Short Answer:

A Francis turbine is a reaction type mixed flow turbine. It works by converting both pressure energy and kinetic energy of water into mechanical energy. Water enters the runner radially and exits axially, making it a mixed flow turbine. It is designed to operate efficiently under medium head and medium discharge conditions, making it the most commonly used turbine in hydroelectric power plants.

The Francis turbine belongs to the reaction type category because part of the energy is converted in the stationary blades (guide vanes) and the rest in the moving blades (runner). It is ideal for heads ranging between 10 to 650 meters and offers high efficiency with smooth operation.

Detailed Explanation :

Type of Turbine

The Francis turbine is classified as a reaction type turbine and more specifically, as a mixed flow reaction turbine. It is one of the most commonly used turbines in hydroelectric power plants across the world due to its adaptability to various water head and discharge conditions. The turbine is named after James B. Francis, who developed it in 1849 to improve energy conversion efficiency in water power systems.

In a reaction turbine, water flows through the blades of the runner under both pressure and velocity heads. Unlike impulse turbines, where all energy is converted into kinetic energy before striking the blades, reaction turbines utilize both pressure energy and kinetic energy of the water to produce mechanical rotation.

The Francis turbine works with a mixed flow, meaning that the water enters the runner in a radial direction and leaves it in an axial direction. This combination allows it to extract maximum energy from the water stream efficiently.

Classification Basis

Turbines are classified according to three major criteria — type of energy conversion, direction of flow through the runner, and head of water. On this basis, the Francis turbine can be described as follows:

1. According to Energy Conversion:
   The Francis turbine is a reaction turbine, since both pressure and velocity energy are converted into mechanical work.
2. According to Flow Direction:
   It is a mixed flow turbine because the water flows radially inward at the inlet and axially outward at the exit.
3. According to Head:
   It is designed for medium head conditions, typically between 10 m and 650 m.
4. According to Shaft Position:
   It can have either a horizontal or vertical shaft arrangement depending on the design and installation site.

Working Principle of Reaction Turbine

In the Francis turbine, the total energy of the water (pressure and kinetic) is partially converted into mechanical energy in both stationary and moving parts. The guide vanes or wicket gates control and direct the water flow to the runner blades at the correct angle. As the water moves through the curved blades, it changes direction and loses energy. This loss of energy causes a reaction force on the blades, which produces torque on the runner shaft.

The water exits the runner through a draft tube, which helps to recover part of the kinetic energy by converting it into pressure energy, thus increasing the overall efficiency of the turbine.

This principle makes the Francis turbine more efficient than impulse turbines like the Pelton wheel for medium head conditions.

Reasons for Reaction Type Design

The Francis turbine is designed as a reaction turbine for the following reasons:

1. Efficient Energy Conversion:
   Utilizing both pressure and velocity energy allows more complete energy extraction from water.
2. Smooth Operation:
   The continuous water flow through blades provides stable and quiet operation.
3. High Efficiency Range:
   The turbine maintains high efficiency (85–95%) over a wide range of heads and loads.
4. Compact Structure:
   The design is compact, reducing the space required for installation compared to impulse turbines.
5. Adaptability:
   It can be used for different heads and discharges, making it suitable for most hydroelectric power stations.

Comparison with Other Turbines

• Pelton Turbine (Impulse Type):
  Works on high heads and uses only kinetic energy. The flow strikes the bucket directly, and pressure remains constant throughout.
• Kaplan Turbine (Reaction Type, Axial Flow):
  Suitable for low heads with axial water flow. It is similar in working to the Francis turbine but handles larger discharges.

The Francis turbine lies between these two types — it operates under medium head conditions with mixed flow characteristics, combining the benefits of both.

Example

If a hydroelectric plant has a water head of about 150 meters and a moderate discharge, a Francis turbine is ideal. The water enters the turbine radially, interacts with curved blades, and exits axially, transferring energy efficiently to rotate the generator.

Conclusion:

The Francis turbine is a reaction type mixed flow turbine designed to utilize both the pressure and kinetic energy of water. It operates efficiently under medium head conditions and converts water energy into mechanical energy through its curved runner blades. Its smooth operation, high efficiency, and ability to work under variable conditions make it the most popular turbine type in hydroelectric power generation systems.