Short Answer:

A Francis turbine is a reaction-type hydraulic turbine that converts both the pressure energy and kinetic energy of water into mechanical energy. It is the most commonly used turbine in hydroelectric power plants for medium head and large discharge conditions.

In a Francis turbine, water enters radially and exits axially through curved blades fixed on the runner. The turbine operates fully submerged in water, and its efficiency is very high. It is suitable for medium-head applications (30 to 600 meters) and is widely used in modern power stations.

Detailed Explanation :

Francis Turbine

The Francis turbine is one of the most widely used turbines in hydroelectric power plants. It was designed by James B. Francis in 1849. It is a reaction turbine, meaning that it works with both pressure and kinetic energy of water. Water flows through the turbine blades, and the pressure decreases as it transfers energy to the runner.

The Francis turbine is suitable for medium heads and high discharge rates. It is an inward flow turbine, where water enters the runner radially and leaves axially. The turbine operates under water and is completely enclosed to maintain pressure. Its compact design, high efficiency, and versatility make it ideal for a wide range of hydropower installations.

Construction of Francis Turbine

A Francis turbine consists of the following main components:

Spiral Casing (Scroll Casing):
The spiral casing is a circular, spiral-shaped passage that surrounds the runner. It distributes water uniformly around the runner through the guide vanes. The area of the casing gradually decreases to maintain a constant velocity of flow around the runner.
Guide Vanes or Stay Vanes:
These vanes control the flow and direction of water before it enters the runner blades. They guide water at the correct angle to ensure maximum efficiency. The guide vanes are adjustable, allowing control of the water flow according to the load on the turbine.
Runner and Blades:
The runner is the rotating part of the turbine, having a series of curved blades mounted on it. When water flows over these blades, it transfers its energy to the runner, causing it to rotate. The runner blades are designed to allow smooth entry and exit of water with minimal losses.
Draft Tube:
The draft tube is a gradually expanding tube fitted at the exit of the runner. It helps to recover the kinetic energy of the discharged water by converting it into pressure energy. The draft tube also allows the turbine to be placed above the tailrace level without losing suction head.
Shaft:
The runner is mounted on a shaft, which transfers the mechanical energy from the turbine to the generator. Depending on design requirements, the shaft can be vertical or horizontal.
Casing and Foundation:
The entire turbine assembly is enclosed in a water-tight casing to prevent leakage and to maintain operating pressure. The turbine is supported on a solid foundation to withstand vibrations and hydraulic forces.

Working of Francis Turbine

The working principle of the Francis turbine is based on the conversion of pressure and velocity energy of water into mechanical energy. The process involves the following steps:

Water Flow to Spiral Casing:
Water from the reservoir flows through the penstock and enters the spiral casing surrounding the turbine. The casing ensures uniform water distribution around the runner.
Flow through Guide Vanes:
From the casing, water passes through adjustable guide vanes that control the amount and angle of water entering the runner blades.
Energy Conversion in the Runner:
Water enters the runner radially and flows through curved blades toward the turbine axis. As it passes over the blades, both its pressure and velocity decrease, transferring energy to the runner. The runner rotates as a result of this energy exchange.
Exit through Draft Tube:
The water leaving the runner enters the draft tube, which gradually expands in diameter. This helps in recovering some kinetic energy of water and ensures smooth discharge into the tailrace at atmospheric pressure.
Power Generation:
The runner’s rotation drives the turbine shaft, which is connected to a generator. The generator converts mechanical energy into electrical energy.

Working Principle

The Francis turbine works on the reaction principle. The energy is generated due to both the pressure difference and the reaction force produced by the flow of water over the runner blades. The flow in a Francis turbine is both radial and axial, which ensures smooth and efficient energy conversion.

The pressure energy and kinetic energy of water are partly converted into rotational motion of the runner, resulting in high efficiency and stable performance under varying load conditions.

Operating Conditions

Type: Reaction turbine
Head range: 30 to 600 meters
Discharge: Medium to high
Efficiency: 85% to 90%
Direction of flow: Inward radial at entry and axial at exit
Position: Fully submerged under water

Advantages of Francis Turbine

High efficiency over a wide range of loads.
Compact design with less space requirement.
Suitable for medium head and large discharge applications.
Operates smoothly with adjustable guide vanes.
Can be installed vertically or horizontally based on site conditions.

Disadvantages of Francis Turbine

Complex design and manufacturing process.
Sensitive to changes in water flow and sediment in water.
Maintenance is more difficult compared to impulse turbines.

Applications of Francis Turbine

Widely used in medium-head hydroelectric power plants around the world.
Installed in both run-of-river and dam-based power stations.
Suitable for power stations requiring continuous and stable energy output.
Used in medium to large-scale electricity generation systems.

Importance of Francis Turbine in Hydropower

The Francis turbine is the most commonly used turbine type in hydropower plants because of its adaptability and high efficiency. Its ability to handle varying heads and discharges makes it suitable for a wide range of sites. Modern turbines are designed with computer-optimized blades and adjustable guide vanes to maintain efficiency under fluctuating conditions.

The turbine’s reliability, compactness, and ability to operate over long periods make it an ideal choice for sustainable and continuous power generation.

Conclusion :

A Francis turbine is a mixed-flow reaction turbine that efficiently converts both pressure and kinetic energy of water into mechanical energy. It is suitable for medium head and large discharge applications and is widely used in hydroelectric power plants across the world.

Its high efficiency, compact design, and operational flexibility make it one of the most successful turbines ever developed. The Francis turbine continues to play a key role in modern hydropower generation, contributing significantly to clean and renewable energy production.