Transient Thermal Effects of Radiant Energy in Translucent Materials

Abstract

When a solid or stationary fluid is translucent, energy can be transferred internally by radiation in addition to heat conduction. Since radiant propagation is very rapid, it can provide energy within a material more quickly than diffusion by heat conduction. Radiation emitted in a hot material can also be distributed rapidly in the interior. The result is that transient temperature responses including radiation can be significantly different from those by conduction alone. This is important for evaluating the thermal performance of translucent materials that are at elevated temperatures, are in high temperature surroundings, or are subjected to large incident radiation. Detailed transient solutions are necessary to examine heat transfer for forming and tempering of glass windows, evaluating ceramic components and thermal protection coatings, studying highly backscattering heat shields for atmospheric reentry, porous ceramic insulation systems, ignition and flame spread for translucent plastics, removal of ice layers, and other scientific and engineering applications involving heating and forming of optical materials. Radiation effects have been studied less for transients than for steady state because of the additional mathematical and computational complexities, but an appreciable literature has gradually developed. This paper will review the applications, types of conditions, and geometries that have been studied. Results from the literature are used to illustrate typical radiation effects on transient temperatures, and comparisons are made of transient measurements with numerical solutions.