Radiative Heat Transfer in Fibre Insulations

Abstract

<p>A first principles method of calculating combined radiative and conductive heat flow in fibrous insulation is presented. Using the measured complex refractive index for the bulk material, the scattering and absorption cross-sections are calculated for an isolated cylinder from expressions analogous to those of Mie theory for spheres. An average over fibre angles gives the cross-sections for the insulation material. Results for extinction and absorption compare well with direct measurements on polyester insulation materials. The scattering is found to be highly anisotropic. A properly weighted average over scattering angles, combined with the calculated absorption cross-section, gives the parameters needed for a diffusion model of radiative heat transport.</p> <p>The equations describing combined radiative and conductive heat flow are solved by an approximate method, and the results are compared with measurements of thermal resistance on several samples of commercial polyester-fibre insulation. The excellent agreement, with no adjustable parameters in the theory, indicates that the diffusion model adequately describes radiative heat transport in such materials. The method is used to predict the effect of possible alterations to the fibres.</p>