Measurement of Heat Conduction through Metal Spheres

Abstract

This paper describes the results of the measurements of heat conduction through a bed of packed metal spheres. Spheres were packed in a fiberglass-epoxy cylinder, 24.4 mm in diameter and 55 nun in length, The cold end of the packed bed was cooled by a Gifford-McMahon (GM) cryocooler to cryogenic temperatures, while the hot end was maintained at room temperature. Heat conduction through the spheres was determined from the temperature gradient in a calibrated heat flow sensor mounted between the cold end of the packed bed and the GM cryocooler. The samples used for these experiments consisted of stainless steel spheres, lead spheres, and copper spheres. The spheres were screened to obtain a uniform diameter of 80 to 120 mum. Porosities of the packed beds varied between 0.371 and 0.398. The measurements to determine the thermal conductance were carried out with various pressures of helium gas in the void space. The results indicated, as expected, that the helium gas between each sphere enhances the heat conduction across the contacts between the individual spheres by several orders of magnitude compared with vacuum in the void space. The conduction degradation factor, defined as the ratio of actual heat conduction to the heat conduction if the metal were in the form of a solid rod of the same metal cross-sectional area, was about 0.11 for stainless steel, 0.08 for lead, and 0.02 for copper. The conduction degradation factor of 0.11 for stainless steel spheres agrees very well with the factor of 0.10 for stainless steel screen measured previously in the same apparatus.