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Chapter 7
External Forced Convection
769
Water is heated by a bundle of resistance heater rods. The number of tube rows is to be determined.
Assumptions
1
Steady operating conditions exist.
2
The surface temperature of the rods is constant.
Properties
The properties of water at the mean temperature of (15
°
C +65
°
C)/2=40
°
C are (Table A9):
k
= 0.631 W/mK
ρ
= 992.1 kg/m
3
C
p
=4.179 kJ/kgK
Pr = 4.32
μ
= 0.653
×
10
3
kg/ms
Pr
s
= Pr
@
Ts
= 1.96
Also, the density of water at the inlet temperature of 15
°
C (for use in the mass flow rate calculation at the
inlet) is
ρ
i
=999.1 kg/m
3
.
Analysis
It is given that
D
= 0.01 m,
S
L
= 0.04 m and
S
T
= 0.03 m, and V
= 0.8 m/s.
Then the maximum velocity and the Reynolds number
S
L
S
T
V=0.8 m/s
T
i
=15
°
C
T
s
=90
°
C
D
based on the maximum velocity become
m/s
20
.
1
m/s)
8
.
0
(
01
.
0
03
.
0
03
.
0
max
=
−
=
−
=
V
V
D
S
S
T
T
232
,
18
s
kg/m
10
653
.
0
m)
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Resistance, Convection, Force, Mass, Heat

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