This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Absorptivity”.
1. Energy strikes a vertical hemispherical plate with an intensity of 640 W/m2. The absorptivity is thrice the transmissivity and twice the reflectivity. Determine the rate of transmission
a) 236.74 W/m2
b) 116.37 W/m2
c) 187.87 W/m2
d) 456.09 W/m2
View Answer
Explanation: Rate os transmission = (0.5455) (640)/3 = 116.37 W/m2.
2. The absorptivity of a surface depends upon
(i) Direction of reflected radiation
(ii) Temperature of the surface
(iii) Composition
Identify the correct statements
a) i, ii and iii
b) i and iii
c) ii only
d) ii and iii
View Answer
Explanation: It depend upon the direction of incident radiation. Absorptivity is the fraction of total energy absorbed by the body.
3. What is the absorptivity of the gray body?
a) Below unity
b) Unity
c) 2
d) 2.5
View Answer
Explanation: Gray body absorbs a certain fixed percentage of impinging radiations.
4. A body that reflects all the incident thermal radiations is called a
a) Opaque body
b) Gases
c) Black body
d) Specular body
View Answer
Explanation: In this case reflection is regular. For such bodies reflectivity is unity and transmissivity is equal to zero.
5. Energy strikes a vertical hemispherical plate with an intensity of 640 W/m2. The absorptivity is thrice the transmissivity and twice the reflectivity. Determine the rate of absorption
a) 449.12 W/m2
b) 349.12 W/m2
c) 249.12 W/m2
d) 149.12 W/m2
View Answer
Explanation: Rate of absorption = (0.5455) (640) = 349.12 W/m2.
6. Let 220 W/m2 of radiant energy is absorbed by a convex surface, 90 W/m2 is reflected and 40 W/m2 is transmitted through it. What is the value of absorptivity?
a) 0.72
b) 0.62
c) 0.52
d) 0.42
View Answer
Explanation: It is the ratio of incident energy flow to that of absorbed radiations. Absorptivity = 220/(220 + 90 + 40) = 0.62.
7. For a grey surface
a) Reflectivity equals emissivity
b) Emissivity equals transmissivity
c) Absorptivity equals reflectivity
d) Emissivity is constant
View Answer
Explanation: Emissivity is constant only for grey surfaces because grey surface radiates much more than any of the other surface.
8. Which quantity can be neglected for gases?
a) Transmissivity
b) Reflectivity
c) Absorptivity
d) None can be neglected
View Answer
Explanation: Gases are known to reflect very little of the radiation incident on their surface.
9. Absorptivity of a body is equal to its emissivity
a) Under thermal equilibrium conditions
b) For a polished surface
c) At one particular temperature
d) At shorter wavelengths
View Answer
Explanation: In most case absorptivity of a body is equal to its emissivity under thermal equilibrium conditions.
10. Electromagnetic waves strikes a hot body maintained at 900 degree Celsius which has a reflectivity of 0.66 and a transmissivity of 0.022. Let the absorbed flux be 60 W/m2. Determine the rate of incident flux
a) 168.67 W/m2
b) 178.67 W/m2
c) 188.67 W/m2
d) 198.67 W/m2
View Answer
Explanation: Absorptivity + Reflectivity + Transmissivity = 1. So, incident flux = 60/0.318 = 188.67 W/m2.
Sanfoundry Global Education & Learning Series – Heat Transfer.
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