Water turbines/hydraulic turbines are rotary prime movers which convert the potential or kinetic energy of water into mechanical energy in the form of rotational energy.
A water turbine when coupled with an electrical generator produces electrical energy. It is one of the most suitable means of electric power generation system. It is estimated that about 20% of the total electric power in the world comes from hydro power plants. The only limitation is that it can be operated through the turbine, if there is a continuous flow of water.
In the areas which are surrounded by hills and mountains known as catchment area, water turbine systems can be installed. The small rivers form a big river to flow. By constructing a machinery dam across flowing rivers, a water reservoir can be formed. The water is carried from the reservoir to water turbine by a long pipe known as penstock and the hydraulic energy possessed by water is converted into mechanical energy and then to electrical energy.
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Depending upon the head of water and available discharge, different types of water turbines are classified.
They are basically of two types:
(a) Impulse turbine and
(b) Reaction turbine (Fig. 5.4).
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(a) Impulse Turbine:
In an impulse turbine, the total potential energy available with water is fully converted into kinetic energy by means of nozzle. The turbine is quite suitable for high head and low discharge available with it.
In this type of turbine, there is a water nozzle which converts the total potential energy available with water into kinetic energy. Water is discharged from the nozzle in the form of water jet and high kinetic energy.
The high kinetic energy jet is made to strike, as shown in Fig. 5.5, on a series of curved buckets or blades mounted on the periphery of a wheel which is placed on the turbine shaft. This is the type of impulse turbine which requires high head and less water availability. Pelton wheel is one of the most commonly used impulse turbines.
Figure 5.6 shows the installation of Pelton wheel. The wheel, so called runner, consists of a circular wheel mounted with large number of buckets or blades fixed on the periphery. The buckets which are used are known as double hemispherical buckets shown in Fig. 5.7. The buckets are made of cast iron, bronze, or stainless steel. The flow of water through nozzle is controlled by the to-and-fro movement of spear.
The impulsive force, which acts tangentially, imparted on a series of buckets one after the another is made to rotate the runner in the direction of water jet impingement. The entire combination is put inside a casing to prevent the scattering of water. The pressure energy is fully converted to kinetic energy through nozzle. The high velocity of jet gives impingement over the bucket. The impulsive force given by the jet rotates the wheel. The pressure inside casing remains atmospheric.
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(b) Reaction Turbine:
Reaction turbine is quite suitable for low head and high discharge. The water supplied to the reaction turbine possesses both pressure as well as kinetic energy. The total pressure energy is not fully converted to kinetic energy initially, as it happens in impulse turbine. The water flows first of all to guide blades which supply water in a proper direction and then it is passed through moving blades which are mounted on the wheel.
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A part of the pressure energy of water, when flowing through the moving blades, is converted into kinetic energy which is absorbed by the turbine wheel. The water leaving the moving blades is at low pressure. Thus, there is a difference in pressure between the entrance and exit of the moving blades.
Due to this difference in pressure, there is an increase in kinetic energy and hence a reaction is developed in opposite direction which acts on the moving blades. The rotation of the wheel is set up in opposite direction. In case of reaction turbine, the water is discharged at the tail race through draft tube.
i. Francis Turbine:
Francis turbine is also called medium head turbine. In this turbine, water flows radially and finally discharges axially. Hence, this turbine is also called mixed flow turbine. It consists of a spiral casing inside which there are large numbers of stationary guide blades/guide vanes. They are fixed all around the circumference of an inner ring of moving vanes called runner (Fig. 5.8). The runner is fixed on the turbine shaft.
The runner consists of a series of curved blades numbering 16-24. The runner vanes are so well-designed in shape that water enters the runner radially and leaves the runner axially. Water with pressure energy enters through the passage into the casing radially through the guide vanes. It flows from the outer periphery of the runner in the radial direction over the moving vanes and finally it is discharged at the center axially at low pressure.
The kinetic energy is imparted to the runner when it flows over the moving vanes which produce rotation to the shaft. Water is then discharged at lower pressure through a diverging conical tube known as draft tube, which is fitted at the center of the runner (Fig. 5.9).
The draft tube converts kinetic energy into pressure energy and hence the pressure available at the exit of draft tube is the atmospheric pressure. The other end of the tube is immersed in water known as tail race.
ii. Kaplan Turbine:
Kaplan turbine is also called as low head reaction turbine which is suitable for comparatively low discharge and is known as axial flow reaction turbine. It is similar to Francis turbine. It consists of a spiral casing in which there are large numbers of stationery guide vanes. They are fixed all around the circumference of an inner ring of moving vanes called runner.
High-pressure water enters the turbine casing and enters into the guide vanes. The water strikes the runner and flows axially through guide vanes and imparts kinetic energy to the runner which produces rotation. The water is then discharged at the center of the runner in axial direction into the draft tube.
The outlet of the draft tube is immersed in water (Fig. 5.10). The construction of Kaplan turbine is just similar to Francis turbine except the shape of runner. The runner of Kaplan turbine runner has only 3, 4, or 6 blades, either fixed or adjustable on hub (Fig. 5.11). The latter is known as propeller turbine.
