A Francis turbine is a type of turbine used most frequently in medium- or large-scale hydroelectric plants. These turbines can be used for heads as low as 2 meters and as high as 300 meters. Additionally, these turbines are beneficial as they work equally well when positioned horizontally as they do when they are oriented vertically.
Water enters these turbines radially meaning that it enters the turbine perpendicular to the rotational axis. Once entering the turbine, the water always flows inwards, towards the center. Once the water has flown through the turbine, it exits axially - parallel to the rotational axis. Francis turbines were the first hydraulic turbines that had a radial inflow, designed by American scientist James Francis.
Francis turbines have a fairly simplistic design. A flat, circular plate is attached to a rotating shaft that rests on this plate vertically. The plate itself has curved blades on it, and this combined unit of blades, plate, and rotating shaft are known collectively as the runner. Surrounding the runner is a channel known as a volute. The volute is simply a curved tube that diminishes in size as it approaches the runner, slightly resembling a snail shell. Blades known as guide vanes do not move, and are set into the inner surface of the volute. These vanes serve the purpose of directing the water through the volute to the runner at an appropriate angle.
The designs of these turbines can result in efficiencies as high as 95%, but only in optimum conditions. Water must enter the turbine at the appropriate speed and direction for the runner blades to operate properly.
Francis turbines are used frequently in hydroelectric power plants. In these plants, high pressure water enters the turbine through the snail-shell casing (the volute). This lowers the pressure as the water curls through the tube, but the speed of the water is maintained. After passing through the volute, the water moves through the guide vanes and is directed towards the blades on the runner at optimum angles.
As the water crosses the specially curved blades of the runner, the water is deflected sideways slightly. This causes the water to lose some of its "whirl" motion. The water is also deflected axially, so that it exits out of a draft tube to the tail race. This tube reduces the exit velocity of the water to obtain the maximum amount of energy from the input water. The process of water being deflected through the runner blades results in a force that pushes the blades in the opposite direction as the water is deflected. This reaction force (like Newton's third law) is what causes power to be transferred from the water to the turbine's shaft, maintaining rotation. Because the turbine moves as a result of this reaction force, Francis turbines are known as reaction turbines. The process of changing the direction of the water flow also results in a decrease in pressure within the turbine itself.
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