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Chapter 8
Internal Forced Convection
849
A circuit board is cooled by passing cool helium gas through a channel drilled into the board. The
maximum total power of the electronic components is to be determined.
Assumptions
1
Steady operating conditions exist.
2
The heat flux at the top surface of the channel is
uniform, and heat transfer through other surfaces is negligible.
3
The inner surfaces of the channel are
smooth.
4
Helium is an ideal gas.
5
The pressure of helium in the channel is 1 atm.
Properties
The properties of helium at the estimated average temperature of 25
°
C are (Table A16)
669
.
0
Pr
C
J/kg.
5193
/s
m
10
233
.
1
C
W/m.
1565
.
0
kg/m
1635
.
0
2
4

3
=
°
=
×
=
°
=
=
p
C
k
υ
ρ
He
15
°
C
4 m/s
T
e
Electronic components,
50
°
C
L
= 20 cm
Air channel
0.2 cm
×
14 cm
Analysis
The crosssectional and heat transfer surface areas are
2
2
m
028
.
0
)
m
2
.
0
)(
m
14
.
0
(
m
00028
.
0
)
m
14
.
0
)(
m
002
.
0
(
=
=
=
=
s
c
A
A
To determine heat transfer coefficient, we need to first find the Reynolds number
m
003944
.
0
m)
0.14
+
m
002
.
0
(
2
)
m
00028
.
0
(
4
4
2
=
=
=
P
A
D
c
h
9
.
127
/s
m
10
233
.
1
m)
944
m/s)(0.003
(4
Re
2
4
=
×
=
=
−
h
m
D
V
which is less than 2300. Therefore, the flow is laminar and the thermal entry length is
m
0.20
<<
m
0.01687
=
m)
003944
.
0
)(
669
.
0
)(
9
.
127
(
05
.
0
Pr
Re
05
.
0
=
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This note was uploaded on 01/22/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.
 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Power

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