Short Answer:

A Kaplan turbine is an axial flow reaction turbine that works on the principle of converting the energy of flowing water into mechanical energy using adjustable blades. It is mainly used for low-head and high-discharge applications. The turbine consists of a runner with movable blades that can change their angle to suit the water flow, maintaining high efficiency under varying load conditions. The water enters through the guide vanes and strikes the runner blades, causing them to rotate and produce mechanical power.

In the Kaplan turbine, both the guide vanes and runner blades can be adjusted, which helps control the flow and optimize performance. The turbine is directly connected to a generator that converts mechanical energy into electrical energy. Because of its adjustable mechanism, it maintains high efficiency even with fluctuating water levels, making it ideal for large hydroelectric plants.

Detailed Explanation:

Working Principle of Kaplan Turbine

The Kaplan turbine works on the principle of reaction and axial flow. In this type of turbine, water flows along the axis of the turbine shaft. The energy of water is partly converted into kinetic energy and partly into pressure energy before entering the turbine. The combination of these two types of energies results in smooth and efficient turbine operation. The turbine consists mainly of a scroll casing, guide vanes, runner with adjustable blades, draft tube, and shaft.

1. Flow of Water:
   Water from the dam or penstock enters the spiral-shaped scroll casing, which distributes the flow evenly around the turbine. The water then passes through guide vanes, which direct it toward the runner blades at an optimal angle. The guide vanes control both the direction and quantity of water entering the runner.
2. Action on Runner Blades:
   The runner of the Kaplan turbine consists of a set of movable blades, usually made of stainless steel or bronze. These blades can rotate on their axis to adjust their pitch angle according to the water flow conditions. As the water passes over the runner blades, both pressure energy and kinetic energy of water are converted into mechanical energy. The force of water causes the runner to rotate, transmitting torque to the turbine shaft.
3. Energy Conversion Process:
   The Kaplan turbine operates on reaction principle, meaning part of the energy conversion occurs due to the change in pressure across the turbine. The pressure of water decreases as it flows through the runner, resulting in a reaction force that makes the runner spin. The turbine shaft is coupled directly to the generator, converting the mechanical rotation into electrical energy.
4. Role of Adjustable Blades and Guide Vanes:
   One of the unique features of the Kaplan turbine is its adjustable runner blades. The blades can automatically change their angle according to the water discharge. This feature ensures that the turbine operates at maximum efficiency under different load conditions. Similarly, the guide vanes also adjust to regulate the flow of water entering the runner. The synchronized operation of guide vanes and runner blades helps maintain a consistent output even when the water level varies.
5. Draft Tube Function:
   After passing through the runner, water enters the draft tube, which gradually expands in diameter. The draft tube helps in recovering some of the kinetic energy by converting it into pressure energy, thus improving the overall efficiency of the turbine. The water finally discharges back into the tailrace.
6. Efficiency and Applications:
   The Kaplan turbine is known for its high efficiency, often above 90%, even under variable flow conditions. It is mainly used for low-head (10–70 meters) and high-discharge applications such as large river dams and run-of-river power plants. The design and construction make it suitable for both horizontal and vertical shaft arrangements.
7. Advantages:
   • High efficiency under varying load and flow conditions
   • Adjustable blades provide flexibility
   • Suitable for low heads and large water flow
   • Compact design and smooth operation
   • Long service life and low maintenance
8. Limitations:
   • High manufacturing and installation cost
   • Requires complex control mechanism for blade adjustment
   • Not suitable for very high head applications

Conclusion:

The Kaplan turbine operates on the reaction principle and is an advanced version of the propeller turbine. Its unique feature of adjustable runner blades and guide vanes allows it to maintain high efficiency even when water flow changes. It is best suited for low-head, high-discharge hydroelectric plants, making it one of the most efficient and widely used turbines in modern hydroelectric power generation.