# Heat Transfer Questions and Answers – Heat Exchange Between Black Bodies

This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Heat Exchange Between Black Bodies”.

1. Engineering problems of practical interest are involved with heat exchange between two or more surfaces, and this exchange is strongly dependent upon
(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. 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
(ii) Geometrical factor
(iii) Configuration factor
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. The interchange factor is also known as
a) Equivalent emissivity
d) Shape factor

Explanation: The interchange factor is also known as equivalent emissivity.

4. 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. 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
a) 0.25
b) 0.50
c) 0.75
d) 1.25

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)].
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6. The grey body shape factor for radiant heat exchange between a small body (emissivity = 0.4) in a large enclosure (emissivity = 0.5) is
a) 0.1
b) 0.2
c) 0.4
d) 0.5

Explanation: (F) 12 = 1/ (1 – E 1 + 1 + 0).

7. 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?
a) 2
b) 4
c) 1
d) 3

Explanation: Without shields/with shield = 1/N + 1.

8. 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
a) 0.50
b) 0.75
c) 0.1
d) 0.25

Explanation: F 11 + F 12 + F 13 + F 14 = 1.

9. The value of shape factor depends on how many factors?
a) 4
b) 1
c) 2
d) 3

Explanation: Geometry and orientation.

10. 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

a) 0.03
b) 0.04
c) 0.05
d) 0.06

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.

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