Thermal radiation in semitransparent media

Asemitransparent medium is one that absorbs, possibly scatters, and emits thermal radiation in volume. It is also said to be a medium that participates in radiation. This type of environment is found, for instance, in combustion, where high temperature gases emit and absorb radiation on a volume basis. In addition, when these gases are loaded with particles (soot, droplets), they also contribute to radiative transfer by absorbing, emitting, and scattering photons. Basically, radiation can be considered from either a wave perspective (propagation of electromagnetic waves) or as a particle flow (tracking of energy packets, in this case photons). Each approach has its own advantages and disadvantages. They are used in a complementary way.

There are several areas of great industrial interest where radiation through a “semitransparent” medium plays a dominant role, such as high temperature processes (glass furnaces, boilers), aircraft or rocket engines (combustion chamber thermics, infrared remote sensing), insulation design, fire protection, and so on.

This article introduces the basic concepts of radiative transfer in media that absorb, emit, and scatter photons. The various mechanisms are described in detail to provide the basis for the physical model on which the major thermal radiation modeling methods are based.

The present text builds on the knowledge presented in [BE 8 210] (in French) for opaque media.