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The Francis turbine is a water wheel that harnesses the energy of running water and turns it into electricity. It is an inward flow reaction turbine, also known as a radial flow turbine. This means that water flows from the outside of the wheel through guide vanes that direct it toward the center of the wheel, where it exits the machine. As the water flows, it turns the turbine's runner, a central cylinder with an average of nine to 19 curved, blade-like spokes. This motion is transmitted by a shaft to a generator, where the water's energy is converted into electricity.
The machine was invented in 1849 by American scientist James Francis, after whom the machine is named. He conducted extensive experiments on the design of water turbines. His findings allowed him to improve upon the existing inward flow reaction turbine, increasing its efficiency to over 90%. It is the most efficient water turbine in the world, and it is also the most widely used.
Francis's modification of the standard turbine design made use of changing water pressure and the law of conservation of angular momentum. Water enters the turbine from a high-pressure source and flows through a spiral with a gradually decreasing radius, causing its pressure and angular momentum to drop and its speed to increase. The fast-moving water spins the runner with ever more force as it nears its center, capturing the water's energy with little waste. The water then passes out of the runner and exits through a tube designed to slow the water back down. This allows the water to be returned to its source gently, without excessive currents or eddying.
In addition to its incredible efficiency, the Francis turbine is popular because of its versatility. Adjustable guide vanes regulate the incoming water flow, keeping the runner speed, and, therefore, the power output, constant regardless of changes in the flow rate of the source water. Hydraulic "head" is a measure of a fluid's energy per unit weight, and the Francis turbine can be designed to work in a head range of approximately 10 to 700 meters. It can also generate power within a broad range from a few kilowatts to nearly a gigawatt.
Unlike an old-fashioned water wheel, the axis of a Francis turbine is vertical. Seen from above or a horizontal cross-section, the machine looks a bit like a snail's shell. In addition to generating power, this kind of turbine can also be used to pump water into reservoirs at higher elevation, where it can later be directed back through the turbine to make more electricity.