Why does the cross sectional area of the Spiral casing gradually decrease along the circumference of the Francis turbine from the entrance to the tip?

Francis Turbine topics include: Francis turbine velocity diagrams, francis turbine work and efficiency. A Francis turbine is a type of reaction turbine invented by James B. Francis around 1855. It is the most common type of turbine used today. Francis turbines are used in about 60% of the world's hydropower capacity.  Here's how a Francis turbine works: Water enters the turbine in a radial direction, flowing towards its axis. The water is deflected 90 degrees in the runner, which sets the runner and the generator in a rotary motion. The energy released in this way can achieve an... Show more

Francis Turbine topics include: Francis turbine velocity diagrams, francis turbine work and efficiency.

A Francis turbine is a type of reaction turbine invented by James B. Francis around 1855. It is the most common type of turbine used today. Francis turbines are used in about 60% of the world's hydropower capacity. 

Here's how a Francis turbine works:
Water enters the turbine in a radial direction, flowing towards its axis.
The water is deflected 90 degrees in the runner, which sets the runner and the generator in a rotary motion.
The energy released in this way can achieve an efficiency of up to 95%. 

Here are some other features of Francis turbines:
They have a radial flow runner with fixed buckets, usually nine or more.
The turbine must be completely immersed when in operation.
The wicket gate around the outside of the turbines rotating runner controls the rate of water flow through the turbine for different power production rates.
The turbine runner is connected to an electrical generator by a common shaft.
As the runner rotates, so does the generator rotor.
The rotor rotates through an electromagnetic field, which induces current in the stator windings.

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