Radiant heat transfer reduction methods in heat insulation of power equipment

Abstract

One common challenge for designers of power equipment to be operated at high temperatures is the reduction of radiation heat transfer. As the emissivity factor of radiating heated surfaces of such equipment in most cases can be reasonably approximated by a constant, the power emitted by them is directly proportional to the fourth power of their absolute temperature. A solution of this problem is the use of thermal radiation shields made from low-emissivity and highly reflective materials. Traditionally heat insulation structures employ aluminum foil as such a shield. However, aluminum is soft and rather expensive, and it also has a low tensile strength. An alternative to the use of aluminum foil is the application to the external surface of the thermal insulation structure of a reflective metal layer via ion-plasma spraying (bombardment) in vacuum or metal spraying (metalizing) by means of an electric arc. In order to find out which method of reduction of radiation heat transfer offers the best results, we have compared a number of methods of thermal radiation shielding (foil winding, ion-plasma spraying in vacuum or electric arc wire spraying) in terms of thermal resistance of proper heat insulation structure. In our paper we have used heat insulation structures with thinfilm heat insulation coatings. The experiments were carried out in conditions simulating real conditions of operation of pipelines and power equipment in Russia. The results of the experiments show that the alternative methods of thermal radiation shielding are more efficient than the traditional one (aluminum foil winding). In particular, the use of an aluminum coating applied to the surface of a heat insulation structure via ion-plasma spraying allows the thermal resistance to increase by 35%; in case of metalizing via electric arc wire spraying the thermal resistance increased by 15% or more.

Processing of polymer‐derived, aerogel‐filled, SiC foams for high‐temperature insulation

“…14 As temperature rises, radiative energy transfer becomes predominant according to the black-body radiation law, and in order to limit it, a good insulator should have an optical thickness larger than the mean free path of photons, in addition to low emissivity and high reflectance. 15,16 In the broad manufacturing context of porous ceramics, the PDCs route is of particular interest: it allows obtaining ceramic foams and aerogels of different systems (e.g., SiOC, SiC, SiCN), with superior thermal stability, resistance to sintering and a reduced high temperature creep rate. 12,17 Moreover, even bulk PDCs possess low thermal conductivity, 18,19 and the presence of a free carbon phase in PDCs allows to reduce the thermal conductivity of the aerogels at high temperature, where the heat transfer is dominated by radiation, as they are substantially opaque to the electromagnetic radiation in the visible and near-infrared region.…”

“…12,17 Moreover, even bulk PDCs possess low thermal conductivity, 18,19 and the presence of a free carbon phase in PDCs allows to reduce the thermal conductivity of the aerogels at high temperature, where the heat transfer is dominated by radiation, as they are substantially opaque to the electromagnetic radiation in the visible and near-infrared region. 15 However, PDCs aerogels possess two main drawbacks: their mechanical resistance is modest making their handling even more difficult in some cases, and they cannot be easily shaped, as they derive from wet gels. On the other hand, ceramic foams can be easily obtained by the PDC route, have acceptable mechanical properties, but are substantially transparent to radiation due to the presence of large and continuous macropores.…”

“…In these structures, thermal insulation is boosted by the fact that the mean free path of gas molecules is in the same dimensional range of pores, so that convection is dramatically reduced and conduction through particles bottlenecks remains the only effective heat transmission mechanism 14 . As temperature rises, radiative energy transfer becomes predominant according to the black‐body radiation law, and in order to limit it, a good insulator should have an optical thickness larger than the mean free path of photons, in addition to low emissivity and high reflectance 15,16 …”

Aspects Regarding of a UGV Fire Fighting Thermal Shield

“…From our research point of view, the task involves to the possibility of robot design imperfections occurrence, since it will be TRL2, in the research laboratory; the lack of specialized utilities at the level of the laboratory for mechanical processing of high temperatures resistant; establishing the geometrical shape of the protection system so that, from the thermodynamic point of view, refraction and reflexion coefficients corresponding to reducing the thermal energy of the robot can be obtained [57][58][59][60]; establishing a numerical-analytical model for calculating the thermal transfer processes…”