Heat Transfer MCQ’s – Heat Exchange Between Black Bodies
1 - Question
Engineering problems of practical interest are involved with heat exchange between two or more surfaces, and this exchange is strongly dependent upon (i) Radiative properties (ii) Temperature levels (iii) Surface geometrics Identify the correct statements
a) i and ii
b) i and iii
c) ii and iii
d) i, ii and iii
Explanation: For black surface, it is necessary to determine what portion of radiation emitted by one will be intercepted by the other.
2 - Question
The fraction of the radiative energy that is diffused from one surface element and strikes the other surface directly with no intervening reflections is called (i) Radiation shape factor (ii) Geometrical factor (iii) Configuration factor Choose the correct answer
a) i only
b) ii only
c) i, ii and iii
d) iii only
Explanation: Radiation shape factor, geometrical factor and configuration factor are all same.
3 - Question
The interchange factor is also known as
a) Equivalent emissivity
d) Shape factor
Explanation: The interchange factor is also known as equivalent emissivity.
4 - Question
For the same type of shapes, the value of the radiation shape factor will be higher when
a) Surfaces are closer
b) Surfaces are larger and held closer
c) Surfaces are moved further apart
d) Surfaces are smaller and held closer
Explanation: Obviously the value of radiation shape factor will be higher when surfaces are larger and held closer.
5 - Question
A thin shield of emissivity E 3 on both sides is placed between two infinite parallel plates of emissivities E 1 and E 2 and temperatures T 1 and T 2. If E 1 = E 2 = E 3, then the fraction radiant energy transfer without shield takes the value
Explanation: The ratio of radiant energy transfer without and with shield is given by (1/E 1 + 1/E 2 – 1)/ [(1/E 1 + 1/E 3 – 1) + (1/E 3 + 1/E 2 – 1)].
6 - Question
The grey body shape factor for radiant heat exchange between a small body (emissivity = 0.4) in a large enclosure (emissivity = 0.5) is
Explanation: (F) 12 = 1/ (1 – E 1 + 1 + 0).
7 - Question
Two long parallel surfaces, each of emissivity 0.7 are at different temperatures and accordingly have radiation exchange between them. It is desired to reduce 75% of this radiant heat transfer by inserting thin parallel shields of equal emissivity 0.7 on both sides. What should be the number of shields?
Explanation: Without shields/with shield = 1/N + 1.
8 - Question
An enclosure consists of four surfaces 1, 2, 3 and 4. The view factors for radiation heat transfers are F 11 = 0.1 F 12 = 0.4 F 13 = 0.25 The surface areas A 1 and A 2 are 4 m2 and 2 m2. The view factor F 41 is
Explanation: F 11 + F 12 + F 13 + F 14 = 1.
9 - Question
The value of shape factor depends on how many factors?
Explanation: Geometry and orientation.
10 - Question
Find the shape factor F 12 for the arrangement shown in the figure. The areas A 1 and A 2 are perpendicular but do not share the common edge
Explanation: A 5 = A 1 + A 3 and A 6 = A 2 + A 4. The sequence of the solution is, A 5 F 56 = A 1 F 16 + A 3 F 36.