Editorial: Editor’s challenge in heat transfer mechanisms and applications: 2022

In the study of many heat transfer processes, it is necessary to consider the interaction of heat conduction, natural or forced convection, and heat transfer by thermal radiation. The greatest difficulties in the computational modeling of combined heat transfer are related to time-consuming calculations of radiative transfer in absorbing and scattering media. Such media are, for example, gases or liquids with suspended particles, as well as dispersed materials and solids with microcracks or bubbles. Natural objects of study include the Earth’s atmosphere and ocean, snow and ice, powders or dust, ordinary sand, and even biological tissues with optically heterogeneous living cells. In thermal engineering, these are combustion products containing soot and fly ash particles, porous ceramics and heatshielding materials, particles in thermochemical reactors, and melt droplets from a possible severe nuclear reactor accident. Thermal radiation has a wide spectral range in which the optical properties of substances and materials are usually substantially dependent on the radiation wavelength. Therefore, in order to calculate the contribution of thermal radiation to heat transfer, radiative transfer calculations must be carried out for a large set of different wavelengths. In the numerical solution of transient heat transfer problems, such calculations, carried out at each time step, are the main factor influencing the computation time. It is also important that the numerical solution of the integrodifferential radiative transfer equation (RTE) regarding the radiation intensity, which is dependent not only on the coordinates but also on the direction, is a very complex procedure (Coelho, 2014). This means that the use of simple but sufficiently accurate models of radiative transfer in scattering media is absolutely essential for solving many problems of combined heat transfer. Fortunately, heat transfer problems (unlike optical diagnostics problems) have a number of physical features that allow simpler mathematical models. Note that we are usually dealing with multiple scattering of radiation in a medium when the angular distribution of the radiation in a single scattering is irrelevant. In this case, the so-called transport approximation can be used (Dombrovsky, 2012); the integral term in RTE is missing and the scattering anisotropy is taken into account by a transport scattering coefficient. The high accuracy of the transport approximation has been confirmed for diverse problems (Dombrovsky, 2010; Dombrovsky, 2019). OPEN ACCESS