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Unformatted text preview: 0.09 hfg g(
( ) l 1/4 and fully turbulent for Re 1800. Heat transfer coefficients in
the wavylaminar and turbulent flow regions are determined
from 2 ) l The heat flux for stable film boiling on the outside of a horizontal cylinder or sphere of diameter D is given by
q·film gk3 Cfilm
(Ts ( )[hfg 0.4Cp (Ts
D(Ts Tsat) l Tsat) hvert, wavy Re kl
1.08 Re1.22 1/4 hvert, turbulent 8750 g 1/3 ,
5.2 2
l
Re kl
58 Pr 0.5 (Re0.75 Tsat) 30 Re 1800 l g 1/3 ,
253) 2
l
Re 1800
l where the constant Cfilm
0.62 for horizontal cylinders and
0.67 for spheres, and k is the thermal conductivity of the
vapor. The vapor properties are to be evaluated at the film temperature Tf (Tsat Ts)/2, which is the average temperature
of the vapor film. The liquid properties and hfg are to be evaluated at the saturation temperature at the specified pressure.
Two distinct forms of condensation are observed in nature:
film condensation and dropwise condensation. In film condensation, the condensate wets the surface and forms a liquid film
on the surface that slides down under the influence of gravity.
In dropwise condensation, the condensed vapor forms countless droplets of varying diameters on the surface instead of a
continuous film.
The Reynolds number for the condensate flow is defined as
Dh Re l l l 4A
p l l
l ·
4m
pl and
·
4Q conden
p l h*
fg Re 4 As h(Tsat Ts)
p l h*
fg where h* is the modified latent heat of vaporization, defined as
fg
h*
fg hfg 0.68Cpl (Tsat 0.943 g l(
l l (Ts ) h* k3
fg l
Tsat)L hhoriz 1/4 (W/m2 · °C) All properties of the liquid are to be evaluated at the film temperature Tf (Tsat Ts)/2. The hfg and are to be evaluated at
Tsat. Condensate flow is smooth and wavefree laminar for
about Re 30, wavylaminar in the range of 30 Re 1800, 0.729 g l( l
l (Ts ) h* k3
fg l
Tsat)D 1/4 (W/m2 · °C) where D is the diameter of the horizontal tube. This relation
can easily be modified for a sphere by replacing the constant
0.729 by 0.815. It can also be used for N horizontal tubes
stacked on top of each other by replacing D in the denomin...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
 Spring '10
 Ghaz

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