MPULSE AND REACTION TURBINES
Hydraulic turbines are required to transform fluid energy into usable mechanical energy as effi-ciently 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 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. 1.16). 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 tur-bine etc., Pelton wheel is predominantly used at present.
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.
Fig. 1.17 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.
The following table lists salient points of difference between the impulse and reaction turbines with regard to their operation and application.
1. All the available energy of the fluid is converted into kinetic energy by an efficient nozzle that forms a free jet.
2. The jet is unconfined and at atmospheric pressure throughout the action of water on the runner, and during its subsequent flow to the tail race.
3. Blades are only in action when they are in front of the nozzle.
4. Water may be allowed to enter a part or whole of the wheel circumference.
5. The wheel does not run full and air has free access to the buckets.
6. Casing has no hydraulic function to perform; it only serves to prevent splashing and to guide the water to the tail race.
7. Unit is installed above the tail race.
8. Flow regulation is possible without loss.
9. When water glides over the moving blades, its relative velocity either remains constant or reduces slightly due to friction.
1. Only a portion of the fluid energy is transformed into kinetic energy before the fluid enters the turbine runner.
2. Water enters the runner with an excess pressure, and then both the velocity and pressure change as water passes through the runner.
3. Blades are in action all the time.
4. Water is admitted over the circumference of the wheel.
5. Water completely fills the vane passages throughout the operation of the turbine.
6. Pressure at inlet to the turbine is much higher than the pressure at outlet ; unit has to be sealed from atmospheric conditions and, therefore, casing is absolutely essential.
7. Unit is kept entirely submerged in water below the tail race.
8. Flow regulation is always accompanied by loss.
9. Since there is continuous drop in pressure during flow through the blade passages, the relative velocity does increase.
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