Hydraulics and Fluid Mechanics in ME
The overall efficiency of a reaction turbine is the ratio of

Work-done on the wheel to the energy (or head of water) actually supplied to the turbine
Actual work available at the turbine to the energy imparted to the wheel
None of these
Power produced by the turbine to the energy actually supplied by the turbine

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Hydraulics and Fluid Mechanics in ME
In a reaction turbine, the draft tube is used

To run the turbine full
To transport water to downstream
To increase the head of water by an amount equal to the height of the runner outlet above the tail race
To prevent air to enter the turbine

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