In many operations, radiation is the dominant heat transfer mode. The heated material's geometry may be simple (such as a sheet) or complex and/or discontinuous. It may be stationary (batch-processed) or moving.

Radiant heating can be supplied directly via combustion or indirectly with electric or gas-heated elements. In general, the treated material and the surrounding gaseous environment are both radiatively participating. If the material is semitransparent to thermal radiation, it may be cold (negligible volumetric emission) or may be at a high enough temperature so that significant volumetric emission occurs.

Processing with inert gases to prevent surface oxidation or combustion is common, so convection and conduction can occur in conjunction with radiative transfer, and these modes are coupled to the radiative exchange through heat transfer interactions at various solid surfaces. For other processes (such as chemical vapor deposition), specific gases are introduced at selected locations so that, even though the heat transfer may be radiatively dominated, understanding the advective transport of important chemical species is of equal or primary concern.

A review of technologies in which heating is induced primarily by radiation is available. Material temperatures induced by radiation heat transfer depend upon:

1. The spectral-directional absorption, reflection, and transmission characteristics of the load
2. The spectral-directional emission characteristics of the radiation source (or sink)
3. The spectral radiation characteristics of the medium separating the source and load
4. The geometrical configuration of the load relative to the source
5. The thermophysical properties of the load
6. Associated convection and/or conduction heat transfer processes

Items 1-4 determine the degree to which the radiation source and material load are thermally coupled and can be addressed with the heat transfer analysis. Items 5 and 6 may be quantified with an analysis, which takes into account the multimode heat transfer effects discussed elsewhere in this handbook.

Because of the nonlinear nature of radiative heat transfer, few correlations exist that can be applied to relevant materials processing situations.

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