Draft tube is used in reaction turbine to

Explanation:

Draft Tube: The draft tube is a diverging tube connects the runner exit to the tailrace. At the exit of a turbine runner, the fluid has appreciable kinetic energy and to recover some of this kinetic energy which would otherwise be wasted the flow enters an expanding area diffuser called a draft tube. It is used with reaction turbines only.

The draft tube has two purposes as follows:

1. It permits a negative or suction head to be established at the runner exit, thus making it possible to install the turbine above the tailrace level without loss head.
2. It is used to convert kinetic energy at the exit of the turbine to the pressure energy.

​A draft tube is made divergent so as to reduce the velocity at the outlet to a minimum. Therefore, a draft tube is basically a diffuser and should be designed properly with the angle between the walls of the tube to be limited to about 8 degrees so as to prevent the flow separation from the wall and to reduce accordingly the loss of energy in the tube.

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