Short Answer:

Adjustable blades are special turbine blades whose angle can be changed during operation to control the flow of water and maintain efficient performance. These blades are commonly used in reaction turbines like the Kaplan turbine. By adjusting their pitch angle, the blades can adapt to variations in water flow and load, ensuring the turbine works smoothly under different operating conditions.

The main purpose of adjustable blades is to keep the turbine efficiency high even when the flow of water changes. They help control the water velocity striking the blades, reducing losses and improving energy conversion. This feature makes turbines more flexible and suitable for varying head and discharge conditions in hydroelectric plants.

Detailed Explanation:

Adjustable Blades

Adjustable blades are turbine blades designed to change their angle or pitch according to the flow of water. They are mainly used in reaction turbines, such as the Kaplan turbine, to maintain maximum efficiency at varying loads and flow conditions. The concept of adjustable blades was introduced to overcome the limitations of fixed-blade turbines, which cannot perform efficiently under changing water flow.

1. Purpose of Adjustable Blades:
   The main function of adjustable blades is to regulate the water flow striking the blades at the most efficient angle. In a hydro turbine, water enters the runner with varying velocities and directions depending on the available head and discharge. If the blades are fixed, they cannot handle these variations effectively, leading to energy losses. Adjustable blades solve this problem by changing their pitch automatically or manually to ensure that water always strikes the blades at the correct angle for optimal power generation.
2. Construction of Adjustable Blades:
   The adjustable blades are attached to the runner hub, and each blade can rotate about its own axis. The mechanism for adjustment includes hydraulic servomotors or mechanical linkages that control the angle of the blades. In modern turbines, the control system automatically adjusts the blade position based on flow rate and load demand. The blades are usually made of high-strength materials like stainless steel or bronze to withstand high pressure and continuous water contact.
3. Working Mechanism:
   When the water discharge or load on the turbine changes, the control system detects the variation. The hydraulic servomechanism adjusts the pitch of the blades accordingly. For example:
   • If water flow increases, the blades turn to a position that allows more water to pass through, maintaining efficiency.
   • If water flow decreases, the blades change to a narrower angle to prevent energy losses.
     This automatic adjustment ensures that the angle of attack of the water on the blades remains optimal, reducing turbulence and increasing power output.
4. Role in Kaplan Turbine:
   In the Kaplan turbine, both guide vanes and runner blades are adjustable. The guide vanes control the quantity and direction of water entering the runner, while the runner blades adjust their pitch to utilize the incoming water efficiently. The coordinated adjustment of these components is managed by a governing mechanism, ensuring smooth operation under fluctuating water conditions. This feature is what makes the Kaplan turbine highly efficient even at partial loads.
5. Advantages of Adjustable Blades:
   • High Efficiency: They maintain constant efficiency over a wide range of water flow and load conditions.
   • Flexibility: Can adapt to changes in discharge and head without affecting output.
   • Smooth Operation: Reduces vibration and mechanical stress by maintaining proper water entry angles.
   • Better Control: Allows better speed regulation in hydroelectric plants.
   • Long Life: Reduces wear and tear caused by uneven water pressure.
6. Applications:
   Adjustable blades are widely used in hydro turbines where the water flow and head vary throughout the year. They are ideal for run-of-river plants, Kaplan turbines, and some marine propeller systems where changing water velocity is common.
7. Difference from Fixed Blades:
   • Fixed blades have a constant angle and cannot adjust according to flow changes.
   • Adjustable blades, on the other hand, change their position automatically or manually to handle variations in flow, giving them superior performance and control.
8. Efficiency Consideration:
   The efficiency of turbines with adjustable blades remains high (around 90–94%) under most working conditions. This is because the water always strikes the blades at the right angle, minimizing hydraulic losses. Additionally, adjustable blades contribute to better load control and smoother power generation.
9. Control Mechanism:
   The blade pitch control system uses oil-pressure servomotors, connected by linkages to each blade. A governor detects changes in speed or flow and sends a signal to adjust the blade position. This synchronization ensures that all blades adjust uniformly, preventing imbalance or uneven loading on the turbine shaft.

Conclusion:

Adjustable blades play a vital role in maintaining the efficiency and stability of reaction turbines, especially under variable flow and load conditions. By allowing the pitch of the blades to change, they help ensure smooth water entry and energy conversion. Their ability to adapt automatically makes them highly effective for modern hydroelectric power systems, providing flexibility, durability, and reliable performance.