Heat transfer properties of hollow-fiber insulation materials at high temperatures
This paper presents a numerical study of heat transfer at high temperatures in thermal insulation materials made of cylindrical ceramic fibers. In particular, the properties of hollow and solid fibers in the temperature range between 1000 and 2000°C have been compared. Considering fibers made of ceramic oxides like Al2O3, light scattering is the key process to suppress radiative heat transfer because the materials have very low absorptivity in the wavelength range of substantial thermal radiation at these temperatures. In a treatment based on Maxwell's equations, the general differences of hollow- and solid-fiber scattering are evaluated in the single fiber limit. After applying the model to the special case of Al2O3 fibers, the comparison is expanded to multifiber configurations. Thereby, dependent scattering effects and conductive heat transfer are considered. Careful inspection of the results leads to the conclusion that thermal insulations made of hollow ceramic fibers can provide significantly lower effective thermal conductivity than insulations made of solid fibers at the same volume fraction of fiber material. This statement holds in particular at temperatures above 1000°C, where heat loss could be reduced up to a factor of two in wide ranges of practical interest.