20. Radiant Heat Transfer

Radiant heat transfer consists of the transfer of energy through the electromagnetic waves that are emitted by any material object as a consequence of its temperature. Unlike the other modality of energy transport, i.e., convection and diffusion, radiation does not need a medium, such as air or a metal, to propagate and, in fact, it can move across the void, as it happens with the solar energy reaching the earth surface. Also, radiation is much more dependent on temperature, compared to heat convection and diffusion. Accordingly, radiation is the dominant form of energy transport in furnaces, because of their high temperature, and in cryogenic insulation, because of the vacuum existing between particles. So, for example, gases in a combustion chamber lose more than 90 % of their energy by radiation. In this Section, we will consider the particular form of electromagnetic radiation that is connected to heat transfer, that is thermal radiation. Most energy of this type is in the infra-red region of the electromagnetic spectrum, although some of it is in the visible region, and should not be confused with other forms of electromagnetic radiation, from such as radio waves, X-rays, or gamma rays.