Working of Francis Turbine

Francis turbines are the most preferred hydraulic turbines it contributes about 60% of the global hydropower capacity mainly because it can work efficiently under a wide range of operating conditions most important part of Francis turbine is its runner it is fitted with a collection of complex shaped blades in runner water is entered radially and leaves axially during the course of flow water glides over runner blades Francis turbine blades are specially shaped you can note that blades are having thin airfoil cross-section so when water flows over it a low pressure will be produced on one side and high pressure on the other side this will result in a lift force you can also note one more peculiar thing about the blade it is having a bucket kind of shape towards the outlet so water will hit and produce an impulse force before leaving the runner both impulse force and lift force will make the runner rotate so Francis turbine is not a pure reaction turbine a portion of force comes from impulse action also thus as water flows over runner blades both its kinetic and pressure energy will come down since flow is entering radially and leaves axially they are also called mixed flow turbine runner is connected to generator via a shaft for electricity production this arrangement is fitted inside a spiral casing flow is entered via an inlet nozzle flow rate of water will get reduced along length of casing since water is drawn into the runner but decreasing area of spiral casing will make sure that flow is entered to run a region almost at a uniform velocity stay vanes and guide vanes are fitted at entrance of runner the basic purpose of them is to convert one part of pressure energy to kinetic energy flow which is coming from the casing meets stay vanes they are fixed stay vanes steers the flow towards the runner section thus it reduces swirl of Inlet flow demand for power may vary over time the guide vein mechanism is used to control water flow rate and make sure that power production is synchronized with power demand apart from controlling flow rate guide veins also control flow angle to inlet portion of runner blade thus guide vanes make sure that Inlet flow angle is that optimum angle of attack for maximum power extraction from fluid low pressure at exit side of turbine may cause serious problems of cavitation so a carefully designed draft tube is fitted at exit side to discharge the fluid out draft tube will transform velocity head to static head due to its increasing area and will reduce effect of cavitation hope you had a nice session on working of Francis turbines thank you

Channel: Lesics

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Keywords: runner, runner blade, turbomachinery, Francis Turbine, hydraulic machines, draft tube, spiral casing, Hydro turbines, Turbine (Engine Category)

Id: 3BCiFeykRzo

Length: 4min 13sec (253 seconds)

Published: Wed Jul 24 2013