The 'draft tube' in a reaction turbine (Francis/Kaplan) serves to:
Choose the correct answer
Supply water to the turbine from the reservoir
Recover kinetic energy at runner exit AND allow turbine above tailwater level — maintains full net head utilisation
Regulate turbine speed
Filter debris from water
Correct Answer
B. Recover kinetic energy at runner exit AND allow turbine above tailwater level — maintains full net head utilisation
Why is this the answer?
Draft tube: diverging passage connecting runner exit to tailrace. Functions: (1) Converts kinetic energy at runner exit to pressure (reduces exit velocity loss); (2) Allows turbine to be set ABOVE tailwater level without losing net head (creates suction at runner by extending head through draft tube). Recovery: velocity head at runner exit (V²/2g) recovered. Without draft tube: runner exit velocity wasted.
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