Engineering Questions with Answers - Multiple Choice Questions

# Thermal Engineering MCQ’s – Heat Transfer by Radiation

1 - Question

What is the equivalent emissivity for radiant heat exchange between a small body (emissivity = 0.4) in a very large enclosure (emissivity = 0.5)?
a) 0.5
b) 0.4
c) 0.2
d) 0.1

Explanation: When body 1 is completely enclosed by body 2,body 1 is very small.
ε=11ε1+A1/A2(1ε2−1)
Since, A1/A2 is very less whole term is neglected.
ε = 0.4.

2 - Question

If a body is at 2000K, the wavelength at which the body emits maximum amount of radiation is __________
a) 1.45µm
b) 1.45cm
c) 0.345cm
d) 0.345µm

Explanation: From Wien’s displacement law,
λmax T = 2898µmk
λmax ×2000 = 2898µmk
λmax = 1.45µm.

3 - Question

Which of the following parameter is not responsible for loss of heat from a hot surface in room?
a) Temperature of the surface
b) Emissivity of the surface
c) Temperature of the air in the room
d) Dimensions of the room

Q = ∈σ(T14-T24)
And A=πDL
Hence heat transfer will depend upon temperature of the surface and surrounding, surface Emissivity and length and diameter of the pipe.

4 - Question

Hot coffee in a cup is allowed to cool. Its cooling rate is measured and found to be greater than the value calculated by conduction, convection and radiation measurement. The difference is due to:
a) Properties of coffee changing with temperature
b) Currents of air flow in the room
c) Underestimation of the emissivity of coffee
d) Evaporation
Explanation: Hot coffee in a cup is allowed to cool, while measuring its cooling rate evaporation is considered so its value is greater than the value of calculated by conduction, convection and radiation.

5 - Question

Which of the following statement is incorrect?
a) For metals, the value of absorptivity is high
b) For non-conducting materials, reflectivity is low
c) For polished surfaces, reflectivity is high
d) For gases, reflectivity is very low