Francis turbines are very versatile. These are reaction turbines, i.e. during energy transfer from water to the runner there is a drop in static pressure as well as a drop in velocity head. Initially, the design of the slow runner (N; '" 60) was of the radial flow type, but dow they are of the mixed flow variety with radial entry and axial exit. Water from the penstock enters a spiral -or scroll casing which surrounds the runner (Fig. 10.28). The cross-section of the spiral diminishes uniformly along the circumference to ke~ the water velocity contant along its path. The water then enters the -guide vanes or wicket gates which are pivoted and can be turned suitably to regulate the flow and ostput. The guide vanes impart a tangential velocity or angular momentum to the water before entering the runner. The runner has a number of curved blades (12 to 22), welded. to the shrouds. The velocity of water gradually changes from radial to axial. 'JUter flowing past the runner, the water leaves through the draft tube, a closed flaring conduit, either straight or elbow type, increasing the pressure and reducing the velocity before falling into the tailrace.
In Fran!is turbine the pressure of water at the inlet is more than that at the outlet. Thus, the water in the turbine must flow in a closed conduit. Unlike the Pelton wheel where the water strikes only a few of the runner buckets at a time, in the Francis turbine the runner is always full of water. After doing its work the water is discharged to the tailrace through the closed tube of gradually enlarging section, the draft tube, which does not allow water to fall freely to tailrace as in the Pelton turbine. The free end of the draft tube is submerged deep into the tail water to make the entire water passage from the head race to the tailrace totally enclosed.
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