Chapter 8
Internal Forced Convection
851
Air enters a rectangular duct. The exit temperature of the air, the rate of heat transfer, and the fan
power are to be determined.
Assumptions
1
Steady operating conditions exist.
2
The inner surfaces of the duct are smooth.
3
Air is an
ideal gas with constant properties.
4
The pressure of air in the duct is 1 atm.
Properties
We assume the bulk mean temperature for air to be 40
°
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 this temperature and 1 atm are (Table A15)
/s
m
10
702
.
1
C
W/m.
02662
.
0
kg/m
127
.
1
2
5

3
×
=
υ
°
=
=
ρ
k
7255
.
0
Pr
C
J/kg.
1007
=
°
=
p
C
T
s
=
10
°
C
L
= 7 m
Air duct
15 cm
×
20 cm
Analysis
(
a
) The hydraulic diameter, the mean velocity of air, and
the Reynolds number are
D
A
P
h
c
==
=
4
4015
20
1
5
01714
(.
(.
.
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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, Heat

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