History
The Francis turbine was invented by James B Francis in the 1850s. Waterwheels had been used to complete useful work for over 1,000 years, but they were inefficient. James Francis applied mathematical principles when designing the Francis turbine and created a turbine with an efficiency exceeding 90%.
Although there are different variations of the Francis turbine now available, the fundamental working principles remain the same as those used over 100 years ago. As the electrical power industry grew over the past 150 years, the Francis turbine was able to compete with other prime movers in terms of cost, which lead to its widespread use for many applications.
Different Francis Turbine Variations
Introduction
Due to their versatile design, Francis turbines are suitable for a wide range of pressures and flow rates. Due to this wide operating range, the Francis turbine is the most common type of hydroelectric turbine employed today.
The Francis runner can be operated as both a hydraulic pump and hydraulic turbine, this is a unique feature compared to other common hydroelectric runners such as the Kaplan and Pelton runners. Pumped storage power plants employ Francis runners exclusively, due to this unique feature.
The efficiency of Francis turbines often exceeds 90% when the right operating conditions are met. This efficiency does not decrease until the load is below 40%.
Theory of Operation
Francis turbines convert potential energy to mechanical energy. This type of turbine is classified as a reaction turbine, as it operates within a pressure system and relies upon a continuous body of water from the suction to pressure side of the turbine.
Construction
The Francis turbine consists of a series of blades mounted between a runner crown and runner band. Gaps between the blades allow water to flow from the outer periphery of the runner to the inner section of the runner; this type of flow is known as radial flow.
Francis Turbine Runner
A spiral case -also known as a scroll case- is used to deliver an even flow of water to the entire runner. Even flow is achieved due to the gradually decreasing cross-sectional area of the casing. As the cross-sectional area decreases, the velocity of the water in the case is maintained and an even flow of water is delivered to the runner.
Francis Turbine Scroll Case
Guide vanes direct water towards the runner. The purpose of the guide vanes is to convert the potential energy of the water to kinetic energy, and to direct the water into the runner at an optimal angle.
A draft tube is used to convert some of the kinetic energy of the discharged water back into pressure energy. This conversion increases the overall operating efficiency of the turbine.
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How Francis Turbines Work
The below video is an extract from our Introduction to Hydroelectric Power Plants Online Video Course.
Water is delivered through a pressurised water conductor known as a penstock. The water travels along the spiral case and passes through the guide vanes. The guide vanes convert the water’s potential energy to kinetic energy and can also be used to start, stop and regulate flow to the runner.
The water enters the Francis runner radially, flowing from the outside to inside of the runner. The water is then discharged downwards, out of the base of the runner; this type of flow is known as axial flow due to the flow being in a parallel direction to the runner shaft.
Francis Turbine Cross Section
Because flow is both radial at entry and axial at discharge, Francis turbines are classified as mixed flow turbines.
The draft tube converts some of the water’s remaining kinetic energy to potential energy; the water is then discharged to the tailrace.
Power Generation
Once water is flowing through the runner, the blades convert the potential energy of the water to mechanical energy. The mechanical energy is applied as torque on the main runner shaft and the shaft begins to rotate.
A common shaft connects the runner to a generator, so as the runner rotates, so too does the generator rotor. The generator rotor rotates within an electromagnetic field, as the rotor moves through the magnetic field, current is induced in the generator stator windings, at this point the mechanical energy supplied by the Francis runner has been converted to electrical energy. The electrical energy can now be transferred through a national grid to end consumers.
The entire power generation process is continuous, which leads to a constant, renewable and reliable form of power generation.
How Pumped Storage Power Plants Work
The below video is an extract from our Introduction to Hydroelectric Power Plants Online Video Course.
See our pumped storage article for more detailed information.
Additional Resources
https://en.wikipedia.org/wiki/Francis_turbine
https://theconstructor.org/practical-guide/francis-turbines-components-application/2900
https://lesics.com/how-does-francis-turbine-work.html
https://blog.gridpro.com/understanding-the-flow-through-francis-turbine