[kach22i]

Working of Francis Turbine
https://www.youtube.com/watch?v=3BCiFeykRzo

Comments section........................
vancebarcelona
7 months ago

Francis turbines are the most preferred hydraulic turbines. It contributes about 60% of the global hydro power capacity mainly because it can work efficiently under wide range of operation conditions. The most important part of a Francis turbine is its runner. It is fitted with complex shade blades. In the runner, water enters radially and exits axially. During the course of flow, water goes through the runner blades. Francis turbine blades are specially shaped. We can note that blades are having thin airfoil cross section. So, when water flows over it, low pressure will be produced on one site and high pressure in the other site. This will result in a lift force. We 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 producing 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 come impulse action also. Thus, when water flows over runner blades, both its kinetic and pressure energy will come down. Since flow is entering radially and leaving axially, they are also called mixed flow turbine. Runner is connected to the generator via shaft for electricity production. This arrangement is fitted in a spiral casing. Flow enters via inlet nozzle. Flow rate of water will get reduced along length of casing, since water is drown into the runner. But decreasing area of spiral casing will make sure that water flow is entered to runner region almost at uniform velocity. Stay vanes and guide vanes are fitted at the 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 runner section. Thus, it reduces inlet flow. Demand for power changes over time. The guide vane mechanism is used to control flow rate and make sure that power production is synchronized with power demand. Apart from controlling flow rate, guide vanes also control flow angle to inlet portion of runner blade. Thus, guide vanes make sure inlet flow angle is the optimum angle of attack for maximum power extraction from fluid. Low pressure at the outlet of the turbine may cause serious problems of cavitation. So, a special draft tube is designed is fitted at exit site to discharge the fluid out. Draft tube will transform velocity head to static head due its increasing area that will reduce effect of cavitation.