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.
Influence of Water on the Total Heat Transfer in ‘Evacuated’ Insulations
The impact of water vapor on the thermal conductivity of vacuum insulation panels (VIPs) filled with fumed silica kernels was investigated and found to be surprisingly large. Besides thermal transport by water vapor within the pore space, the question was whether water in the adsorbed phase also contributed significantly to the total heat transfer. In order to quantify these effects, several series of thermal conductivity measurements were performed in an evacuable guarded-hot-plate. The fumed silica specimens were first investigated in the dry state. Then water vapor was added into the vacuum system in different quantities (valves closed and no pumping), yielding water content in the VIP-kernel of up to 4% by mass. Most of the water is adsorbed in the specimens; smaller amounts are to be found in the gas phase. The parameter varied was the temperature; accordingly, the partial pressure of water vapor changed. An empirical equation is presented that describes the influence of water vapor and adsorbed water on the total heat transfer.
Composite monolayer fabrication by an arc-spray process
A single layer (monotape) technique for fabricating complex high-temperature tungsten-fiber-reinforced superalloy composites is proposed. The fabrication of sheets of arc-sprayed monotape 38 cm wide and 122 cm long has been demonstrated. Composites fabricated using the method are shown to have equal tensile strength and a cleaner matrix than composites fabricated from powder metal cloth monotapes, and the present technique is less expensive than the powder metal fabrication techniques.