Thermal contact conductance under low applied load in a vacuum environment

Abstract

The thermal contact conductance in a vacuum environment under low applied load was studied with square test plates (100 X 100 mm) made of aluminum alloy (A6061 and A5052). Two kinds of contact geometries were examined: the contact between a practically flat rough surface and an approximately spherical one, and the contact of similarly flat rough surfaces. Heat transfer experiments were carried out in a vacuum (< 13.3 Pa) in a contact pressure range of 0.1–0.6 MPa. These experimental conditions simulated an actual heat transfer surface of a cold-plate heat exchanger that is scheduled to be used in outer space. By using a pressure-measuring film that is capable of visualizing contact pressure distributions, a new technique for predicting the thermal contact conductance was developed. The technique evaluates the microscopic and macroscopic thermal constriction resistances from the real contact pressure distribution, which is measured by means of digital image processing from the color density pattern appearing in the film. A procedure was also devised to reevaluate the real contact pressure distribution that would occur in direct metal-to-metal contact without the pressure-measuring film. It is shown that the values of thermal contact conductance predicted by the present technique are in excellent agreement with those obtained by the heat transfer experiment, thus demonstrating the usefulness of the present technique as a practical tool for predicting thermal contact conductance. In contrast, the theory and laboratory correlations reported in previous studies were not applicable under the conditions examined here owing to insufficient accuracy. The present results clearly indicate that the macroscopic constriction resistance caused by the surface waviness and/or deformation of the substrate is predominant under low applied loading, and, therefore, its evaluation is crucial to the better design of efficient heat exchangers for use in spacecraft applications.