Chapter 8
Internal Forced Convection
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)
ρ
υ
μ
μ
=
=°
=×
=
−
−
1143
0 0268
167
1006
0710
189 10
208 10
5
350
5
.
.
.
Pr
.
.
.
,@
kg / m
W/m. C
10 m / s
J/kg
. C
kg / m.s
kg / m.s
3
5
2
K
k
C
p
b
s
Air
32
°
C
0.8 L/s
T
e
Electronic components,
35 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
3
3
3

4
&
&
()
&
&
Qm
CT T
T T
Q
mC
pe i
e i
p
=−
→
=
+
=
°
×°
−
32
914 10
4
C+
35 W
kg / s)(1006 J / kg. C)
70.1 C
(b) The mean fluid velocity and hydraulic diameter are
V
V
A
D
A
P
m
c
h
c
×
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat

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