الفهرس 
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Abstract
An experimental and theoretical
investigation on the
nucleate pool boiling at an isolated site under saturation
conditions was conducted to study the factors affecting the
frequency and the diameter of the departing bubble . These
factors represent the most important parameters affecting the
heat transfer coefficient in nucleate pool boiling .
An experimental apparatus was designed and constructed
for this purpose. Test runs were carried out using R-12 aa a
boiling refrigerant , three heating surface metal. ( copper ,
brass and stainless steel ) were tried . Boilina was effected
in a pressure range of 0.11 to 0.44 of the critical pressure
and heat flux up to 51 kW/m2, from artificial cylindrical
cavities These cavities had the
dimensions of
100,150,200,250 and 325 ~m radii and a depth equal to five
times the cavity diameter
The experimental results showed :
1- The frequency of bubble formation from each artificial
cavity increases as follows:
i-When the saturation pressure increases at the same heat
flux and the same heating surface material.
ii-When the parameter J(PAClm/(PACt increases at the same
saturation pressure and the same heat flux •
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•
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iii-When the heat flux increases
for the same saturation
pressure and the same heating surface material.
2- The bubble frequency decreases as the cavity radius
increases at the same heating surface material, the same
saturation pressure and the same heat flux.
3- The departing bubble diameter from each artificial cavity
decreases as follows:
i-When the saturation pressure increases at the same heat
flux and the same heating surface material .
ii-When the parameter J(P}”C)m/(P}..C~ increases at the same
saturation pressure and the same heat flux.
iii-When the heat flux increases for the same saturation
pressure and the same heating surface material •
4- The departing bubble diameter increases as the cavity
radius
increases at the same heat flux, the same
saturation
material.
pressure and
the same heating surface
A theoretical model for calculating the waiting time
from an active site in nucleate boiling is advanced . A
unique feature of this model is the surface temperature
variations throughout the waiting time could be obtained .
Parametric studies of this model showed
1- The important role of the combined thermal properties of
boiling liquid and heat transfer material in determininc
the waitina time as well as the axial h.at flux durinc
this wait ina time •
-v-
2- The waiting time decreases as the dimensionless group
increases.
3- The model is able not only to calculate the axial rate of
heat transfer at any moment during the waiting time as
Mikic and Rohsenow’s (9J and Shai and Rohsenow’s (11J
models but also to predict the waiting time itself •
•
A correlation includes all the parameters affecting the
.’
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•
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frequency ( f and departing bubble diameter ( Da ) as (
cavity radius heat flux , thermophysical propertie. of
fluid and heating material ) may be expressed mathematically
as follows
o 66, 060~ .,0.856 QI44 UI44 \1 1.33
f D~ = ( (q Yf’ )/(6.Twi\( f1 Ce 20(0.5383+0.4632 ~(PAC)cu/(PAC~)))
Good agreement has been found between experimental ,
theoretical results and available previous data .