A thermal conductivity model for microporous insulations in gaseous environments


Ceramic insulations are often used to mitigate heat losses from high temperature systems. In the case of thermal batteries, insulation is exposed to high temperatures, high axial loads, shock and vibration scenarios, and alternate gaseous environments. All of these conditions influence the effective thermal conductivity of the material and subsequently the performance of the battery. To properly balance the thermal design of these systems, a model that can capture the influence of these variables on the effective thermal conductivity is necessary. A previous publication presented the development of an effective thermal conductivity model for two common materials used for thermal batteries. This paper investigates the application of this model to Min-K TE1400 board, whose microstructure interrupts gas phase conduction to minimize thermal conductivity. This necessitates modifications to the model to accurately represent effective thermal conductivity for this type of material. Thermal conductivity was measured as a function of temperature and compression in air, helium, and partial vacuum environments using the Transient Plane Source (TPS) technique. The trends observed in the measured thermal conductivity are explained through pore size measurements of compressed samples. The effective medium model was tuned using this data and is shown to agree with experimental data. Lastly, the anisotropy of the material was evaluated using the TPS data and specific heat estimations. This showed that axial conductivity is consistently less than radial thermal conductivity, as is corroborated by X-ray CT analysis of the fiber orientations in the material. The results and implications of the anisotropic results are also discussed.

Publication Title

International Journal of Heat and Mass Transfer