Heat transfer in the core of highly evacuated vacuum insulation panel (VIP) occurs by conduction through the solid structure and radiation through the pores. Radiation shields are often inserted in a VIP to reduce the radiation. In this study, heat transfer in the core of VIP with radiation shields is investigated.
The effect of contact resistance between the interstitial material and the shield is studied with combined heat transfer of conduction and radiation. Numerical analysis shows enhancement of radiation when it interacts with conduction, while the introduction of contact resistance flattens temperature profile and reduces both of the conductive and the radiative heat transfer. In VIPs, if the interstitial material is purely absorbing/emitting and well contacting the radiation shield, the center-of-panel thermal conductivity can be more than twice larger than that estimated by separate analysis, especially when the wall emissivity is low. This is certainly surprising and care should be paid in this case. On the other hand, the difference decreases with increased scattering albedo and reduced conduction/radiation interaction. Further, contact resistance between the interstitial material and the shield reduces the heat transfer substantially. This result suggests that contact resistance is effective in reducing heat transfer for low emissivity radiation shields.
A new idea of providing an embossing to the radiation shield to add the contact resistance is investigated. The near- and far-field radiation which occurs in the gap between the interstitial material and the shield is analyzed. The local heat transfer coefficient increases by more than 2 times of far-field radiation at nanometric distance. However, this enhancement of radiation is limited only to the region close to the contact spot and no significant change in total radiative heat transfer is found. A series of experiments are also made to confirm the analysis. Fair agreement with theoretical model with the additive solution is found.