Radiant Heat Transfer in Fibrous Thermal Insulation

Abstract

Expressions are derived for calculating radiant heat transfer rates in laminar arrays of high‐emissivity fibers having diameters much larger than the wavelengths of the emitted radiation. New experimental results are given for a sample composed of 280‐μ fibers, with the mean sample temperature near or below room temperature, so the dominant wavelengths of fiber emission are of the order of 10 μ. The data show general agreement with the theory, and establish clearly that the rate of radiant heat transfer is proportional to the cube of the mean absolute temperature of the sample. A brief review of published data suggests that the derived expressions also give correct functional dependence on insulation parameters for finer fibers having diameters equal to, or less than, the wavelengths of the emitted energy. But the absolute magnitude of the values reported for finer fibers is generally larger than that predicted by the large‐diameter expression, and in some cases is larger by a whole order of magnitude. Reflection, diffraction, and fiber transparency effects are probable causes of the higher values. Since these effects are neglected in this analysis, it is suggested that the expressions given here may represent a lower limiting value for smaller‐diameter fibers.