A-to-Z Guide to Thermodynamics,
Heat & Mass Transfer, and Fluids Engineering
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It seems natural to explain first which particles will be considered in numerous articles following from the present introductory article. The most evident example is a homogeneous spherical particle of an absorbing and refracting material. It is assumed that this particle is surrounded by a non-absorbing and non-refracting medium. There is a classical analytical solution obtained by Gustav Mie for the radiation absorption characteristics of such ideal particle. This solution presented in the article “Mie solution for spherical particles” can be easily generalized to include the case of a two-layer or hollow particle immersed in a non-absorbing or weakly absorbing refracting medium. The solutions for two-layer spherical particles as well as the limiting cases of a perfectly reflecting particle or two-layer particle with a perfectly reflecting core are also given in the article “Mie solution for spherical particles”. A spherical cavity (or bubble) in a homogeneous absorbing and refracting medium can be also considered as a particle and one can use the same general solution to calculate the characteristics of such bubble. The same general theory is applicable to the case of hollow particles which are also called bubbles when the particle wall is relatively thin. Nevertheless, the simplified expressions are recommended for calculations of radiative properties of hollow particles.

One can consider an infinite circular cylinder as another type of particles. The cylindrical particles are not so often observed in nature, but industrial applications of various fibrous materials make the solution for long cylinders to be very important for engineering practice. This analytical solution for cylinders at normal and oblique illumination by the incident radiation is presented in the article “Scattering problem for cylindrical particles”. As in the case of spherical particles, the solution is given not only for homogenous cylinders but also for hollow and two-layer cylinders at arbitrary incidence.

It should be noted that the radiative properties of some particles and fibers, which are of interest for engineering applications, are presented in some details in a set of specific articles following from the general articles “Mie solution for spherical particles” and “Scattering problem for cylindrical particles”.

The radiative properties of complex-shape particles, particle clusters, and agglomerates of spherical particles are not so simple, and one needs a special mathematical technique even to compute the integral (over the angles) characteristics of such absorbing and scattering objects. This is a subject of a separate article entitled “Agglomerates and complex shape particles”. The latter article is considered as an important component of the general topic under discussion because of the well-know applications, such as soot agglomerates in combustion, particle clusters in the atmosphere, agglomerates of the pigment particles in paint coatings, and aggregates of nanoparticles in advanced thermal super-insulations.

It goes without saying that the majority of real disperse systems are composed of particle of various sizes. It means that one should use some average local characteristics of these polydisperse particles in the continuous radiative transfer models. The relations between the properties of single independently scattering particles and the resulting absorption and scattering characteristics of polydisperse systems with the known size distribution of particles are discussed in the article “Radiative properties of polydisperse systems of independent particles”. Particularly, it is shown that in some (but not all) cases, there is no need in detailed data for the size distribution and one can use a monodisperse approximation with an effective average size of particles to determine the radiative properties of real polydisperse systems.