Thermal properties of sand and mineral flours
The determination of the thermal conductivity and the thermal diffusivity of granular dry material is exposed to the problem of heat transfer between the heat source of an instrument and the media of investigation. This problem can be solved by filling the pore space with various fluids. An empirical relation between the thermal conductivity and the diffusivity of the composed media and the thermal conductivity of the pore fluid is extrapolated to the value of air as a pore filling. A further method provides the heat contact conductance of the fluid–solid interface during a non-stationary state which allows the estimation of the thermal diffusivity for air in the pore space. The validity of the geometric mean, which is often used to estimate the thermal conductivity of composed media, is limited to a heat conductivity contrast of up to 1:20. The structural constant of Wiener’s mixing rule represents not only a fixed structure of the pore space, but also relates to the heat transfer properties at the fluid–solid interface. It strongly varies with the surface tension of the fluid as well as with its wetting property.