Studies of the radiative properties of high temperature ceramic fibre insulation materials

The main object of the present study was to obtain useful basic radiative property data for a range of commonly used high temperature ceramic fibre insulation materials. These radiative properties are : (1) the emissivity, which is of importantance in connection with external heat transfer and (2) the extinction coefficient, scattering albedo and back-scattered fraction factor, which affect internal heat transfer. There is a real need for data concerning these radiative properties for heat transfer analysis of systems utilising ceramic fibre materials especially in high temperature applications. Ten materials have been studied in the work reported here. In order to obtain the emissivity data, a specially designed experimental apparatus has been constructed and developed. Proving tests clearly showed that the new equipment was working well and capable of yielding accurate results. Good agreement was obtained in these tests between emissivities measured using the present method and using well established proven methods in the case of stainless steel, graphite and an alumino silicate refractory material. In the course of subsequent work with ceramic fibre insulation materials well-defined distributions of total and spectral emissivities were then obtained as functions, respectively, of temperature from 100 to 950°C and wavelength from 1.2 to 19.8 pm. In addition, the spectral transmittance has also been measured. After a detailed survey and evaluation of earlier work, a new data-deduction scheme based on a semi-analytical method has been used. It combines the model of Tong et al. with the emissivity correlations of Andersen. Data obtained from direct spectral measurements of radiation transmission with specimens of various thickness is combined with the spectral emissivity data to deduce the extinction coefficient, scattering albedo and back-scattered fraction factor using a nonlinear least squares method. Values of these properties have been obtained over the wavelength range from 1.2 to 15 mum.