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Chapter 8
Internal Forced Convection
866
Air enters the underwater section of a duct. The outlet temperature of the air and the fan power
needed to overcome the flow resistance 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
The surface of the duct is at the temperature of the water.
5
Air is an
ideal gas with constant properties.
6
The pressure of air is 1 atm.
Properties
We assume the bulk mean temperature for air to be 20
°
C since the mean temperature of air at
the inlet will drop somewhat as a result of heat loss through the duct whose surface is at a lower
temperature. The properties of air at 1 atm and this temperature are (Table A15)
7309
.
0
Pr
C
J/kg.
1007
/s
m
10
516
.
1
C
W/m.
02514
.
0
kg/m
204
.
1
2
5

3
=
°
=
×
=
°
=
=
p
C
k
υ
ρ
Air
25
°
C
3 m/s
River water
15
°
C
L
= 15 m
D
= 20 cm
Analysis
The Reynolds number is
4
2
5
10
959
.
3
/s
m
10
516
.
1
m)
m/s)(0.2
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This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.
 Fall '10
 Dr.DanielArenas
 Thermodynamics, Resistance, Convection, Force, Mass, Power

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