Short Answer:

A Francis turbine is a type of reaction turbine that is widely used in medium and large hydroelectric power plants. It works efficiently under medium heads and converts the potential energy of water into mechanical energy. Water flows radially inward and exits axially, driving the runner blades connected to the shaft.

It was invented by James B. Francis and is known for its high efficiency and reliability. The turbine is placed between the Pelton and Kaplan turbines in terms of head range. It operates efficiently for heads ranging from 10 meters to 300 meters and is commonly used in dams and hydropower stations worldwide.

Detailed Explanation :

Francis Turbine

A Francis turbine is a type of reaction turbine, meaning it operates with both pressure and kinetic energy of water. It is the most commonly used hydraulic turbine in hydroelectric power stations because of its wide range of efficiency and adaptability to different head and flow conditions. It converts the energy of flowing water into mechanical rotational energy, which is then transformed into electrical energy by a generator.

The Francis turbine was designed by James B. Francis in 1849. It is an inward-flow, mixed-flow turbine because the water enters radially and exits axially through the runner. This mixed-flow arrangement allows it to handle a large range of heads and flow rates efficiently. It is placed between Pelton and Kaplan turbines in terms of head usage, making it suitable for medium head (10–300 meters) applications.

Construction of Francis Turbine

The main components of a Francis turbine include:

1. Spiral Casing (Volute):
   The spiral casing is a circular housing that surrounds the runner. Water from the penstock enters the casing tangentially and flows around the runner. The design ensures that the velocity of water remains constant as it moves around the casing.
2. Guide Vanes (Stay Vanes and Wicket Gates):
   The guide vanes control the flow of water entering the runner blades. They can be adjusted to regulate the water quantity according to the load demand. The guide vanes also direct the water at the correct angle to strike the runner blades efficiently.
3. Runner and Blades:
   The runner is the rotating part of the turbine. It consists of specially shaped blades or buckets that convert the kinetic and pressure energy of water into mechanical energy. The water flows radially inward through the runner and exits axially.
4. Draft Tube:
   The draft tube is fitted at the exit of the runner and helps recover the kinetic energy of water leaving the runner by converting it into pressure energy. It also allows the turbine to be placed above the tailrace level without a large loss of head.
5. Shaft and Bearings:
   The shaft connects the runner to the generator. Bearings are provided to support the shaft and reduce friction during rotation.

Working Principle of Francis Turbine

The working of a Francis turbine is based on the principle of reaction and impulse forces. Water from the reservoir is directed to the turbine through the penstock. The high-pressure water enters the spiral casing and flows through the guide vanes, which control the flow rate and direct the water at the proper angle to the runner blades.

As the water strikes the curved blades of the runner, both its pressure and velocity change. The change in momentum of water produces a reaction force on the blades, causing the runner to rotate. This mechanical energy is transmitted through the shaft to the generator, which produces electricity.

After passing through the runner, the water enters the draft tube and finally discharges into the tailrace with minimum energy loss. The draft tube helps in recovering the remaining kinetic energy of water and increases the efficiency of the turbine.

Advantages of Francis Turbine

1. High efficiency over a wide range of heads and flow conditions.
2. Suitable for medium head and large discharge.
3. Compact design and requires less space compared to Pelton turbines.
4. Can be used for both small and large hydroelectric power plants.
5. Easy to control the flow using adjustable guide vanes.

Applications of Francis Turbine

Francis turbines are mainly used in hydroelectric power stations where the head is moderate and the flow of water is steady. Examples include power plants built on rivers with medium waterfalls or dams. They are used in most of the world’s hydro plants due to their adaptability and efficiency, ranging between 85% to 95%.

Conclusion

In conclusion, the Francis turbine is a highly efficient reaction turbine designed to convert the energy of flowing water into mechanical energy. Its mixed-flow design, adjustable guide vanes, and efficient energy recovery through the draft tube make it ideal for a wide range of hydroelectric applications. Because of its reliability, efficiency, and ability to handle varying heads and discharges, the Francis turbine remains the most widely used turbine in modern hydroelectric power generation.