Combined heat transfer in multi-layered radiation shields for vacuum insulation panels: Theoretical/numerical analyses and experiment

Abstract

Radiation and conduction heat transfer in stacked radiation shields to be used in the VIP (vacuum insulation panel) is investigated. Test radiation shields are multi-layered films of 32 nm Al, 12 mu m PET and 32 nm Al thicknesses, folded with regular span and stacked in staggered manner. Radius of curvature of the folded parts is measured by a three-dimensional scanner and the contact radius is calculated using Hertz contact theory. Depthwise conduction around the contact spot and two-dimensional radial conduction models are adopted for the theoretical and the numerical analyses, together with measured surface emissivity. Measurement of the effective thermal conductivity of radiation shields is conducted using a vacuum guarded hot plate apparatus. Measurements show very low values between 0.3 and 1.0 mW/m K. Theoretical and numerical results agree with measurements with maximum relative error of 29.1% and 18.3%, respectively. A simplified conduction model is also proposed and shown to be very useful for practical applications. We find that the stacked radiation shields have very high insulation performance, the numerical model is fairly reliable and finally, conduction is negligibly small compared with radiation for this shield. (C) 2012 Elsevier Ltd. All rights reserved.