ABSTRACT
Cross Flow Heat Exchanger is very important in many industrial applications and
many equations show that Reynolds number has some effects on the surface heat
transfer coefficient. The main objective of this report is to study the effect of
Reynolds number on the surface heat transfer coefficient in turbulent cross flow of air
stream around a single copper tube.
A collection of some data at fixed element surface temperature 70
o
C and
atmospheric pressure 764 mm Hg was recorded by a computer. However, by plotting
ln(Nu) vs. ln(Re) according to Nu=C*Re
m
the error obtained in C was 72 % and in m
was 18 %.
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View Full DocumentINTRODUCTION
The overall heat transfer coefficient in turbulent cross flow heat exchanger is
basically contained three components:
1)
Tube side heat transfer coefficient.
2)
Thermal conductivity and thickness of the tube material.
3)
Shell side heat transfer coefficient.
In order to improve the first two components, the flow velocity in the tubes should
be increased and the wall thickness should be reduced. Arranging the tube layout to
have maximum turbulent flow or increasing the stream velocity to have high
Reynolds number can increase the third component.
THEORETICAL BACKGROUND
The heat transfer rate can be calculated by:
Q=VI
(1)
Consequently, heat flux is
φ
=Q/As
(2)
The mean surface heat transfer coefficient is calculated by:
h=
φ
/(TsTa)
(5)
The duct air velocity will be evaluated by:
U=74.294√(TaPd/Pa)
(6)
Reynolds and Nusselt numbers can be found by:
Re=(UD/
ν29,
Nu=(hD/k) respictivly.
(7),(8)
The heat transfer coefficient can be manipulated by:
Nu=C Re
m
Pr
n
(9)
In gases Pr may be assumed as a part of constant C therefore,
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 Fall '09
 M.Elgaily

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