Thermal conductivity measurements of particulate materials: 4. Effect of bulk density for granular particles

Abstract

Thermal conductivities were measured with a line‐heat source for three granular particulates of different particle sizes under low pressures of a carbon dioxide atmosphere and various bulk densities. A 25–30 μm size fraction of crushed quartz exhibited a small, linear increase in thermal conductivity with increasing bulk density. For the range of atmospheric pressures appropriate for Mars, doubling the bulk density increased the thermal conductivity by 20–35%. In contrast, the thermal conductivities of 150–180 μm crushed olivine exhibited an average slight decrease (−3%) for a 5% increase in bulk density. Less than 11 μm silica glass beads exhibited a small, nonlinear increase in the thermal conductivity of 9% with a factor of 2 increase in bulk density. Two minima occurred in the data from the glass beads. One minimum occurred at a porosity of 76%, where the decrease in thermal conductivity averaged ∼7%; the second minimum occurred at a porosity of 63%, with an average decrease of ∼2%. The second minimum corresponds to the same porosity as a minimum reported by previous investigators, but is below the level of significance. An increase in the bulk density will decrease the “heat transfer average distance,” and therefore decrease the thermal conductivity. Counteracting this effect is the increased mass within the same volume, which will increase the solid conduction. The latter effect appears to have an advantage, but the competition of these effects minimizes the influence of bulk density on the thermal conductivity under Martian atmospheric pressures.