RADIATIVE COOLING OF PARTICLE FLOW IN VACUUM
In many articles concerning the thermal radiation of disperse systems, the temperature of a disperse system was assumed to be known, and the only problem was to determine the radiation field. Strictly speaking, a nonequilibrium thermal radiation field, when the radiation flux is not equal to zero at least at one point, will lead to variations of the medium temperature. Certainly, these variations may be small. In this case, the problem statement based on the known temperature field, as in the article Radiation of nonisothermal layer of scattering medium, is correct. For example, simple estimates show that radiation heat transfer cannot lead to an essential variation of gas and particle temperature in the combustion chamber and supersonic nozzle of a solid propellant rocket engine. At the same time, a two-phase exhaust jet of the rocket engine may be cooled by various conditions: at low altitudes — due to mixing with ambient air (the role of radiation is not important, see article Thermal radiation of two-phase exhaust jet), but at high altitude — sbecause of the thermal radiation to surrounding space.
When thermal radiation takes part in formation of the medium temperature field alongside other heat transfer modes, i.e., in the case of combined heat transfer, the resulting radiation field can be calculated only by solving the coupled radiative transfer and energy equations. In the articles Radiative boundary layer and Liquid droplet radiator for space applications, problems of the radiation heat transfer in moving media are considered, when there is an interaction between the radiation and convective heat transfer, but heat conduction in the medium can be neglected. In the first of the above-mentioned articles, the known model problem of a radiative boundary layer on a plate is considered. As in some other articles of the Radiation area, the differential approximations are employed. The abstraction of the physical problem statement has proved to be highly useful in solving some practical problems. An example of such problem is presented in the article Liquid droplet radiator for space applications.