Francis turbine is from the reaction turbine class of hydraulic turbine.It is name after its inventor James Francis. It was the first turbine with the radial flow, water enter the turbine radially and leave axially. High pressure water entering the turbine strike the blades of turbine and reaction of which blades of turbine rotates.
Parts of Francis Turbine
Pen stockIt is a large size pipe which take water from the reservoir and take it to the turbine runner
Spiral / scroll casingIt is a close end passage with maximum cross section area at the inlet which decrease continuously along the direction of flow and finally zero at the outlet
Guide vanes / wicket gatesMain function of the guide vanes is to guide the water toward the runner at an angle appropriate for the design
Governing mechanismMain function of the governing mechanism is to change the angle of attack at runner by guide vanes when flow rate of the water is changing
Runner and Runner bladesThere are 16 to 24 runner blades attached to the runner. Runner blades the the direct action of water and rotate the runner. runner rotate due to both impulse and reaction forces
Bottom Ringprovide drainage space
Shaft SealPrevent water leakage between main shaft and head cover.
Draft TubeIt is gradually expanding tube which discharge water passing through the runner, to the tail race
Working of Francis turbineAs water enter the turbine through the inlet nozzle its velocity will decrease as it moves forward in the turbine but the continuous decrease in the area of spiral casing will not let it happen. Design of spiral casing makes shore that the water strike the each blade of the turbine as a constant speed.
Blades are mounted on the runner. Design of the blade is like a thin air foil so when water flow over it low pressure is created at one point and high pressure is created on other side
and motion take place from high pressure to low pressure. Lower side of the blade is like a bucket which use the impulse forces of water in the rotation.
Advantages of Francis turbine
- Easy control on changing head
- Size of runner and generator is small
- Small change in efficiency over time
- The operating head can be utilized even when the variation in the tail water level is relatively large when compared to total head
Disadvantages of Francis turbine
- Unclean water can create wear
- Inspection method is very difficult
- Cavitation is an ever present danger
- Water hammer effect can create some problems
- Head 50% lower can have harmful effect on the efficiency as well as cavitation danger became more serious