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Chapter 8
Internal Forced Convection
857
Air enters a hollowcore printed circuit board. The exit temperature of the air and the highest
temperature on the inner surface are to be determined.
√
Assumptions
1
Steady flow conditions exist.
2
Heat generated is uniformly distributed over the two
surfaces of the PCB.
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 310 K since the mean temperature of air at
the inlet will rise somewhat as a result of heat gain through the hollow core whose surface is exposed to a
constant heat flux. The properties of air at 1 atm and this temperature are (Table A15)
kg/m.s
10
08
.
2
kg/m.s
10
89
.
1
710
.
0
Pr
C
J/kg.
1006
/s
m
10
67
.
1
C
W/m.
0268
.
0
kg/m
143
.
1
5
K
350
@
,
5
2
5

3
−
−
×
=
×
=
=
°
=
×
=
°
=
=
s
b
p
C
k
μ
υ
ρ
Air
32
°
C
0.8 L/s
T
e
Electronic components,
20 W
L
= 18 cm
Air channel
0.25 cm
×
12 cm
Analysis
(a) The mass flow rate of air and the exit temperature are determined from
&
&
(.
mV
==
×
×
ρ
1143 kg / m )(0.8 10 m / s) = 9.14 10 kg / s
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat

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