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This set of Thermal Engineering Questions & Answers for Exams focuses on “Heat Transfer by Radiation”.

1. 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=$$\frac{1}{\frac{1}{ε1}+A1/A2(\frac{1}{ε2}-1)}$$
Since, A1/A2 is very less whole term is neglected.
ε = 0.4.

2. 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. 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. 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. 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

Explanation: The emissivity of metallic surface is generally small. Therefore absorptivity will be high. Emissivity of non conducting material is high so reflectivity low. For highly polished surface reflectivity is high.