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Chapter 8
Internal Forced Convection
856
The components of an electronic system located in a circular horizontal duct are cooled by forced air.
The exit temperature of the air and the highest component surface temperature are to be determined.
Assumptions
1
Steady flow conditions exist.
2
The inner surfaces of the duct are smooth.
3
The thermal
resistance of the duct is negligible.
4
Air is an ideal gas with constant properties.
5
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 duct whose surface is exposed to a constant
heat flux. The properties of air at 1 atm and this temperature are (Table A15)
ρ
υ
=
=°
=×
=
1143
00268
167
1006
0710
.
.
.
Pr
.
kg / m
W/m. C
10 m / s
J/kg. C
3
5
2
k
C
p
Analysis
(a) The mass flow rate of air and the exit temperature are determined from
Air
32
°
C
0.65 m
3
/min
T
e
Electronics, 90 W
L
= 1 m
D
= 15 cm
&
&
(.
mV
==
ρ
1143 kg / m )(0.65 m / min) = 0.74295 kg / min = 0.0124 kg / s
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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

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