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Chapter 8
Internal Forced Convection
860
A computer is cooled by a fan blowing air through its case. The flow rate of the air, the fraction of the
temperature rise of air that is due to heat generated by the fan, and the highest allowable inlet air
temperature are to be determined.
√
Assumptions
1
Steady flow conditions exist.
2
Heat flux is uniformly distributed.
3
Air is an ideal gas with
constant properties.
4
The pressure of air is 1 atm.
Properties
We assume the bulk mean temperature for air to be 300 K. The properties of air at 1 atm and
this temperature are (Table A15)
C
J/kg.
1005
/s
m
10
57
.
1
C
W/m.
0261
.
0
kg/m
177
.
1
2
5

3
°
=
×
=
°
=
=
p
C
k
υ
ρ
kg/m.s
10
08
.
2
kg/m.s
10
85
.
1
712
.
0
Pr
5
K
350
@
,
5
−
−
×
=
×
=
=
s
b
μ
Analysis
(
a
) Noting that the electric energy consumed by the fan is converted to thermal energy, the mass
flow rate of air is
kg/s
0.01045
=
°
°
+
×
=
−
+
=
→
−
=
C)
C)(10
J/kg.
(1005
W
25)
10
(8
)
(
)
(
fan
elect,
i
e
p
i
e
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat

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