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Dynamics of Two-Phase Flows

ISBN Druckformat: 0-8493-9925-4

NUCLEATION AND FLASHING OF INITIALLY SUBCOOLED LIQUIDS IN NOZZLES: 1. A DISTRIBUTED NUCLEATION MODEL

Abstrakt

It is well known that both number and size of bubbles must be accurately determined for initial calculation of flashing void development downstream of restrictions. This paper presents a new method of accurately determining both which results in accurate calculation of downstream void development.
A wall cavity model is described for use in the calculation of nucleation rates and bubble number densities at flashing inception in nozzles, and subsequently in the calculation of void development downstream of minimum area zones. The model is based on the physics of the nucleation phenomena in flashing and considers transient conduction to be the sole means of heat transfer from the superheated liquid to the vapor bubble.
The activation criterion developed for site nucleation is one sided due to the uniform superheat rather than two sided as in boiling. A figure of merit for the particular fluid solid combination is then determined which yields minimum nucleation surface energy per site. Characteristic site nucleation frequencies, and number densities of nucleation sites of given sizes are then obtained from data.
A bubble transport equation is used to predict the number density and size of bubbles at the throat. Throat superheats are calculated with a standard deviation of 1.9K for throat superheats up to ∼100K and expansion rates between 0.2 bar/sec to over 1 Mbar/sec, extending previous correlations by more than three Orders of magnitude. Throat void fractions for all data found in the literature are less than 1% confirming earlier assumptions and allowing nozzle critical flow rates to be calculated with an accuracy of ∼3%.
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