Short Answer:

The angle of attack in a turbine refers to the angle at which the fluid enters the blades of the turbine. If this angle is correct, the fluid flows smoothly over the blade surface, ensuring proper energy transfer. But if the angle is too steep or too flat, it can cause flow separation, turbulence, and energy loss.

A correct angle of attack helps in maximizing efficiency and reducing wear and tear on the blades. It ensures that the fluid hits the blade in the most effective direction, improving turbine performance and increasing the output power of the machine.

Detailed Explanation:

Angle of Attack Impact on Turbine Performance

In fluid machines like turbines, the angle of attack is the angle between the direction of incoming fluid and the chord line (straight line connecting the leading and trailing edges) of the turbine blade. It is one of the most important factors that affects how efficiently the turbine extracts energy from the moving fluid.

When the fluid strikes the turbine blade at the correct angle, it moves smoothly along the curved surface of the blade. This allows for maximum momentum change, leading to high energy transfer and better efficiency. If the angle is wrong, the fluid flow may separate from the blade surface, leading to turbulence and poor performance.

1. Correct Angle of Attack Ensures Efficient Flow
   When the angle of attack is properly aligned:

• The fluid flows along the blade without sudden changes in direction.
• The energy is transferred effectively from the fluid to the rotating shaft.
• The blade experiences less vibration and pressure fluctuations.
• This results in smooth, quiet, and energy-efficient operation.

A well-designed blade with the right angle of attack can handle higher loads and generate more power with less resistance.

2. Too High or Too Low Angle Reduces Performance
   An incorrect angle of attack can lead to several problems:

• Too high angle: The fluid hits the blade too steeply, causing flow separation and backflow. This creates turbulence and pressure drag, reducing energy transfer and increasing vibration.
• Too low angle: The fluid does not interact properly with the blade, resulting in less force generation and inefficient rotation. It may also lead to slippage.

In both cases, the result is reduced turbine performance, higher wear on the blades, and possible damage over time.

3. Relation with Blade Design and Velocity Triangles
   The correct angle of attack is determined during the blade design phase using velocity triangles. These triangles show the relation between the absolute velocity of the fluid, blade velocity, and relative velocity.

• Blade angles are adjusted so that the fluid enters and exits the blade at the correct angle.
• This alignment ensures that the angle of attack remains within an optimal range under different flow conditions.

Proper matching of the angle of attack with design flow improves the hydraulic efficiency of the turbine.

4. Role in Avoiding Flow Separation and Cavitation
   A well-set angle of attack helps in:

• Avoiding flow separation, where fluid loses contact with the blade.
• Reducing the risk of cavitation, which occurs due to pressure drops and can damage the blade surface.
• Improving stability and lifespan of the turbine blades.

Controlling the angle is especially important in adjustable-blade turbines like Kaplan turbines, where the blades change their angle based on the load and flow.

Conclusion

The angle of attack has a direct effect on turbine performance by influencing how the fluid strikes and flows over the blades. A correct angle improves energy transfer, reduces losses, and increases turbine efficiency. Incorrect angles lead to turbulence, wear, and reduced output. Proper design and control of the angle of attack are essential for smooth and effective turbine operation.