Question

Arrange the fluids in the decreasing order of heat transfer coefficient \(h\) in forced convection regime for similar operating conditions: A. Air
B. Water
C. Mercury
D. Boiling water

• A. D > C > B > A
• B. C > D > A > B
• C. A > B > C > D
• D. B > A > D > C

Hint:

Phase change (boiling/condensation) gives highest heat transfer coefficient.

Answer 1

The Correct Option is A

Concept: Heat transfer coefficient \(h\) depends on:
• Thermal conductivity of fluid
• Specific heat
• Density
• Viscosity
• Phase change effects (if any)

Step 1: Understand relative heat transfer ability.

• Gases (like air) → lowest \(h\)
• Liquids (like water) → higher \(h\)
• Liquid metals (like mercury) → very high thermal conductivity
• Boiling → extremely high \(h\) due to phase change

Step 2: Analyze each fluid carefully.
Air (A):
• Low density and conductivity
• Lowest heat transfer coefficient Water (B):
• Higher thermal conductivity than air
• Moderate \(h\) Mercury (C):
• Very high thermal conductivity
• Higher \(h\) than water Boiling water (D):
• Involves latent heat
• Bubble formation enhances heat transfer
• Highest \(h\)

Step 3: Arrange in decreasing order.
\[ D > C > B > A \] Final Answer: \[ \boxed{D > C > B > A} \]