Thermal radiation surface phenomenon

Thermal radiation can take place without a medium. Thermal radiation may be understood as being emitted by matter that is a consequence of the changes in the electronic configurations of its atoms or molecules. Solid surfaces, gases, and liquids all emit, absorb, and transmit thermal radiation to different extents. The radiation heat transfer phenomenon is described macroscopically by a modified form of the Stefan-Boltzmann law, which is... 

Most materials encountered in practice, such as tnclals, wood, and bricks, are opaque to thermal radiation, and radiation is considered to be a surface phenomenon for such materials. That is, thermal radiation is emitted or... 

The radiative heat-flux is generally treated separately from the other heat flux contributions because these physical phenomena are quite different in nature and involve unacquainted mathematics. Besides, the radiative contributions in the bulk of the fluid are limited because this flux is merely a surface phenomenon. Nevertheless, the radiative losses from solid surfaces are often significant in combustion and in particular chemical reactor processes. A brief introduction to the theory of thermal radiation is presented in sect 5.3.6. 

The stated theory of thermal radiation also allows an explanation of a phenomenon that has an influence on life on earth. This is the so-called green-house effect. The sun s radiation (the spectrum is depicted in Figure 6.34), passes through open space, and reaches the external layers of the earth s atmosphere, naturally with a loss of intensity, but without a special change of spectral composition. In the atmosphere there is selective absorption of the sun s radiation by natural and industrial gases. This selectivity is defined by the structure of molecules, by their concentration and properties. It is natural also, that absorption of radiation depends on humidity, dust content and other properties of the atmospheric layers close to the surface of the earth. 

The irradiation of a polymer surface with the high intensity, pulsed, fer-UV radiation of the excimer laser causes spontaneous vaporization of the excited volume. This phenomenon was first described by Srinivasan (1) and called ablative photodecomposition. The attention of many researchers was drawn to the exceptional capabilities of photoablation (2). Etching is confined to the irradiated volume, which can be microscopic or even of submicron dimensions, on heat-sensitive substrates like polymers. In most experimental conditions, there is no macroscopic evidence of thermal damage, even when small volumes are excited with pulses of... 

Infrared (IR) active gases, like water vapor (H20), carbon dioxide (C02), ozone (03), methane (CH4), nitrous oxide (N20), chlorofluorocarbons CFC-11 (CC13F) and CFC-12 (CC12F2) naturally and anthropogenically present in the Earth s atmosphere, absorb thermal IR radiation emitted by the Earth s surface and atmosphere. This phenomenon is known as the atmospheric greenhouse effect , and the IR active... 

Becaii.se of the phenomenon of self-absorption the ideal sample for conventional emission studies is a thin layer (e.g.. a polymer film), on both metal and semiconductor. surfaces [81]. A sample is usually heated from below the emitting surface, the lower surface thus having a higher temperature than the upper one. Therefore, radiation emitted from below the upper surface is absorbed before it reaches the surface, and this self-absorption of previously emitted light severely truncates and alters features in the emission spectra of optically thick samples. This problem is overcome by using a laser for controlled heat generation within a thin surface layer of the sample, self-absorption of radiation thus being minimized. These methods, known as laser-induced thermal emission (LITE) spectroscopy [85], [86] and transient infrared emission spectroscopy (TIRES) [87], [88] can produce analytically useful emission spectra from optically thick samples. Quantitative applications of infrared emission spectroscopy are described in [89]-[91]. 

Water s unique temperature-density relationship results in the formation of distinct layers within nonflowing bodies of water, as shown in Figure 11.4. During the summer a surface layer (epilimnion) is heated by solar radiation and, because of its lower density, floats upon the bottom layer, or hypolimnion. This phenomenon is called thermal stratification. When an appreciable temperature difference exists between the two layers, they do not mix, but behave independently and have very dif-... 

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