Whilst conduction and convection both represent significant modes of heat transfer in industrial equipment, radiation can be, under certain conditions, the dominant mode. This is particularly the case for heat exchange occurring in industrial furnaces and in combustion chambers.

It is also thanks to thermal radiation that the energy emitted by the Sun propagates through different media before reaching the Earth, passing through interstellar spaces comprising the extremely diffuse gases and dust of the Milky Way, the interastral voids, and the Earth’s atmosphere.

Yet transfer by radiation is characterized by an essential specific feature that differentiates it from conduction and convection. Indeed, as we saw in Volume 1 of this series, heat transfer by radiation can occur between bodies, at a distance, even without a “support medium” to convey energy. In reality, in this energy transfer, heat can even be exchanged between surfaces separated by vacuum. Of course, radiation can also take place between surfaces separated by air or by any homogeneous or non-homogeneous medium.

This energy transfer mode can therefore occur without the need for either contact-continuity (as with conduction), or a carrier fluid (as with convection). In actual fact, this characteristic is inextricably linked to the very nature of the phenomena governing radiation heat transfer and, above all, to the very essence of radiant energy. As we will demonstrate in Chapter 1, radiant energy is essentially wave-based in nature. It is generated through the transfer of electromagnetic waves between surfaces. Given that waves transport photons, radiant energy is also corpuscular in nature.

This volume aims at analyzing in detail the basics of energy transfer by radiation, according to the perspective of determining design equations for industrial equipment such as furnaces, boiler heaters, etc. This analysis uses a set of parameters that are specific to this energy transfer mode. These parameters are presented in Chapter 2 of the present volume.

Moreover, as we will see in Chapter 3, the study of radiation of matter is greatly facilitated by the introduction of a virtual component having an ideal radiative behavior. This component is referred to as a black body, the radiation of which is entirely governed by laws such as Planck’s law, the Stefan-Boltzmann law or the Wien laws.

In reality, introducing the black body constitutes a tool enabling the studying of nonvirtual, nonideal real bodies. Indeed radiation of any real surface is studied by linking it to that of the ideal black body. This is accomplished through the introduction of a specific parameter, emissivity (Chapter 4); which is defined as the ratio between the energy radiated by a real surface at a given temperature divided by the energy which would be emitted by a black body under the same temperature.

The different parameters defined in chapters 2 to 4 make it possible to analyze radiative energy transfers between surfaces separated by a transparent medium (Chapter 5).

Moreover, radiative energy exchange between surfaces depends on the geometric positions occupied by these surfaces in space. This specific feature is taken into account using angle factors, whose practical calculation methods are covered in Chapter 6.

In practical engineering calculations, it is sometimes justified to assume that some surfaces have a black body behavior. It is in this perspective that Chapter 7 is reserved for energy balances for radiations between black surfaces.

It is necessary however, to underline that the “black body assumption” does not hold for all surfaces. Those surfaces which cannot be assumed to be black are called gray surfaces. Radiative energy balances between gray surfaces represent therefore most of the practical situations encountered in engineering calculations. These cases are analyzed in Chapter 8.

From a computational point of view, radiative energy balances often lead to large systems of equations to be resolved. This can be rather complicated when more than two surfaces are involved. But the introduction of electrical analogs (see Chapter 9) can lead to easier ways of resolution.

In most practical situations, we wish to maximize energy transfer between surfaces. But in certain cases, we may wish to reduce the transfers by radiation between surfaces. This is for example the case when you try to reduce energy input to a building from its glazing. This task is generally accomplished by introducing filters. Chapter 10 shows that interposing filters between surfaces can lead to significant reductions in the energy exchanged.

On a different note, in furnaces and boilers we often encounter energy transfer by radiation between surfaces which are separated by non-transparent media. Indeed, the latter are often charged with molecules of carbon dioxide (CO₂), water vapor (H₂O) and traces of SO₂, NO_x and unburned hydrocarbons. Under these conditions (see Chapter 11), the medium contributes to the exchange by absorbing a portion of the radiation and by reflecting another portion.

Lastly, in order to help the reader assimilate the calculation methods presented in this book, Chapter 12 is devoted to a series of practical exercises and to the presentation of their solutions; meanwhile the physical data required for the calculations are grouped together in the Appendix (database).