Kaplan turbine is a turbine which has adjustable blades to achieve efficiency over a wide range of flow and water level. It was an evolution of Francis Turbine. It works on the principle of axial flow.In axial flow turbines, the water flows through the runner along the direction parallel to the axis of rotation of runner. It is also known as propeller turbine and evolved from Francis Turbine. This turbine can work efficiently at low head and high flow rates which was not possible with Francis Turbine.
Main Parts Of Kaplan Turbine :-
1) Scroll Casing. 2) Guide Vane Mechanism. . 3) Runner Blades.
4) Draft Tube
Function Of these parts :-
1) Scroll Casing :- The spiral casing around the runner of the turbine is known as scroll casing. The cross-section area of the spiral casing decreases uniformly to maintain the velocity.
Image Source : https://mediatoget.blogspot.com/2011/11/kaplan-turbine.html
2) Guide Vanes :- Guide Vanes are present between scroll casing and runner. This is the gate that guides water from scroll casing to runner.
3) Runner Blades :- Runner Blades are the blades that are rotated by the flow of water. In kaplan turbines, runner blades has twist along its length and are adjustable to an optimum angle for maximum power output.
4) Draft Tube :- The draft tube is channel that connects runner exit to tail race from which water is discharged from the turbine. Its main function is to reduce the velocity of water discharged to minimize the loss of kinetic energy at the outlet.
Working of Kaplan Turbine :-
At first water from pen stock is entered into scroll casing. Scroll casing is made in such a way that the flow pressure is not lost. Cross-section of scroll casing decreases uniformly to maintain the pressure of water.
Then from the scroll casing the guide vanes direct the water to the runner. In kaplan turbine, the guide vanes are adjustable and can be adjusted to meet the required flow rate. If guide vanes were not present the flow would be highly swirling in nature due to its tangential entry and would reduce the performance of turbine, so guide vanes also control the swirl of the water flow .
When the guided vanes direct water towards runner, water enters the main area where it meets the runner blades. Runner blades are adjustable in kaplan turbine to maintain optimum angle of attack with varying flow rate. When flow rate is high the velocity of water after leaving runner blade i.e relative velocity is more axial , so the runner blades should be adjusted vertically to have optimum angle of attack. Similarly, when flow rate is low the relative velocity is more tangential and the runner blades should be pitched tangentially .
From the runner blades, water enters into the draft tube where the kinetic and pressure energy of turbine reduces. Kinetic Energy is converted into pressure energy due to which pressure of water increases and finally water is discharged from the turbine through trail race.
Rotation of runner blades rotates the shaft to which runner blades are attached. This rotation of shaft is used for electric production.