Micro- and macro-phenomena in nucleate pool boiling on graphite-copper composite materials.

Abstract

An experimental study is performed to investigate micro- and macro-phenomena in nucleate pool boiling of Freon 113 on Graphite-Copper (Gr-Cu) composite materials under atmospheric pressure. The material consists of a multitude of ultra-high thermal conductivity graphite fibers of 8 to 10 micron in diameter, uniformly consolidated and parallel to the axis of a cylindrical copper matrix. The side surface of the composite cylinder is insulated with the lower end contacting a heat source and upper end serving as the boiling surface, resulting in a boiling surface normal to the graphite fibers. The composite of graphite fibers in the copper matrix is varied (near 0%, 25%, and 50% of graphite in volume) to determine its effects on the boiling phenomena. Temperature profiles in the boundary layer near the boiling surface are measured by a moving micro thermocouple with the aid of a three-dimensional micro manipulator and a digital micro indicator. Results reveal that the graphite-fiber reinforced copper materials provide not only super-high heat transfer performance (compared with pure copper), but also distinctive features in their nucleate boiling curves. The mechanisms which cause these results are disclosed by an application of the vacuum evaporation method aided by visualization using a scanning electronic microscope and optical color video imaging. A two-tier model is developed to describe these mechanisms, through which both the microlayer and macrolayer thicknesses are determined. This model can be extended to explain nuclear pool boiling performance on the surface of pure materials. The enhancement mechanisms of heat transfer for nuclear pool boiling on the micro configured surface are analyzed. The study has potential applications to electronic cooling, power generation, distillation processes, and others, with advantages of tailoring non-homogeneous boiling heat fluxes to meet specific requirements by varying the local density of graphite fibers in the copper matrix.