Mechanisms of nucleate pool boiling on composite surfaces

Abstract

A previous experimental study [1] has disclosed that nucleate pool boiling heat transfer coefficients on a graphite-copper composite surface are three to six times higher than those on a pure copper surface. Three programs are executed in order to explore the cause for such a high thermal performance: An experiment is conducted on nucleate pool boiling of R113 on a composite surface, a computer program is developed to determine the temperature distribution inside a composite cylinder heated from the lower end with incipient boiling on the upper end, and the microstructure of the composite surface is examined by means of a scanning electronic microscope (SEM). The computer results indicate that (i) Each fiber acts like a fin transporting higher heat flux, resulting in the composite body having a higher average temperature at any cross sections than that of a pure base material, and (ii) There exists a criterion for the tip of graphite fibers to function as a hot spot, i.e. a potential site for macro-bubble nucleation. The SEM discovers micro trenches and intermingled winding trails on the matrix surface between graphite fiber tips which may serve as the nucleation sites for microbubbles. The experiments in support of both hypotheses about the nucleation site.