What is a reaction turbine?

Short Answer:

A reaction turbine is a type of hydraulic turbine that works on the principle of reaction, where energy is developed by the pressure difference across the turbine blades. In this turbine, water possesses both kinetic and pressure energy when it enters the runner. The flow of water over the blades produces a reaction force, which rotates the runner and generates mechanical energy.

Reaction turbines are mainly used for medium and low-head water applications with large discharge rates. Common examples include Francis turbine and Kaplan turbine, which are widely used in hydroelectric power plants.

Detailed Explanation:

Reaction Turbine

A reaction turbine is a hydraulic machine that converts the energy of flowing water into mechanical energy by using both pressure energy and kinetic energy. Unlike an impulse turbine, where all pressure energy is converted into kinetic energy before hitting the blades, in a reaction turbine the conversion takes place gradually as the water flows through the runner blades. The pressure of water continuously decreases while transferring energy to the runner.

The working of a reaction turbine depends on Newton’s third law of motion, which states that “for every action, there is an equal and opposite reaction.” The action of water on the blades produces a reaction force that causes the runner to rotate. This rotation is then used to drive a generator for electricity production. Reaction turbines are enclosed in a casing that maintains water pressure and ensures efficient operation.

Working Principle

The principle of operation of a reaction turbine is based on the reaction of water flow. When water passes through the runner blades, it changes its velocity and direction. This change in momentum results in an equal and opposite force acting on the blades, which makes the runner rotate. The flow of water through the turbine is a combination of both radial and axial directions depending on the design type.

In a reaction turbine, water enters the runner with high pressure and moderate velocity. As the water flows through the runner blades, part of its pressure energy converts into kinetic energy, which produces the reaction force. This gradual energy conversion makes the reaction turbine efficient and smooth in operation, especially for low and medium heads.

Construction of Reaction Turbine

The main components of a reaction turbine are described below:

  1. Casing:
    The casing encloses the turbine and prevents water leakage. It is usually spiral-shaped to distribute water uniformly around the runner. It also helps maintain pressure within the turbine.
  2. Guide Vanes (or Stay Vanes):
    The guide vanes direct the flow of water onto the runner blades at the proper angle. They also control the discharge and help regulate the turbine speed by adjusting the flow area.
  3. Runner and Blades:
    The runner consists of curved blades or buckets mounted on a central hub. The shape of the blades allows water to flow smoothly and transfer energy efficiently. As water moves through the blades, both pressure and velocity energy contribute to rotation.
  4. Draft Tube:
    The draft tube is a gradually expanding pipe that connects the runner exit to the tailrace. It helps recover part of the kinetic energy of water leaving the runner and converts it into pressure energy, improving efficiency.
  5. Shaft:
    The runner is mounted on a shaft, which transmits the rotational motion to the generator or mechanical device.

Working Process

  1. Water from the reservoir is supplied to the turbine through a penstock under controlled conditions.
  2. The water enters the casing, which distributes it evenly to the guide vanes.
  3. The guide vanes control the direction and amount of water entering the runner.
  4. Water passes through the runner blades, where its pressure energy gradually decreases while transferring energy to the runner.
  5. The reaction force generated by the change in velocity and pressure causes the runner to rotate.
  6. The water then exits through the draft tube, where remaining kinetic energy is converted into pressure energy before being discharged into the tailrace.
  7. The rotating shaft connected to the runner is used to drive an electric generator to produce electricity.

Types of Reaction Turbines

There are two main types of reaction turbines based on the direction of water flow through the runner:

  1. Francis Turbine:
    It is a mixed-flow reaction turbine in which water enters radially and leaves axially. It is used for medium head (30 to 450 m) and large discharge conditions. The Francis turbine is the most widely used reaction turbine in hydroelectric power plants.
  2. Kaplan Turbine:
    It is an axial-flow reaction turbine where water enters and leaves the runner axially. The Kaplan turbine has adjustable blades and guide vanes, which make it suitable for low-head (below 70 m) and high-discharge applications.

Characteristics of Reaction Turbines

  • Operates fully submerged in water under pressure.
  • Water pressure decreases continuously as it passes through the runner.
  • Requires a draft tube to recover kinetic energy and improve efficiency.
  • Suitable for medium and low heads with high discharge rates.
  • More complex design compared to impulse turbines but provides smoother and more efficient operation.
  • Used widely in hydroelectric power plants for consistent power generation.
Conclusion:

A reaction turbine is a hydraulic turbine that converts both pressure and kinetic energy of water into mechanical energy through the principle of reaction. As water flows through the runner blades, the change in velocity and pressure creates a reaction force that rotates the runner. These turbines are highly efficient and suitable for medium and low-head applications with high discharge. Common examples like the Francis and Kaplan turbines are essential components of modern hydroelectric power systems, providing reliable and continuous energy generation.