Theory of Operation of Francis Turbine
The Francis Turbine is the most used Turbine type.The Francis turbine is a reaction turbine, which means that the working fluid changes pressure as it moves through the turbine, giving up its energy. A casement is needed to contain the water flow. The turbine is located between the high pressure water source and the low pressure water exit, usually at the base of a dam.
The inlet is spiral shaped. Guide vanes direct the water tangentially to the turbine wheel, known as a runner. This radial flow acts on the runner's vanes, causing the runner to spin. The guide vanes may be adjustable to allow efficient operation for a range of water flow. As the water moves through the runner its spinning radius decreases, further acting on the
runner. This property, in addition to the water's pressure, helps Francis and other inward- flow turbines harness water energy efficiently.
At the exit, water acts on cup shaped runner features, leaving with no swirl and very little kinetic or potential energy. The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.
more detail on Francis Turbines