Short Answer:

A Francis turbine works by converting the energy of flowing water into mechanical energy using a combination of pressure energy and kinetic energy. Water enters the turbine through guide vanes, flows over curved runner blades, and causes the runner to rotate, producing power.

The water enters radially and exits axially through the draft tube. As it flows through the turbine, both pressure and velocity change, and the curved blades of the runner help in smooth energy conversion. Francis turbines are commonly used in medium-head hydropower plants due to their high efficiency and reliable performance.

Detailed Explanation

How a Francis Turbine Works

The Francis turbine is a reaction-type, mixed-flow hydraulic turbine designed for medium-head hydroelectric projects (typically from 30 to 300 meters). It operates by utilizing both pressure and kinetic energy of water to rotate a runner and generate mechanical energy, which is then converted to electricity using a generator. The unique feature of the Francis turbine is that water enters radially and exits axially, making it efficient for a wide range of operating conditions.

Main Working Principle

The working of a Francis turbine involves the smooth transition of water from the spiral casing to the runner blades, using both pressure and velocity to generate rotation.

Step-by-step working process:

1. Water Entry through Spiral Casing
   Water from a reservoir is guided into the spiral-shaped scroll casing, which surrounds the runner and evenly distributes water to the guide vanes.
2. Control by Guide Vanes
   The adjustable guide vanes (also known as wicket gates) regulate the flow and angle of water entering the turbine. These vanes help in controlling the turbine output and maintaining efficiency during varying load conditions.
3. Flow through Runner Blades
   Water then passes through the curved runner blades. These blades are designed to extract energy from both the pressure and velocity of the water. As water flows through the runner, it causes the runner to spin.
4. Energy Conversion
   The pressure and kinetic energy of the water are transferred to the runner, converting hydraulic energy into mechanical rotational energy.
5. Discharge through Draft Tube
   After rotating the runner, the water exits through a draft tube, which helps recover some kinetic energy and allows the turbine to be installed above the tailwater level.
6. Power Generation
   The rotating shaft connected to the runner is linked to a generator, which converts mechanical energy into electrical energy.

Operating Features

• Water flows radially inward and axially outward.
• Operates under medium-head conditions (30–300 meters).
• Has high efficiency even with varying flow rates.
• Requires precise design and maintenance for best performance.

Applications in Civil Engineering

Francis turbines are commonly used in:

• Dams and hydroelectric power stations.
• Medium-head run-of-the-river projects.
• Pumped storage plants for grid balancing.

They are suitable for a wide range of flows and heads, making them one of the most widely used turbines in the world for power generation.

Conclusion

A Francis turbine works by allowing water to enter through guide vanes and flow over curved runner blades, converting both pressure and kinetic energy into mechanical rotation. The water enters radially and exits axially, making the process smooth and efficient. These turbines are ideal for medium-head hydroelectric projects and play a vital role in modern civil engineering hydropower systems.