What are axial flow turbines?

Short Answer:

Axial flow turbines are reaction turbines in which water flows parallel to the axis of the turbine shaft. In this type, the flow direction remains along the axis both before and after passing through the runner blades. Kaplan turbines are the most common example of axial flow turbines, where adjustable blades are used to handle varying loads efficiently.

These turbines are mainly used for low-head and high-discharge applications such as in large hydroelectric power plants. The design allows smooth and continuous energy conversion, ensuring higher efficiency and stable operation even under fluctuating water flow conditions.

Detailed Explanation :

Axial Flow Turbines

Meaning of Axial Flow Turbines

Axial flow turbines are a type of reaction turbine in which the water enters and leaves the runner flowing in a direction parallel to the axis of the turbine shaft. The name “axial flow” comes from the fact that the motion of water remains along the same direction as the turbine axis, unlike radial flow turbines where water flows radially inward or outward.

In these turbines, the water’s pressure and velocity both change as it passes through the runner. The runner blades are designed like aerofoils, allowing smooth flow and efficient conversion of hydraulic energy into mechanical energy. The Kaplan turbine, which is widely used in hydroelectric power plants, is a prime example of an axial flow turbine.

Construction of Axial Flow Turbines

An axial flow turbine consists of the following main components:

  1. Casing: It is a watertight cover that directs the flow of water towards the runner and protects the internal parts from damage.
  2. Guide Vanes (or Wicket Gates): These control the amount and direction of water entering the runner, ensuring efficient operation at different loads.
  3. Runner and Blades: The runner has several blades shaped like propeller blades. Water flows axially through the runner, imparting torque to rotate the shaft.
  4. Shaft: It is connected to the runner and transmits mechanical energy to the generator for producing electricity.
  5. Draft Tube: This is a diverging pipe fitted at the runner exit, which helps in converting leftover kinetic energy into pressure energy and improves overall efficiency.

Each of these parts is designed to ensure smooth axial flow of water and minimize energy losses due to turbulence or friction.

Working Principle of Axial Flow Turbines

The working of an axial flow turbine is based on the principle of reaction. When high-pressure water from the penstock enters the turbine, it passes through the guide vanes which control and direct the flow onto the runner blades. The pressure energy of water decreases, and its velocity increases as it enters the runner.

As the water flows over the curved blades of the runner, it exerts force due to change in momentum, causing the runner to rotate. During this process, both pressure and velocity of water are reduced, while mechanical energy is transferred to the shaft. The water finally passes into the draft tube where the remaining kinetic energy is converted into pressure energy before being discharged into the tailrace.

This smooth conversion of energy with minimal losses is a key feature of axial flow turbines, making them efficient and reliable.

Characteristics of Axial Flow Turbines

  1. Flow Direction: The water enters and leaves the runner in a direction parallel to the shaft.
  2. Type of Energy Conversion: Both pressure and velocity energy are converted into mechanical energy, which classifies them as reaction turbines.
  3. Head Range: These turbines are suitable for low to medium heads, generally ranging from 10 meters to 70 meters.
  4. Discharge: They can handle very large quantities of water, making them ideal for high-flow conditions.
  5. Efficiency: They offer high efficiency even at part-load operations due to adjustable blades.

Types of Axial Flow Turbines

There are mainly two types of axial flow turbines based on blade movement:

  1. Propeller Turbine: The blades are fixed, and the flow of water remains axial. It is suitable for constant head and discharge conditions.
  2. Kaplan Turbine: It has adjustable blades that can be set at different angles to suit varying load conditions. This type provides better efficiency over a wide range of discharges.

Applications of Axial Flow Turbines

  • Widely used in low-head hydroelectric power plants.
  • Suitable for river-based power stations with large water flow.
  • Used in irrigation projects and tidal power plants.
  • Effective where steady and continuous flow of water is available.

These turbines are commonly installed in vertical or slightly inclined positions for easy alignment with the water flow.

Advantages of Axial Flow Turbines

  1. High efficiency at varying loads.
  2. Compact design requiring less space.
  3. Capable of handling large water discharge.
  4. Smooth and stable operation.
  5. Suitable for low-head hydroelectric plants.

Limitations of Axial Flow Turbines

  1. Not suitable for high-head conditions.
  2. Initial cost is high due to complex design.
  3. Requires precise control of blade angles and flow direction.
  4. Maintenance can be challenging because of underwater installation.
Conclusion

Axial flow turbines are efficient reaction turbines where the flow of water remains parallel to the axis of the turbine shaft. They are best suited for low-head and high-discharge conditions, making them an important component in modern hydroelectric power generation. Kaplan turbines, as an advanced form of axial flow turbines, further enhance performance by using adjustable blades. Despite their high cost and complex design, their ability to operate efficiently over a wide range of conditions makes them a preferred choice in large hydroelectric systems.