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Chapter 7
External Forced Convection
765
Combustion air is preheated by hot water in a tube bank. The rate of heat transfer to air and the
pressure drop of air are to be determined.
Assumptions
1
Steady operating conditions exist.
2
The surface temperature of the tubes is equal to the
temperature of hot water.
Properties
The exit temperature of air, and thus the mean temperature, is not known. We evaluate the air
properties at the assumed mean temperature of 20
°
C (will be checked later) and 1 atm (Table A15):
k
= 0.02514 W/mK
ρ
= 1.204 kg/m
3
C
p
=1.007 kJ/kgK
Pr = 0.7309
μ
= 1.825
×
10
5
kg/ms
Pr
s
= Pr
@
Ts
= 0.7132
Also, the density of air at the inlet temperature of 15
°
C (for use in the mass flow rate calculation at the
inlet) is
ρ
i
= 1.225 kg/m
3
.
Analysis
It is given that
D
= 0.021 m,
S
L
=
S
T
= 0.05 m, and V
= 3.8 m/s.
Then the maximum velocity and the Reynolds number based on
the maximum velocity become
m/s
552
.
6
m/s)
8
.
3
(
021
.
0
05
.
0
05
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This note was uploaded on 01/22/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.
 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Force, Mass, Heat

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