The Francis turbine is a hydraulic machine designed to generate high flow of water from low head of pressure. It is mainly used at hydroelectric power plant to drive AC current generators. In this type of turbine, the water is fed radially to the runner from guide vanes, which are located around the rim of the turning wheel; the runner consists of the peripheral rotating elements (rim + vanes). The angles of these vanes can be altered in order to control the turbine output as the water is evenly distributed to them by a spiral casing.
Having been directed onto the runner, the water flows radially inwards as it is turned by the runner blades so that it discharges axially into the draft tube. The function of the draft tube is to permit efficient recovery of the velocity energy that leaves the turbine so that the turbine efficiency is maximized. Both horizontal and vertical shaft mounting can be utilized. The maximum efficiency of the Francis turbine ranges from 94 to 95%.
Control of end thrust is important as it derives from the different pressures between the front and rear shrouds and their respective casings. The source of pressurization for the rear shroud is exclusively the high pressure at guide vane exit. On the other hand, the front shroud pressure is influenced by the guide vane exit pressure at the outer diameter as well as by the draft tube pressure at the inner diameter (or front shroud seal). The front face of the impeller also receives draft tube pressure over the whole eye.
Rolling bearings are often utilized on small turbines. On big turbines, however, and particularly on vertical shaft machines, where bearing stability can be a problem, tilting pad journal bearings are often employed. Meanwhile, machine output is controlled by the guide vanes, with an average number of 24 being utilized.
Below, diagram of a Francis turbine
