In this introductory article, we will briefly discuss basic knowledge about thermal radiation and radiative heat transfer phenomena. A specific of presentation of this important and complex branch of science and engineering in the Thermopedia will be also characterized below.
First of all, one should remember the thermal radiation emitted by the sun and transferred through the space to our planet. The solar heat is a real source of life on the Earth. This heat cannot be transferred to the Earth by conduction and convection. The only mode of heat transfer responsible for this process is a radiation (electromagnetic waves) which can propagate in vacuum. Fortunately, the temperature of the sun surface layer is very high (about six thousand degrees). As a result, the main part of the radiation power is emitted in the visible spectral range and we can see the world around us. The great role of solar radiation in human life has been seen many centuries ago. It is sufficient to remember the sun-worship in ancient Egypt.
Everybody knows the role of fire in the early history of mankind. It is difficult to explain, but even people of the modern time like to look at campfire in the dark or seat near the fireplace and feel the radiative heat on the face and hands. One can easily continue a set of well-known examples when the radiation is visible. At the same time, it is important not to forget about invisible radiation, particularly in the so-called infrared spectral range. It is known that thermal radiation of not too hot bodies takes place in the infrared range. A good example is the infrared radiation of the Earth surface. In the case of a cloudy atmosphere, this radiation is scattered and absorbed by cloud droplets and cannot leave our planet. It is the so-called green-house effect. One can also remember an opposite case of a clear sky night when the temperature decreases during the night because of the radiative cooling of the earth surface. As a result, the morning after the clear night appears to be very cold as compared with the yesterday evening. In general, a contribution of the radiation in heat balance of the atmosphere is significant and one cannot predict the weather and climate of our planet without taking into account the radiative heat transfer.
We should go now from the human experience to the modern science history and to talk a bit about the role of knowledge on thermal radiation in present-day physics. One should remember the principal results obtained by Max Planck, who studied the thermal radiation at the turn of the nineteenth and twentieth century. His well-known fundamental law is considered as the beginning of the quantum theory. It is a good time to repeat an instructive story from the young age of Max Planck. The Munich physics professor Philipp von Jolly advised Planck against going into physics, saying, "in this field, almost everything is already discovered, and all that remains is to fill a few holes." Planck replied that he did not wish to discover new things, only to understand the known fundamentals of the field…
In the Thermopedia, we focus mainly on engineering approach to the radiative heat transfer. At the same time, we cannot avoid some physical knowledge especially in the articles concerning the primary radiative properties of various substances. Another principal problem is a choice between two alternative descriptions of the radiation: the first one is based on the geometrical optics (the traditional radiation transfer theory) whereas the second approach (more general and physically sound) takes into account the wave nature of the radiation.
The physical basis of the majority of solutions considered in a set of articles (aside from the topic “Microscale/nanoscale radiative heat transfer” and some other specific articles) is the notion of radiation transfer in an absorbing and scattering medium as a macroscopic process, which can be described by a phenomenological transfer theory and radiative transfer equation for spectral radiation intensity. The radiation transfer theory has been developed by a number of famous scientists working in physical optics, astrophysics, nuclear reactor theory, and heat transfer theory.
The simplest problems described well by employing the geometrical optics, are presented in a set of articles under the general title “Radiation transfer between the surfaces through a non-participating medium”. Some practical problems can be really solved on the basis this “geometrical” approach. Nevertheless, the absolute majority of the realistic situations are characterized by presence of a participating medium. Many diverse problems of radiative transfer in participating media are analyzed in some details in the topic entitled “Radiation transfer in emitting, absorbing and scattering media”. This involves the physical properties of gases and plasma, the specific of radiation absorption and scattering by particles comparable in size with the radiation wav
In some engineering problems, the desirable distribution of the radiation heat flux is known (like that in material thermal processing by radiative heating) and the question is how to choose the appropriate sources of thermal radiation, their positions and orientations to achieve the expected result. It is a simple example of the so-called inverse problems. Another important example of the inverse problem is an identification of the radiative properties of complex materials (like advanced thermal insulations) from the experimental data for both reflectance and transmittance of the radiation by the material sample. The inverse mathematical problems have been studied in details. It is well-known that some of these problems cannot be solved without additional data concerning the expected solution. In addition, even in the case of complete and correct problem statement, when there is the only physical solution of the inverse problem, one should use a specific mathematical procedure to find this solution. It goes without saying that the engineers and researches should know this technique. For this reason, a topic entitled “Inverse problems in radiation transfer” with several particular articles is involved in the Radiation Area.
While solving many practical heat transfer problems, one should take into account not only the thermal radiation but also heat transfer by conduction and convection in the medium. Most general problems of combined radiative-convective heat transfer are very complicated and their solution is possible only by employing approximate computational models for radiative transfer. A correct choice of an appropriate computational model is not a simple task. This choice should be based on a long-time experience in solving the specific problems for coupled radiation, conduction, and convection. These problems and some important applications of the solutions obtained are considered in the topic “Combined heat transfer by radiation, conduction, and convection”.
The last topic involved is a result of more recent studies of the present-day problems of micro-scale and nano-scale heat transfer. A batch of articles on this subject has a general title “Microscale/nanoscale radiative heat transfer”. A theoretical approach employed in these articles is based on the fluctuational electrodynamics. Obviously, it is a quite different theory as compared with the ray (geometrical) optics.
The Radiation Area of Thermopedia is not completed at the moment. Moreover, this part of the project is comparably new and preparing of some important articles is in progress now. Nevertheless, we present the existing material and ask a reader to visit this area time by time to follow the growing of the “radiation tree”. As a representative of our “radiation” team, I invite you to taste the fruits of our tree at every season. I hope it will be not only interesting and useful for your professional skill but also yield some “vitamins” for your brain…
For a topic which is broad as the one considered in the Radiation Area of Thermopedia, it is very difficult to be comprehensive. However, we hope that enough key references are cited in single articles to enable an interested reader to undertake a more detailed study of specific radiation heat transfer problems.
Heat & Mass Transfer, and Fluids Engineering