly coupled, to the turbine shaft. The power generated by utilizing the potential and kinetic energy of water has the advantages of high efficiency, operational flexibility, low wear tear, and ease of maintenance.

Water (hydraulic) turbines have been broadly classified as,
1. Impulse 
2. Reaction

Working Principle Of Hydraulic Turbine:

Hydraulic turbines are required to transform fluid energy into usable mechanical energy as efficiently
as possible. Further depending on the site, the available fluid energy may vary in its quantum of potential and kinetic energy. Accordingly a suitable type of turbine needs to be selected to perform the required job.

Based upon the basic operating principle, water turbines are categorized into impulse and reaction turbines depending on whether the pressure head available is fully or partially converted into
kinetic energy in the nozzle.

Impulse Turbine:

Impulse Turbine wherein the available hydraulic energy is first converted into kinetic energy by
means of an efficient nozzle. The high velocity jet issuing from the nozzle then strikes a series of
suitably shaped buckets fixed around the rim of a wheel (Fig. ). The buckets change the direction of jet without changing its pressure. The resulting change in momentum sets buckets and wheel into rotary motion and thus mechanical energy is made available at the turbine shaft. The fluid jet leaves the runner with a reduced energy. An impulse turbine operates under atmospheric pressure, there is no change of static pressure across the turbine runner and the unit is often referred to as a free jet turbine. 

Important impulse turbines are: Pelton wheel, Turgo-impulse wheel, Girad turbine, Banki turbine and Jonval turbine etc., 

Pelton wheel is predominantly used at present.

Principle Of Impulse Turbine
Principle Of Impulse Turbine

Reaction Turbine:

Reaction Turbine wherein a part of the total available hydraulic energy is transformed into kinetic energy before the water is taken to the turbine runner. A substantial part remains in the form of pressure energy. Subsequently both the velocity and pressure change simultaneously as water glides along the turbine runner. The flow from inlet to outlet of the turbine is under pressure and, therefore, blades of a reaction turbine are closed passages sealed from atmospheric conditions.

Principle Of Reaction Turbine
Principle Of Reaction Turbine
Above Figures illustrates the working principle of a reaction turbine in which water from the reservoir is taken to the hollow disc through a hollow shaft. The disc has four radial openings, through tubes, which are shaped as nozzles. When the water escapes through these tubes its pressure energy decreases and there is increase in kinetic energy relative to the rotating disc. The resulting reaction force sets the disc in rotation. The disc and shaft rotate in a direction opposite to the direction of water jet. 

Important reaction turbines are, Fourneyron, Thomson, Francis, Kaplan and Propellor turbines 

Francis and Kaplan turbines are widely used at present.

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