Measurement of VIP Edge Conduction Using Vacuum Guarded Hot Plate
Abstract—Vacuum insulation panel (VIP) is a promising thermal insulator for buildings, refrigerator, LNG carrier and so on. In general, it has the thermal conductivity of 2~4 mW/m·K. However, this thermal conductivity is that measured at the center of VIP. The total effective thermal conductivity of VIP is larger than this value due to the edge conduction through the envelope. In this paper, the edge conduction of VIP is examined theoretically, numerically and experimentally. To confirm the existence of the edge conduction, numerical analysis is performed for simple two-dimensional VIP model and a theoretical model is proposed to calculate the edge conductivity. Also, the edge conductivity is measured using the vacuum guarded hot plate and the experiment is validated against numerical analysis. The results show that the edge conductivity is dependent on the width of panel and thickness of Al-foil. To reduce the edge conduction, it is recommended that the VIP should be made as big as possible or made of thin Al film envelope.
Effective thermal conductivity of various filling materials for vacuum insulation panels
Three thermal transport mechanisms of various filling materials for Vacuum Insulation Panels (VIPs) are theoretically investigated with special emphasis on the solid conduction. As the first, the solid conductivities of porous materials such as powder, foam, fiber and staggered beam subject to external atmospheric compression are derived using simplified elementary cell models. The results show that the solid conductivities of the fiber and staggered beam insulation are lower than those of the powder and foam due to the relatively long thermal path. The second mechanism, i.e., gaseous conductivity shows the lowest for the fine powder among the considered materials due to its smallest pore size. The radiative conductivity as the last is calculated using the diffusion approximation. If radiation shields are installed for the staggered beam, the radiation effect can be lowered to a negligible order of magnitude. The predicted total effective conductivities suggest that the fiber and staggered beam structures are promisingly proper filling materials for VIPs.
Thermal imbalance errors and effective area in guarded hot plates
An analytical equation is derived for the error heat flow due to thermal imbalance between the hot plate and the guard in the case of an infinite strip. Finite element calculations show that the same equation can be used for circular plates with negligible error. The equation can also be extended to square plates if corner effects are neglected. By combining the present solution and a previous one by Woodside [Rev. Sci. Instrum. 28, 1033 (1957)], an expression is derived for the effect of the distortion of heat‐flow lines near the gap even when thermal balance is obtained. The availability of this expression opens the possibility of using larger gaps than previously, to reduce thermal imbalance errors. The use of two‐material specimens (test material over hot plate, insulant over guard and gap) would lessen thermal imbalance errors, material requirements, and edge effects.