Investigation of Heat Transfer in Evacuated Foil-Spacer Multilayer Insulations

Abstract

For a comparison of different evacuated foil-spacer insulations, the heat transfer coefficient, as a function of temperature and packing density, was measured using an evacuable, guarded flat-plate apparatus. The mean temperature was varied between 260 K and 550 K, the packing density from 1 to 15 foils per mm. Other experimentally changed parameters were type and emissivity ∈ _f of foils and type of spacer. The measurements allow one to determine contributions from solid conduction and radiative transport, as well as the coupling between these two heat transfer modes. The coupling is characterized by an effective foil emissivity ∈^* _f, with ∈_f ≤ ∈^*_f≤ 1. The goal of the investigation is the optimization of foil-spacer vacuum super-insulations. The lowest heat transfer coefficients achieved were 0.01 W.m^-2.K^-1 at 300 K for a 1 mm thick stack of 7 polyimide foils, aluminized on both sides with glass fiber fabric spacers.