Heat Transfer MCQ’s – Absorptivity
1 - Question
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 - Question
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 - Question
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 - Question
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 - Question
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 - Question
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 - Question
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 - Question
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 - Question
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 - Question
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.