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Chapter 7
External Forced Convection
Flow Across Cylinders And Spheres
735C
For the laminar flow, the heat transfer coefficient will be the highest at the stagnation point
which corresponds to
θ
≈
°
0 . In turbulent flow, on the other hand, it will be highest when
θ
is between
90
°
°
and 120 .
1336C
Turbulence moves the fluid separation point further back on the rear of the body, reducing the
size of the wake, and thus the magnitude of the pressure drag (which is the dominant mode of drag). As
a result, the drag coefficient suddenly drops. In general, turbulence increases the drag coefficient for flat
surfaces, but the drag coefficient usually remains constant at high Reynolds numbers when the flow is
turbulent.
1337C
Friction drag is due to the shear stress at the surface whereas the pressure drag is due to the
pressure differential between the front and back sides of the body when a wake is formed in the rear.
1338C
Flow separation in flow over a cylinder is delayed in turbulent flow because of the extra mixing
due to random fluctuations and the transverse motion.
739
A steam pipe is exposed to windy air. The rate of heat loss from the steam is to be determined.
√
Assumptions
1
Steady operating conditions exist.
2
Radiation effects are negligible.
3
Air is an ideal gas
with constant properties.
Properties
The properties of air at 1 atm and the film temperature of (T
s
+ T
∞
)/2 = (90+7)/2 = 48.5
°
C are
(Table A15)
7232
.
0
Pr
/s
m
10
784
.
1
C
W/m.
02724
.
0
2
5

=
×
=
°
=
υ
k
Analysis
The Reynolds number is
4
2
5
10
228
.
6
/s
m
10
784
.
1
m)
(0.08
]
s/h)
0
m/km)/(360
1000
(
km/h)
(50
[
Re
×
=
×
=
υ
=

∞
D
V
The Nusselt number corresponding to this Reynolds number is
(
29
[
]
(
29
[
]
1
.
159
000
,
282
10
228
.
6
1
7232
.
0
/
4
.
0
1
)
7232
.
0
(
)
10
228
.
6
(
62
.
0
3
.
0
000
,
282
Re
1
Pr
/
4
.
0
1
Pr
Re
62
.
0
3
.
0
5
/
4
8
/
5
4
4
/
1
3
/
2
3
/
1
5
.
0
4
5
/
4
8
/
5
4
/
1
3
/
2
3
/
1
5
.
0
=
×
+
+
×
+
=
+
+
+
=
=
k
hD
Nu
The heat transfer coefficient and the heat transfer rate become
C
.
W/m
17
.
54
)
1
.
159
(
m
08
.
0
C
W/m.
02724
.
0
2
°
=
°
=
=
Nu
D
k
h
length)
m
(per
=
C
7)

)(90
m
C)(0.2513
.
W/m
17
.
54
(
)
(
m
0.2513
=
m)
m)(1
08
.
0
(
2
2
2
W
1130
°
°
=

=
=
=
∞
T
T
hA
Q
DL
A
s
s
conv
s
π
720
Air
V
∞
= 50 km/h
T
∞
= 7
°
C
Pipe
D
= 8 cm
T
s
= 90
°
C
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View Full Document 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
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This homework help was uploaded on 03/19/2008 for the course ME 410 taught by Professor Benard during the Spring '08 term at Michigan State University.
 Spring '08
 BENARD
 Heat Transfer

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