The angle of taper on the draft tube is

Explanation:

Draft Tube: The draft tube is a conduit that connects the runner exit to the tailrace where the water is being finally discharged from the turbine. It is used with reaction turbines only.

The draft tube has two purposes as follows if

i) 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.

ii) It converts a large proportion of velocity energy rejected from the runner into useful 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.