Determination of optical properties of fibrous thermal insulation

Abstract

We present a method of obtaining optical parameters relevant to radiative heat transfer in polyester fiber insulations. The complex index of refraction of the bulk material for frequencies between 100 and 2000 cm−1 is derived from transmission measurements on thin films by an iterative procedure. Absorption and scattering functions for an isolated fiber are calculated from expressions analogous to those of Mie theory for spheres. Results for the extinction and absorption cross section compare well with measurements on the insulation material itself. The calculation also yields the scattering as a function of angle, which is highly anisotropic. A properly weighted average over angle together with the calculated absorption cross section gives the necessary parameters for a diffusion model of radiative heat transport. Approximate differential equations describing combined conductive and radiative heat flow are solved with these parameters, and the results compared with thermal measurements on a commercial polyester insulation. The excellent agreement between theory and experiment indicates that the diffusion model accurately describes radiative heat transport in these materials. Our method can be used to predict the effect of hypothetical alterations to the fibers.