The Thermal Conductivity of Beds of Spheres

Abstract

The thermal conductivities (k) of beds of solid and hollow microspheres were measured using two radial heat flow techniques. One technique provided k-data at 300 K for beds with the void spaces between particles filled with argon, nitrogen, or helium from 5 kPa to 30 MPa. The other technique provided k-data with air at atmospheric pressure from 300 to 1000 K. The 300 K technique was used to study bed systems with high k-values that can be varied by changing the gas type and gas pressure. Such systems can be used to control the operating temperature of an irradiation capsule. The systems studied included beds of 500 µm dia solid Al₂0₃, the same Al₂0₃ spheres mixed with spheres of silica-alumina or with SiC shards, carbon spheres, and nickel spheres.* Both techniques were used to determine the k-value of beds of hollow spheres with solid shells of Al₂O_3, Al₂0₃,•7 w/o Cr₂0₃, and partially stabilized Zr0₂.** The hollow microspheres had diameters from 2100 to 3500 µm and wall thicknesses from 80 to 160 µm.