Radiative properties of fibrous insulations - Theory versus experiment

Abstract

The validity of an exact theoretical model for calculation of the radiative properties of high-porosity, randomly oriented fiber media is examined by comparing experimental measurements with theoretical predictions for spectral reflectance and transmittance. Spectral hemispherical reflectances and spectral normal transmittances were calculated at wavelengths from 1.5 to 10.0 /tm for several thicknesses of three types of thermal insulating materials having randomly oriented silica fibers of different size distributions. Theoretical results are compared with experimental measurements made on each of the three materials. The theoretical methodology, analytical results, and experimental characterizations of geometric parameters and radiative properties of the test materials are presented. The comparison shown between theory and experiment is excellent. Thus, the model is considered to be a valid tool for prediction of radiative properties of high-porosity, randomly oriented fibrous media. Nomenclature d2F = fiber orientation distribution function Fv = fiber volume fraction / = intensity i(rj, ) = isolated fiber scattering intensity distribution Ke = extinction coefficient L = thickness N = number of fiber sizes p = phase function Q = single fiber efficiency R = reflectance r = fiber radius T = transmittance