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Chapter 1
Basics of Heat Transfer
1105
A 1000W iron is left on the iron board with its base exposed to the air at 20°C. The
temperature of
the base of the iron is to be determined in steady operation.
Assumptions
1
Steady operating conditions exist.
2
The thermal properties of the iron base and the
convection heat transfer coefficient are constant and uniform.
3
The temperature of the surrounding
surfaces is the same as the temperature of the surrounding air.
Properties
The emissivity of the base surface is given to be
ε
= 0.6.
Analysis
At steady conditions, the 1000 W energy supplied to the iron will be dissipated to the
surroundings by convection and radiation heat transfer. Therefore,
&&&
QQQ
total
conv
rad
1000 W
=+
=
where
W
K)
293
(
0.7
K)
293
(
)
m
K)(0.02
W/m
(35
2
2
conv
−
=
−
⋅
=
Δ
=
s
s
s
T
T
T
hA
Q
&
and
W
]
K)
(293
[
10
0.06804
]
K)
(293
)[T
K
W/m
10
)(5.67
m
0.6(0.02
)
(
4
4
8
4
4
s
4
2
8
2
4
4
rad
−
×
=
−
⋅
×
=
−
=
−
−
s
o
s
s
T
T
T
A
Q
εσ
&
Iron
1000 W
Substituting,
]
K)
(293
[
10
06804
.
0
)
K
293
(
7
.
0
W
1000
4
4
8
−
×
+
−
=
−
s
s
T
T
Solving by trial and error gives
C
674
K
947
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Mass, Heat

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