Hydraulic Machines MCQ with answer and solution

Concept

Turbines are classified based on the action of water into impulse turbines (like Pelton wheel) and reaction turbines (like Francis and Kaplan turbines).

Step-by-step solution

1. Check the classification of turbines: - Pelton wheel is an impulse turbine. It works under high head and low flow. - Francis turbine is an inward-flow reaction turbine (medium head and medium discharge). - Kaplan and Propeller turbines are axial-flow reaction turbines (low head and high discharge). 2. Review each statement: - Statement (A): Reaction turbines (like Kaplan) are used for low head and high discharge. This is correct. - Statement (B): The total cone angle of a draft tube is kept less than 8° to prevent flow separation. This is correct. - Statement (C): An impulse turbine must be kept above the tailrace level so that water falls freely. This is correct. - Statement (D): A Francis turbine is an impulse turbine. This is incorrect because Francis turbine is a reaction turbine. 3. Identify the wrong statement asked in the question: Statement (D) is wrong. Answer: (D) A Francis turbine is an impulse turbine.

Explanation

A Francis turbine is a reaction turbine, not an impulse turbine. In a reaction turbine, water enters under pressure and flows through closed guide passages and runner vanes. Pelton wheel is the most common example of an impulse turbine.

Why the other options are wrong

A: Reaction turbines such as Kaplan turbines are correctly used for low head and large flow. B: The draft tube taper angle is kept under 8° to avoid boundary layer separation. C: An impulse turbine is installed above the tail race so the runner does not submerge in water.

Shortcut

Remember: Pelton = Impulse, Francis and Kaplan = Reaction.

Common mistake

Students sometimes confuse Pelton and Francis turbines because Francis handles medium head.