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Chapter 12
Radiation Heat Transfer
1243
Two perpendicular rectangular surfaces with a common edge are maintained at specified
temperatures. The net rate of radiation heat transfers between the two surfaces and between the horizontal
surface and the surroundings are to be determined.
Assumptions
1
Steady operating conditions exist
2
The surfaces are opaque, diffuse, and gray.
3
Convection heat transfer is not considered.
Properties
The emissivities of the horizontal rectangle and the surroundings are
ε
= 0.75 and
ε
= 0.85,
respectively.
Analysis
We consider the horizontal rectangle to be surface 1, the vertical rectangle to be surface 2 and the
surroundings to be surface 3. This system can be considered to be a threesurface enclosure. The view
factor from surface 1 to surface 2 is determined from
27
.
0
75
.
0
6
.
1
2
.
1
5
.
0
6
.
1
8
.
0
12
2
1
=
⎪
⎪
⎭
⎪
⎪
⎬
⎫
=
=
=
=
F
W
L
W
L
(Fig. 126)
W
= 1.6 m
(2)
L
2
= 1.2 m
L
1
= 0.8 m
A
1
(1)
A
2
T
3
= 290 K
ε
3
= 0.85
T
2
= 550 K
ε
2
= 1
T
1
=400 K
ε
1
=0.75
(3)
The surface areas are
2
2
2
1
m
92
.
1
)
m
6
.
1
)(
m
2
.
1
(
m
28
.
1
)
m
6
.
1
)(
m
8
.
0
(
=
=
=
=
A
A
2
2
2
3
m
268
.
3
6
.
1
2
.
1
8
.
0
2
8
.
0
2
.
1
2
=
×
+
+
×
×
=
A
Note that the surface area of the surroundings is determined assuming that surroundings forms flat surfaces
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Mass, Heat, Radiation

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