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Chapter 7
External Forced Convection
715
The top surface of a hot block is to be cooled by forced air. The rate of heat transfer is to be
determined for two cases.
Assumptions
1
Steady operating conditions exist.
2
The critical Reynolds
number is Re
cr
= 5
×
10
5
.
3
Radiation effects are negligible.
4
Air is an ideal gas
with constant properties.
Air
V
∞
= 6 m/s
T
∞
= 30
°
C
T
s
= 120
°
C
Properties
The atmospheric pressure in atm is
L
atm
823
.
0
kPa
101.325
atm
1
kPa)
4
.
83
(
=
=
P
For an ideal gas, the thermal conductivity and the Prandtl number are independent of pressure, but the
kinematic viscosity is inversely proportional to the pressure. With these considerations, the properties of air
at 0.823 atm and at the film temperature of (120+30)/2=75
°
C are (Table A15)
7166
.
0
Pr
/s
m
10
2.486
=
823
.
0
/
)
/s
m
10
046
.
2
(
/
C
W/m.
02917
.
0
2
5

2
5
1
@
=
×
×
=
υ
=
υ
°
=
−
atm
atm
P
k
Analysis
(
a
) If the air flows parallel to the 8 m side, the Reynolds number in this case becomes
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat

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