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Chapter 9
Natural Convection
Heat loss by both natural convection and radiation heat can be expressed as
]
)
K
273
20
(
)
273
)[(
10
67
.
5
)(
m
03
.
0
)(
8
.
0
(
C
)
20
)(
m
03
.
0
)(
C
.
W/m
696
.
6
(
W
8
)
(
)
(
4
4
8
2
2
2
4
4
+
−
+
×
+
°
−
°
=
−
+
−
=
−
∞
s
s
surr
s
s
s
s
T
T
T
T
A
T
T
hA
Q
σε
&
Its solution is
T
s
=°
42.6 C
which is sufficiently close to the assumed value of 45
°
C in the evaluation of the properties and
h
.
(c)
Horizontal, hot surface facing down
This time we expect the surface temperature to be higher, and
assume the surface temperature to be 50
. We will check this assumption after obtaining result and
repeat calculations with a better assumption, if necessary. The properties of air at the film temperature of
°
C
C
35
2
20
50
2
°
=
+
=
+
=
∞
T
T
T
s
f
are (Table A15)
1

2
5
K
003247
.
0
K
)
273
35
(
1
1
7268
.
0
Pr
/s
m
10
655
.
1
C
W/m.
02625
.
0
=
+
=
=
=
×
=
°
=
−
f
T
k
β
υ
The characteristic length in this case is, from part (b),
L
c
= 0.0429 m.
Then,
379
,
166
)
7268
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Mass, Heat, Radiation

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