Thermal radiation

Heat radiation is based on electromagnetic waves emitted to the environment by bodies and liquids. This thermal radiation occurs simply because the bodies have a positive thermodynamic temperature. Like all electromagnetic waves, temperature radiation is not bound to the presence of matter. For solids, heat radiation is a surface effect, for most liquids a volume effect.

According to Kirchhoff's radiation law, the emission is equal to the absorption and both are dependent on the surface condition. With regard to its radiative power, it is described with the so-called emission coefficient. Glossy, perfectly reflecting surfaces neither absorb nor give off energy through heat radiation. The exact opposite is true for matt, black surfaces.

For the heat transport between bodies with heat radiation, it is also important how well the radiating surfaces are visible to each other. For the exchange it is therefore not important how large an area is, but how much of it is actually seen. This makes it necessary to define or determine the so-called viewing factors between the surfaces.

Computation time / model size

When simulating temperature fields with heat radiation, a distinction is usually made between two cases: radiation into free space and radiation between several bodies. The first modeling option is a simplification. This requires considerably less numerical effort, as the otherwise necessary viewing factors do not have to be determined in advance.

According to the Stefan Boltzmann law, the heat flow that can be transmitted by heat radiation depends on the fourth power of the surface temperature. Therefore, the consideration of thermal radiation inevitably leads to a slower convergence behavior, which is reflected accordingly in the necessary computing time.

Both the determination of the view factors and the strong non-linearity must be taken into account in the model size.

Relevance

Even if always present, in many technical applications the proportion of heat flow due to heat radiation is relatively small compared to the amount of heat transferred by convection or heat conduction. It can therefore often be neglected. To which extent this can be tolerated depends on the situation and requires careful estimation.