0 1F F F OR F+ = = =∵− − − −=A F AF1 2 2 1− −2Π= × = × =OR 1 1A R2F...
1,
0
1
F
F
F
or F
+
=
=
=
∵
−
−
−
−
=
A F
AF
1
2
2 1
−
−
2
π
=
×
=
×
=
or
1 1
A
r
F
F
1 2
2 1
2
1
1
1
0.5
+
=
= =
F
F
or F
−
−
−
1 1
1 2
1 1
IES-34. Ans. (b)
F
22
=
0;
∴
F
21
=
1
or
1
A F
A F
F
=
=
=
π
=
1 12
2 21
12
2
IES-35. Ans. (d)
F
11
+
F
12
=
1;
∵
F
11
=
0
+
=
⇒
=
0
1
1
12
12
D
⎛ ⎞
⎜ ⎟
π
π
4
2
A F
A F
F
A
=
⇒
=
=
⎝ ⎠
=
1 12
2
21
21
1
2
6
A
D
IES-36. Ans. (c)
Page 92 of 97
IES-37. Ans. (b)
Shape factor F
12
means part of radiation
body 1 radiating and body 2 absorbing
+
=
11
12
or 0
1
F
12
A F
A F
or A F
then
1 12
2
21
2
21
or
0.6
1 0.004
F
A
F
=
×
=
× =
21
1
12
2
A
L
or
0.6
5m
=
=
L
×
6 0.004
1
A
1
2
rl
0.36
IES-38. Ans. (d) Shape factor F
22
=
1
−
= −
=
A
π
rl
IES-39. Ans. (c)
F
11
+
F
12
=
1 as
F
11
=
0 or
F
12
=
1
A F
A F
F
A F
F
1
2
1 12
2
21
21
1 12
22
or
or
3
3
=
=
A
=
=
IES-40. Ans. (b)
IES-41. Ans. (d)
IES-42. Ans. (b)
F
2 2
−
=
0,
∴
F
2 1
−
=
1
A
d
d
/ 4
A F
A F
or F
1 1 2
2
2 1
12
2
2
−
=
−
=
=
=
+
+
A
d
Dh
d
h
4
IES-43. Ans. (b)
F
14
= −
1 0.1 0.4 0.25 0.25
−
−
=
A F
A F
or F
A F
4
0.25 0.5
1 14
4
41
41
1 14
=
=
A
= ×
=
IES-44. Ans. (d)
IES-45. Ans. (c)
IES-46. Ans. (b)
IES-47. Ans. (c) When body 1 is completely enclosed by body 2, body 1 is large.
is given by
1
.
∴ ∈
∈
+
⎛
⎜
⎝
∈
−
⎞
⎟
⎠
A
1
1
1
1
2
2
∈ = ∈
( )4
4
∴
=
∈
=
−
q
A
σ
T
T
1 2
1
1
1
2
−
IES-48. Ans. (b)
IES-49. Ans. (d)
IES-50. Ans. (b) Interchange factor (f
12
)
=
=
1
1
1
2
1
ε
+
−
1
400 k
ε
300 k
ε
ε
0.9
−
= 0.8182
IES-51. Ans. (b) Heat transfer Q =
σ
F F T
e
A
(1
4
−
T
2
4
)W m
/
2
; σ = 5.67 × 10
- 8
W/m
2
K
4
Page 93 of 97
1
1
12
F
e
= effective emissivity coefficient =
1
1
1
1
1
1
23
+
−
+
−
0.8
0.6
Shape
factor
F
A
= 1
8
12
4
4
2
2