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Chapter 7
External Forced Convection
740
A hot stainless steel ball is cooled by forced air. The average convection heat transfer coefficient and
the cooling time are to be determined.
Assumptions
1
Steady operating conditions exist.
2
Radiation effects are negligible.
3
Air is an ideal gas
with constant properties.
4
The outer surface temperature of the ball is uniform at all times.
Properties
The average surface temperature is (350+250)/2 = 300
°
C, and the properties of air at 1 atm
pressure and the free stream temperature of 30
°
C are (Table A15)
7282
.
0
Pr
kg/m.s
10
934
.
2
kg/m.s
10
872
.
1
/s
m
10
608
.
1
C
W/m.
02588
.
0
5
C
300
@
,
5
2
5

=
×
=
×
=
×
=
°
=
−
°
−
∞
s
k
μ
υ
Air
V
∞
= 6 m/s
T
∞
= 30
°
C
D
= 15 cm
T
s
= 350
°
C
D
Analysis
The Reynolds number is
4
2
5
10
597
.
5
/s
m
10
57
.
1
m)
m/s)(0.15
(6
Re
×
=
×
=
υ
=
−
∞
D
V
The Nusselt number corresponding this Reynolds number is determined to be
[]
6
.
145
10
934
.
2
10
872
.
1
)
7282
.
0
(
)
10
597
.
5
(
06
.
0
)
10
597
.
5
(
4
.
0
2
Pr
Re
06
.
0
Re
4
.
0
2
4
/
1
5
5
4
.
0
3
/
2
4
5
.
0
4
4
/
1
4
.
0
3
/
2
5
.
0
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
×
×
+
×
+
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
μ
μ
+
+
=
=
−
−
∞
s
k
hD
Nu
Heat transfer coefficient is
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat, Radiation

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