Influence of scattering pattern on the insulation performance of VIP filler materials

Abstract

Vacuum insulation panel (VIP) is an attractive thermal insulation component to satisfy the needs for high insulation performance. Radiation is a significant heat transfer mode in the VIP. In this study, radiative heat transfer is investigated for each of isotropic and backward-scattering media to evaluate the effect of each scattering pattern on the insulation performance of the VIP. The filler material for practical VIP can be considered as a one-dimensional pure scattering medium between two walls. Both walls are diffuse and have the same emissivity. The resulting insulation performance by each scattering pattern is expressed through the radiative thermal conductivity. A statistical formulation proposed in this paper and a diffusion approximation are used to calculate the radiative thermal conductivity theoretically. Monte Carlo method and discrete ordinate interpolation method (DOIM) are also taken to calculate the radiative transfer numerically. The results of DOIM, Monte Carlo method, and diffusion approximation agree well with those of statistical formulation within 1 % error. The results show that the insulation performance of the VIP can be enhanced by increasing the optical thickness or decreasing the wall emissivity. To decrease the wall emissivity, it is recommended to insert radiation shields or to use an envelope of low emissivity. It is also found that the backward scattering is more effective than the isotropic scattering especially. Therefore, by addingspecular metal scales in the filler material, significant improvement in the insulation performance is expected.