Guia A a Z para Thermodinâmicas, Transferência de calor e massa, e Engenharia de Fluidos
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Thermal radiation is an important mode of energy transfer in many engineering systems. A wide variety of these systems involve semi-transparent media. Some examples of these materials include fluidized and packed beds, combustors, catalytic reactors, surface pigmented coatings, soot and fly-ash, sprayed fluids, and a variety of insulating materials such as fibers, foams, porous and reticulated ceramics, microspheres, and multilayered particles. An early review of thermal radiation phenomena in particulate media has been given by Tien and Drolen (1987). A more recent review of the radiative properties of porous and particulate media was reported by Baillis-Doermann and Sacadura (2000). The very recent book by Dombrovsky and Baillis (2010) presents the state-of-the-art in both theoretical prediction and experimental determination of spectral radiative properties of real disperse media. In the third chapter of this book, the authors focus on the determination of the radiative properties of some important materials such as foams, fibrous materials, various ceramics, polymer coatings containing microspheres, and aerogel superinsulations (see, also, the articles Highly porous cellular foams, Low porosity foams, Highly porous isotropic and anisotropic fibrous materials, Low-porosity ceramics for thermal barrier coatings, etc.)

The most common assumption to consider in order to solve the radiative transfer problem in such complex materials involves treating the disperse medium as continuous and homogeneous media and using the so-called “effective radiative properties” in the radiative transfer equation. Thus, the characterization of radiative properties of porous materials is crucial. Theoretical prediction and the experimental identification method present advantages and drawbacks.

Under the assumption of independent scattering, the equivalent radiative properties of the disperse medium can be calculated by simply adding the spectral radiative characteristics of each particle present in an elementary volume. A theoretical analysis [see the article Radiative properties of particles and fibers (theoretical analysis)] provides better insight into the influence of morphology and the optical constants of the particle and matrix materials on the radiative properties of the medium. However, it is usually based on some specific assumptions concerning the particle shape. It is also important that the hypothesis of independent scattering is considered as a basis of these models. In some practically important cases, the particles are regularly positioned in space or they are in close proximity. As a result, the hypothesis of independent scattering may be invalid. In other words, the interaction of a particle with the radiation is affected by the presence of other particles. This may be a result of the following physical effects:

  1. The far-field interference of radiation scattered by regularly positioned particles.
  2. The near-field interference of radiation scattered by the neighboring particles.
  3. The geometrical effect known as “nonpoint scattering” due to close spacing of the neighboring particles.

Of course, the radiative models for dispersed materials based on the presentation of a real material as a combination of independently scattering particles of simple shape cannot be universal and the reliable spectral results for complex real materials can be obtained only on the basis of the experimental study and subsequent identification of the material radiative characteristics. Obviously, the experimental identification (see the article A basis of experimental characterization and identification procedure) provides knowledge of the real material properties. Basically, the measurements of both spectral directional transmittance and reflectance carried out on thin layers of disperse media can be used to identify the radiative properties using special mathematical procedures for the inverse problems. These methods are suitable for real materials, which are generally much more complex than the particulate media considered in simplified theoretical models. The measurements of radiative characteristics of dispersed materials are crucial to verify the validity of theoretical models. They also permit investigating the limitation of assumptions used in theory. However, as the number of radiative properties to be identified can be high, an optimizing issue concerns using theoretical prediction for some parameters combined with experimental identification of other ones. Combined approaches of this type allow decreasing the number of parameters to be identified. The determination of radiative parameters may be shared in variable proportions into theoretical prediction and experimental identification; the choice being specific to each particular situation, depending on the properties to be determined. Due to the potential complexity and number of radiative properties to be determined, there is no single response to the problem of their determination. Both theoretical prediction and the experimental identification method can be useful and complementary in the determination of “effective radiative properties” of these media (see the article Spectral radiative properties of some important materials: Experimental data and theoretical models).

Of course, many researchers from different countries are involved in experimental and theoretical studies of wide-range radiative properties of advanced dispersed materials and it is impossible to give a complete picture of this branch of science in just several Thermopedia articles. The authors of the present introductory article have tried to compensate this drawback in their recent book, where the reader can find a number of useful references of many interesting papers by other authors (Dombrovsky and Baillis, 2010).

REFERENCES

Baillis-Doermann, D. and Sacadura, J.-F., Thermal radiation properties of dispersed media: Theoretical prediction and experimental characterization, J. Quant. Spectrosc. Radiat. Transf., vol. 67, no. 5, pp. 327-363, 2000.

Dombrovsky, L. A. and Baillis, D., Thermal Radiation in Disperse Systems: An Engineering Approach, Redding, CT: Begell House, 2010.

Tien, C. L. and Drolen, B. L., Thermal radiation in particulate media with dependent and independent scattering, Annu. Rev. Numer. Fluid Mech. Heat Transfer, vol. 1, pp. 1-32, 1987.

References

  1. Baillis-Doermann, D. and Sacadura, J.-F., Thermal radiation properties of dispersed media: Theoretical prediction and experimental characterization, J. Quant. Spectrosc. Radiat. Transf., vol. 67, no. 5, pp. 327-363, 2000.
  2. Dombrovsky, L. A. and Baillis, D., Thermal Radiation in Disperse Systems: An Engineering Approach, Redding, CT: Begell House, 2010.
  3. Tien, C. L. and Drolen, B. L., Thermal radiation in particulate media with dependent and independent scattering, Annu. Rev. Numer. Fluid Mech. Heat Transfer, vol. 1, pp. 1-32, 1987.
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