Short Answer:

The working principle of a Francis turbine is based on the conversion of both pressure energy and kinetic energy of water into mechanical energy through the use of reaction force. Water flows from the guide vanes to the runner blades, changing direction and pressure as it moves. This change in momentum of water produces a force that rotates the runner, which in turn drives the turbine shaft connected to the generator.

In simple terms, the Francis turbine works on the principle of reaction, where the movement of water through the curved blades causes a reaction force that produces rotational motion. This process efficiently converts hydraulic energy into useful mechanical energy used for power generation.

Detailed Explanation :

Working Principle

The working principle of the Francis turbine is based on the reaction principle, which means that both the pressure energy and kinetic energy of water are used to produce mechanical energy. When water flows through the runner blades, its pressure decreases and velocity changes. This change in energy results in a reaction force on the blades, which causes the runner to rotate. The rotation of the runner converts the hydraulic energy of water into mechanical energy that is then used to drive an electric generator.

The turbine operates under medium head conditions (10 m to 650 m) and can handle a wide range of discharge values. The overall working of the Francis turbine can be understood step-by-step as follows.

1. Flow of Water through Spiral Casing

The working process starts when high-pressure water from the penstock enters the spiral casing or volute casing. This casing is designed in a spiral shape to distribute the water evenly around the circumference of the runner. As the water moves through the casing, its cross-sectional area gradually decreases to maintain constant velocity. The spiral casing ensures that water enters all the guide vanes with uniform pressure and speed.

2. Flow Regulation by Guide Vanes

After leaving the spiral casing, water flows through guide vanes (also called wicket gates). These adjustable vanes control the amount and direction of water entering the runner blades. The angle of the guide vanes can be changed according to the load on the turbine to regulate water flow.

The guide vanes convert part of the pressure energy of water into kinetic energy and direct the jet of water toward the runner blades at an appropriate angle. The flow direction at this stage is primarily radial, that is, water moves inward toward the center of the turbine.

3. Flow through Runner Blades

The water jet from the guide vanes strikes the curved blades of the runner. The runner is the rotating part of the turbine where the main energy conversion occurs. When water passes over the curved surfaces of the runner blades, both its direction and velocity change.

This change in momentum of water exerts a reaction force on the blades. According to Newton’s third law, the water exerts an equal and opposite force on the blades, which produces torque and causes the runner to rotate.

In the Francis turbine, the flow of water through the runner is mixed flow — it enters radially inward and leaves axially downward. This design improves efficiency by allowing smoother water discharge and better utilization of pressure energy.

4. Conversion of Energy

The main energy transformation in the Francis turbine is from hydraulic energy to mechanical energy. The water entering the runner has both pressure and kinetic energy, and as it moves through the runner blades, most of this energy is converted into rotational mechanical energy.

The rotating runner is connected to a shaft, which transmits the mechanical energy to an electric generator. The generator then converts the mechanical energy into electrical energy for power supply.

5. Discharge through Draft Tube

After passing through the runner, water still retains some velocity energy. To recover part of this remaining energy, the water is passed through a draft tube — a gradually expanding pipe connected to the runner exit.

The draft tube converts the leftover kinetic energy of the water into pressure energy and helps discharge the water safely into the tailrace (the downstream water level). It also maintains a suction pressure at the runner exit, which ensures continuous flow of water and allows the turbine to be placed above the tailrace level without loss of head.

6. Overall Energy Conversion Process

The entire process of energy conversion in the Francis turbine can be summarized as follows:

• Step 1: Water enters the turbine through the spiral casing under pressure.
• Step 2: Guide vanes regulate and direct the water onto the runner blades.
• Step 3: Water impinges on the runner blades, and a reaction force rotates the runner.
• Step 4: The runner transmits the mechanical energy to the shaft and generator.
• Step 5: The draft tube discharges the water, recovering part of the kinetic energy.

The efficiency of the Francis turbine is very high, usually between 85% and 95%, due to the effective use of both pressure and velocity energy.

Mathematical Expression (Conceptual)

The power developed by the Francis turbine depends on the energy head and discharge rate. The efficiency (η) is given by:

Since the turbine works on a reaction principle, the pressure drop occurs continuously as the water passes through the turbine.

Conclusion:

The working principle of the Francis turbine is based on the reaction principle, where both pressure and kinetic energy of water are converted into mechanical energy through curved runner blades. Water flows radially inward and exits axially, allowing efficient energy transfer with minimal losses. This smooth and continuous operation under medium head conditions makes the Francis turbine the most widely used hydro turbine in power generation plants around the world.