Vacuum insulation panel core materials and modelling the thermal conductivity of granular materials
Two potential alternative core materials, diatomaceous earth (DE) and glass bubbles (GB), were considered for cost effective and long life-time vacuum insulation panels (VIP). The pore size distribution of the DE was determined quantitatively by mercury porosimetry and nitrogen sorption. The majority of pores in DE centered around 1 μm. The average pore size of GB was estimated to be 50 μm via scanning electron microscopy. Subsequently, the relationship between pore size and gaseous thermal conductivity was established using the Kaganer relation. Composites of DE and fumed silica (FS) showed remarkable insulating performance whereby thermal conductivity only rose by 11 mW/m.K as pressure increased to 10,000 Pa. A similar analysis on composites of GB and fumed silica showed an increment of 16 mW/m.K. It was estimated that the duration for a typical VIP to reach 10,000 Pa is approximately 50 100 years depending on the type of barrier used. [30] Hence, both composites are capable of providing long vacuum insulating service life. Furthermore, DE/FS composites would greatly reduce the cost of VIPs because of the inexpensive nature of DE. Both composites show superior life-time when compared to glass fibers, which are already commercially used in VIPs.
A study of the thermal conductivity of granular silica materials for VIPs at different levels of gaseous pressure and external loads
Abstract Fast and reliable methods for the determination of thermal properties of core materials for vacuum insulation panels (VIPs) are needed. It is of great importance to know the thermal performance of a VIP core at different levels of vacuum and external loads. In this study a new self-designed device, consisting of two cylindrical cavities connected to a Transient Plane Source instrument, is used to determine the thermal conductivity of low-density nanoporous silica powders, from atmospheric pressure down to 0.1 mbar while applying different levels of external pressure up to 4 bars. The study includes a brief theoretical discussion of methods. The TPS is validated through comparison with available data for commercial silica as well as through independent stationary measurements with a hot plate apparatus and with a Transient Hot Bridge method. The different materials illustrate clear but different trends for the thermal conductivity as a function of the level of vacuum and external pressure. The analysis of experimental results shows that the transient methods are less suitable for measuring the thermal conductivity of low-density silica powders, especially for the cases when the density is less than a limit at which the heat transfer by radiation becomes dominant compared to pure conduction.
The Parameters Identification Method of Radiation Heat Transfer for Nanoporous Materials
The two flux approximation model is usually used to calculate the heat transfer of radiation for porous materials. In this paper, the parameters identification method of the extinction coefficient and the albedo of scattering in the two flux model is set up by using the identification theory according to the data of the back temperature. The simulated process shows that the convergence rate is fast and the results by the parameters identification are very close to the origin values.