Short Answer:

Reaction machines are hydraulic machines that operate based on the principle of reaction force produced due to the change in pressure and velocity of the flowing fluid. In these machines, the fluid possesses both pressure energy and kinetic energy when passing through the runner. Common examples of reaction machines include Francis turbine, Kaplan turbine, and Propeller turbine.

These machines are generally used for low to medium head and high discharge conditions. They are widely used in hydroelectric power stations where water flow and head are moderate, and high efficiency is required in continuous operation.

Detailed Explanation :

Reaction Machines

Reaction machines are a type of hydraulic machine that works on the principle of Newton’s Third Law of Motion — for every action, there is an equal and opposite reaction. In a reaction machine, the fluid flows over the blades or vanes of the runner while its pressure continuously decreases. The force causing the runner to rotate is produced by the reaction of the fluid pressure and velocity changes as it passes through the blades.

In these machines, the conversion of energy takes place gradually along the passage of the runner. The fluid entering the runner has both pressure energy and kinetic energy, and as it moves through the blades, a part of the pressure energy is converted into kinetic energy. The reaction between the fluid and the blades results in a torque that turns the runner, producing mechanical work.

Working Principle

The working of reaction machines depends on the principle of impulse and reaction combined. Unlike impulse machines where pressure remains constant over the blades, in reaction machines, there is a continuous pressure drop from inlet to outlet of the runner. This pressure difference creates a reaction force that drives the runner.

The fluid in reaction machines is not free; it is always in contact with the blades of the runner. The water passages are designed in such a way that the velocity and direction of the water flow continuously change. This variation in velocity and pressure generates the required reaction force. The turbine casing is airtight and filled with water to maintain pressure and prevent losses due to air or splashing.

The energy conversion process in reaction turbines is smooth, and both the pressure and kinetic energies contribute to the total output power. The efficiency of such machines is very high, especially when operating under designed conditions.

Examples of Reaction Machines

Francis Turbine:
The Francis turbine is a commonly used reaction turbine designed for medium head and discharge conditions. It is an inward flow mixed turbine, meaning the water enters the runner radially and leaves axially. The runner consists of a series of curved blades between a stationary guide vane system. The water flows through the guide vanes, which control the direction and flow rate before entering the runner. As the water passes through the blades, both its pressure and velocity decrease, generating a reaction force that rotates the runner. The Francis turbine is widely used in large hydroelectric power stations due to its high efficiency and versatility.
Kaplan Turbine:
The Kaplan turbine is an axial-flow reaction turbine used for low head and high discharge conditions. It operates similarly to a propeller but with adjustable blades to maintain high efficiency even with variable water flow. Water flows parallel to the axis of the shaft, and the adjustable runner blades allow the turbine to adapt to changing load and flow conditions. The Kaplan turbine is commonly used in run-of-river hydroelectric plants and tidal power installations.
Propeller Turbine:
The propeller turbine is another axial-flow reaction turbine similar in design to the Kaplan turbine but with fixed blades. It is suitable for low head and constant flow rate conditions. The fluid flows parallel to the axis, and the reaction force generated by the pressure drop causes rotation. Propeller turbines are simple, cost-effective, and suitable for small hydropower plants.

Characteristics of Reaction Machines

Both pressure and velocity energy are used to produce power.
Pressure continuously decreases as the fluid passes over the runner blades.
The turbine casing is airtight to maintain pressure differences.
The runner always remains completely submerged in water.
The guide vanes control the flow direction and discharge.
Reaction turbines have higher efficiency compared to impulse turbines under low and medium heads.

Applications

Reaction machines are used extensively in:

Hydroelectric power plants operating under low and medium head.
River-based power generation systems.
Tidal and small dam installations.
Situations where constant and high discharge water is available.

Because of their adaptability and high efficiency under varying flow conditions, reaction turbines like Francis and Kaplan are the most widely used turbines in the world.

Advantages

High efficiency even at low head.
Smooth and continuous flow through the runner.
Compact design suitable for various installation conditions.
Adjustable blades (in Kaplan turbines) allow flexible operation.
Capable of handling high discharge efficiently.

Disadvantages

More complex design compared to impulse turbines.
Requires airtight casing and draft tube for proper operation.
Maintenance is higher due to continuous water contact.
Not suitable for high head and low discharge conditions.

Conclusion

Reaction machines are essential hydraulic machines used to convert both the pressure and kinetic energy of water into mechanical energy. They operate based on the principle of reaction and are most effective in medium and low head conditions with large water flow. Examples like the Francis, Kaplan, and Propeller turbines have become standard in modern hydroelectric power plants due to their excellent efficiency, reliability, and adaptability to variable flow conditions. These turbines play a crucial role in sustainable power generation and efficient water energy utilization.