Chapter 8
Internal Forced Convection
Properties
We assume the water temperature not to drop significantly since the pipe is not very long. We
will check this assumption later. The properties of water at 90
°
C are (Table A9)
96
.
1
Pr
C
J/kg.
4206
/s;
m
10
326
.
0
/
C
W/m.
675
.
0
;
kg/m
3
.
965
2
6

3
=
°
=
×
=
ρ
μ
=
υ
°
=
=
ρ
p
C
k
Analysis
(a) The mass flow rate of water is
kg/s
0.9704
m/s)
8
.
0
(
4
m)
(0.04
)
kg/m
3
.
965
(
2
3
=
π
=
ρ
=
V
A
m
c
&
Water
90
°
C
0.8 m/s
10
°
C
L
= 15 m
D
i
= 4 cm
D
o
= 4.6 cm
The Reynolds number is
062
,
98
/s
m
10
326
.
0
m)
m/s)(0.04
(0.8
Re
2
6
=
×
=
υ
=
−
h
m
D
V
which is greater than 10,000. Therefore, the flow is turbulent and the entry lengths in this case are roughly
m
4
.
0
m)
04
.
0
(
10
10
=
=
≈
≈
D
L
L
t
h
which are much shorter than the total length of the pipe. Therefore, we can assume fully developed
turbulent flow in the entire pipe. The friction factor corresponding to Re = 98,062 and
ε
/D = (0.026 cm)/(4
cm) = 0.0065 is determined from the Moody chart to be
f
= 0.034. Then the Nusselt number becomes
6
.
521
96
.
1
062
,
98
034
.
0
125
.
0
Pr
Re
125
.
0
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 Fall '10
 Dr.DanielArenas
 Fluid Dynamics, Thermodynamics, Convection, Force, Mass, Heat, Heat Transfer, pipe

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