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Axial flow Reaction turbine:
If water flows parallel to the axis of the rotation of the shaft, the turbine is known as the axial flow turbine.
And if the head of the inlet of the turbine is the sum of pressure energy and kinetic energy during the flow of water through a runner a part of pressure energy is converted into kinetic energy, the turbine is known as reaction turbine.
For the axial flow reaction turbine, the shaft of the turbine is vertical.
The lower end of the shaft is made larger which is known as a hub or boss.
The vanes are fixed on the hub and hence hub acts as a runner for axial flow reaction turbine.
The Important flows of axial flow reaction turbine:
- Propeller Turbine and
- Kaplan Turbine.
The Kaplan turbine is propeller what is -type water turbine that has adjustable blades.
It was developed in 1913 by the Austrian professor Viktor Kaplan.
The Kaplan turbine was an evolution of the Francis turbine.
Its invention allowed efficient power production in the low head application that was not possible with Francis turbine.
Kaplan turbine is now widely used throughout the world in high-flow, low head power production.
The Kaplan turbine is an axial flow reaction turbine because the water is moving in the axial direction.
The main parts of the Kaplan turbine are:
- Scroll casing,
- Guide vane mechanism,
- Hub with vanes or runner of the turbine, and
- Draft tube.
The scroll casing is a spiral type of casing that decreases the cross-section area. First, the water from the penstocks enters the scroll casing and then moves to the guide vanes.
From the guide vanes, the water turns through 90° and flows axially through the runner.
The scroll casing protects the runner, runner blades, guide vanes and other internal parts of the turbine from external damage of the turbine.
Guide Vanes Mechanism:
This is the only controlling part of the whole turbine. which opens and closes depending upon the demand of power requirement.
When the more power output requirements, it opens wider to allow more water to hit the blades of the rotor.
And when low power output requires, it closes to cease the flow of water.
When the guide vanes are absent then the turbine cannot work efficiently and so that the efficiency of the turbine decreases.
Hub with vanes or Runner of the turbine:
The term “Runner” in the Kaplan turbine plays an important role. The runner is the rotating part of the turbine in which helps in the production of electricity.
The shaft is connected to the shaft of the generator.
The runner of this turbine has a large boss on which its blades are attached and the blades of the runner are adjustable to an optimum angle of attack for maximum power output.
The blades of the Kaplan turbine have twist along its length.
Twist along its length in the Kaplan turbine is provided because to have always the optimum angle of strike for all cross section of blades and hence to achieve greater efficiency of the turbine.
At the exit of the runner of Reaction Turbine, the pressure available here is generally less than the atmospheric pressure.
The water at the exit cannot be directly discharged to the tailrace.
A tube or pipe which is gradually increasing area and this is used for discharging water from the exit of the turbine to the tailrace.
So, the increasing area of the tube or pipe is called a Draft tube. The draft tube, one end of the tube is connected to the runner outlet and the other end is submerged below the level of water in the tail-race.
The main important point is that the Draft tube is used only in Reaction turbine.
There are 4 types of draft tube:
- Simple Elbow Draft Tube
- Elbow with the varying cross section
- Moody Spreading Draft Tube.
- Conical Diffuser or Divergent Draft Tube
Working Principle or Theory of operation:
The rotation of the turbine is used to rotate the shaft of the generator for electricity production.
Advantages of Kaplan Turbines:
This turbine work more efficiently at low water head and high flow rates as compared with other turbines.
This is smaller in size.
The efficiency of the Kaplan turbine is very high as compares with other types of the hydraulic turbine.
The Kaplan turbine is easy to construct and
Space requirement is less.
Disadvantages of Kaplan Turbines:
The position of the shaft is only in the vertical direction.
Large flow rate must be required.
The main disadvantages are the cavitation process. which occurs due to pressure drop in the draft tube.
Use of draft tube and proper material generally stainless steel for the runner blades may reduce the cavitation problem to a greater extent.
Application of the Kaplan Turbine:
Kaplan turbines are widely used throughout the world for electric power production. They cover the lowest head hydro sites and are especially suited for high flow condition.
Inexpensive microturbines are manufactured for individual power production with as little as two feet of head.
Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%.
They are very expensive to design, manufacture and install but operate for decades.
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