Thermal Conductivity of Evacuated Glass Beads: Line Source Measurements in a Large Volume Bead Bed Between 225 and 300 K

Line source measurements of thermal conductivity have been made in a large tank of evacuated glass beads that were used as a thermal simulator for the Lunar Heat Flow Experiment. Measurements were made with two 3/16 inch diameter probes 87 inches long that contained five (5) evenly spaced thermocouples and a double filament heater wire. The probes were inserted vertically from the top into the bead bed, which allowed ten (10) measurements of conductivity to be made over an interval of depth of 55 inches in tank. Power densities used ranged from 1.2 to 2.2 MW/cm. The bead tank is 21 inches in diameter by 86.75 inches in depth with temperature control at the outer walls. This tank was contained in a vacuum chamber that could be evacuated to gas pressure 10⁻⁵ to 10⁻⁴ Torr. Measurements were made at various bead bed temperatures between 225 and 300 K.

Measurements at 225 K gave a range of conductivity values from 1.0 to 4.0 × 10⁻⁴ watt/cm ∙ K. The conductivity increased uniformity over the interval of depth measured. Extrapolation of the values to the surface of the bead tank indicates an average conductivity of 0.7 × 10⁻⁴ $wattcm K$⁠. Measurements using the Lunar Heat Flow Experiment probes at 225 K yield values in good agreement, ± 10%, with the line source data. All of the probe qualification tests were made at 225 K. These comparisons demonstrate that valid conductivity measurements can be made with the heat flow probe.

The increase in conductivity with depth is due to an increase in compressive stress. However, the observed increase at 225 K is much greater than would be expected from “hydrostatic” loading alone. Measurements at higher bead bed temperatures, 246 K and 258 K, yield lower values of conductivity. If the increase of conductivity with depth is due to “hydrostatic” loading alone, an increase of conductivity with increasing temperature would be expected. The opposite observed effect suggests that compressive stresses from thermal contraction of the bead jacket are important.