An experimental investigation of boiling burnout for subcooled carbon tetrachloride under conditions of constrained vapor space volume

Abstract

It has been determined that boiling heat flux densities may be made to exceed maximum pool boiling values for the same pressure and subcooling. Heat flux density elevations of about 50% were measured, and larger values may be possible. Large fluctuations in the vapor space volume were eliminated by placing a small, rigid enclosure around the boiling surface, and regulating the amount of fluid inside the enclosure. The supposition which prompted this research was that, even for large subcooling, the departure form nucleate boiling is characterized by more than one mode of evaporation. Discrete bubbles, liquid microlayers, dry patches, etc. may individually alternate between prominence and insignificance in a matter of milliseconds. For pool boiling, where the vapor could occupy any volume demanded by thermodynamics and heat transfer, these transients in the modes of evaporation would seem likely to cause considerable fluctuations in the vapor space volume. If so, any means by which the vapor volume fluctuations might be greatly reduced should also produce changes in the overall boiling process, including peak heat flux density. The use of a small enclosure system allows vapor volume to be controlled. This, in a sense, introduces a new variable into the study of boiling heat transfer. In the current experiments, the degree of control was limited to holding the vapor volume relatively constant (at an unknown value) with respect to rapid fluctuations. Improved apparatus could place vapor volume control on a more quantitative basis, and might provide a significant tool for studying the fundamental mechanisms related to boiling, especially if combined with photographic techniques.