Example 121
A
copper tube is soldered to the back of a solar collector,
which is maintained at a uniform temperature of 75°C by
incident sunlight. Water enters the tube at 25°C with a
mass flow rate of 0.011 kg/s. Find the length of tube
needed so that the exit temperature is 55°C.
Solution:
To find the heat transfer coefficient, first find the Reynolds
number
Re
D
ρ
µ
=
V
x
mA
=
±
where
x
A
is crosssectional area
Re
x
Dm
A
=
±
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25
55
40 C
22
ie
mean
TT
T
+
+
==
=
4
kg
viscosity(water,
40,
100)
6.53 10
ms
TP
µ
−
=
=
×
⋅
()
2
4
kg
0.012 m
0.011
s
Re
1787
kg
0.012
6.53×10
π
m
2
⎛⎞
⎜⎟
⎝⎠
⎛
⎞
⎜
⎟
⋅
⎝
⎠
Pr
Prandtl(water,
40,
100)
4.33
=
=
Because Re < 2100, flow is laminar. We assume the tube is
long enough so that the flow is fully developed. For laminar
flow with constant wall temperature
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 Spring '08
 Kaminski

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