Chapter 8
Internal Forced Convection
867
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
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 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
Water
25
°
C
3 m/s
River water
15
°
C
L
= 15 m
D
= 20 cm
Mineral deposit
0.15 mm
Analysis
The Reynolds number is
4
2
5
10
959
.
3
/s
m
10
516
.
1
m)
m/s)(0.2
(3
Re
×
=
×
=
υ
=
−
h
m
D
V
which is greater than 4000. Therefore, the flow is turbulent and the entry lengths in this case are roughly
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 Fall '10
 Dr.DanielArenas
 Fluid Dynamics, Thermodynamics, Resistance, Convection, Force, Mass, Power, Heat, ρ, Rconv, Rmineral

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