An Empirical Modification of Nucleation Theory and Its Application to Boiling Heat Transfer Page: 11 of 38
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Evaporation in this range, from A to G in Fig. 1, is generally called natural
convection boiling. With further increase of superheat, bubbles are formed
from more nuclei on the heating surface, and the contribution to heat trans-
fer due to bubble action becomes gradually more important than that due to
simple convection. The range from G to D is usually called nucleate boiling.
In this range the bubble population increases rapidly with the increase of
superheat. When the bubble population becomes high, the bubbles, before
detaching from or collapsing at the heater, will be flattened and broken up
into smaller bubbles of various sizes. A part of the smaller bubbles will be
entrained by the turbulent liquid adjacent to the heater, forming a phenome-
non more or less similar to the occurrence of "white water" in rapids or on
spillways. Then the heating surface is splashed by the "white liquid" instead
of by liquid alone, and the heat transfer rate starts to decrease. Thus, there
is a critical value of superheat at which the heat transfer rate passes through
a maximum. Previous investigators, however, considered that at this con-
dition the bubbles tend to merge, spreading out over the heating surface and
forming a partial vapor film. Therefore, the range DE has been called partial
nucleate or partial film boiling, although there is no bubble coalescence at all.
When the vapor blankets the entire surface, the process is called complete-
film boiling, which is indicated by EF. The transition from simple convection
to nucleate boiling, in fact, takes place gradually, as is shown by the solid
line BC in Fig. 1. The ranges BC and CD are called feeble and vigorous
boiling, respectively, in this paper.
Although Fig. 1 shows the boiling curve of a heated wire, its basic
character holds for any heater irrespective of its size, shape, or orientation.
When forcing-flow of the liquid is present and the bulk of liquid is maintained
at a temperature below the saturation point, the boiling curve also exhibits
a similar character. From Fig. 1 it is seen that there are, in general, two
critical conditions in boiling heat transfer, one corresponding to the maximum
heat flux in nucleate boiling, and the other to the inception of complete film
boiling. It is convenient to refer to the former as the first critical condition
and to the latter as the second.
An attempt has been made by the author to predict the complete curve
ABCDEF for boiling of saturated or subcooled liquids with or without forced
convection, and the results will be reported in three consecutive papers.
This paper represents the first and deals only with heat transfer in the
ranges from A to D.
2. REVIEW OF PREVIOUS SEMITHEORETICAL INVESTIGATIONS
A great number of formulae for boiling heat transfer have been
empirically correlated in the past few years. Semitheoretical correlations
are mostly based on the assumption that the heat transfer coefficient should
be represented in a form similar to the Nusselt equation for forced convec-
tion. Rohsenow(2) introduced an empirically obtained relation between
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Chang, Y. P. An Empirical Modification of Nucleation Theory and Its Application to Boiling Heat Transfer, report, February 1, 1961; United States. (https://digital.library.unt.edu/ark:/67531/metadc863296/m1/11/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.