Hydraulic Machines MCQ with answer and solution

Concept

The draft tube allows the turbine to be set above the tailrace level without losing head and improves efficiency by reducing the pressure at the runner exit to below atmospheric pressure.

Formula

Efficiency (η) = (Net Head + Suction Head - Losses) / Net Head

Step-by-step solution

1. Identify the function of a draft tube: It is used in reaction turbines (like Francis or Kaplan) at the outlet. 2. Understand the energy conversion: It converts the high kinetic energy (V²/2g) at the runner exit into pressure energy. 3. Analyze the pressure effect: This conversion creates a vacuum or suction head at the runner exit. 4. Determine the outcome: The increased effective head leads to higher power output for the same discharge, thereby increasing the efficiency.

Explanation

A draft tube is a diverging conduit used at the exit of a reaction turbine to recover the kinetic energy of the water leaving the runner. By converting this kinetic energy into pressure energy, it creates a suction (negative pressure) at the runner outlet, effectively increasing the net head and thus the overall efficiency of the turbine.

Why the other options are wrong

A: Discharge is primarily controlled by the guide vanes or wicket gates, not the draft tube. C: While it increases the 'effective' head, the term 'Head' usually refers to the static head available from the source, which remains constant. D: Speed is a function of the runner design and flow rate, governed by the governor system.

Shortcut

Draft tube = Kinetic energy recovery = Increased Efficiency.

Common mistake

Students often think it increases the 'Head' (C) available from the reservoir, but it actually increases the 'Effective Head' or 'Net Head' utilized by the turbine, which ultimately manifests as an increase in efficiency.