To know about the working principle of francis turbine, we will discuss inner flow francis turbine in which water enters from volute casing and leaves through the eye of the runner.
First of all we know that francis turbine operates on medium head and the water came from large reservoir tank through penstock. This water enters in the volute casing of the turbine. It is the type of casing which has converging cross section.
During the travelling of water from large reservoir through penstock, water develops pressure head but partially converts into velocity head as water move towards converging section of the casing.
After that water strikes to the guide vane mechanism. In this mechanism, we have two circular wheels in which the outer wheel is fixed one and the inner wheel is movable. Inner wheel adjust the position with respect to the fixed wheel.
In the end, water will strike to the runner of the turbine with partially pressure head and velocity head and leaves it through draft tube. It is the type of tube which has expansion duct like diffuser.
By using this type of duct, we minimize the velocity of water so that we can reuse water’s energy. Before entering in this duct, water has atmospheric pressure but after leaving it, its pressure is less than atmospheric pressure.