Radiation heat transport, effect

The insertion of low-emissivity floating shields within the evacuated space can effectively reduce the heat transport by radiation. The effect of the shields is to greatly reduce the emissivity factor. For example, for N shields or N + 2) surfaces, an emissivity of the outer and inner surface of and an emissivity of the shields of the emissivity factor reduces to... 

Convection is not a singular heat transport vehicle as are conduction and radiation. Instead, convection increases conduction by constantly circulating warmer material away from hot surfaces and replacing it with colder material. This increases the effective temperature difference, which increases the rate of heat transfer by conduction. 

Due to the Langrangian formulation applied to the solid phase, the use of an effective thermal conductivity as usually applied to porous media is not necessary. In a packed bed heat is transported between solid particles by radiation and conduction. For materials with low thermal conductivity, such as wood, conduction contributes only to a minor extent to the overall heat transport. Furthermore, heat transfer due to convection between the primary air flow through the porous bed and the solid has to be taken into account. Heal transfer due to radiation and conduction between the particles is modelled by the exchange of heat between a particle and its neighbours. The definition of the neighbours depends on the assembly of the particles on the flow field mesh. 

The thermal environment is sometimes very complex. Convection, radiation and conduction are the common means of heat exchange and they vary independently over time and location. The final effects on the surface heat exchange of the human body are important factors for heat balance and for perception of the thermal conditions. Assessment of the thermal environment in a modern office or a car can create difficulties due to the complex interaction of the ventilation system with the situation close to the person and the external, environmental factors (e.g. radiation, air temperature and air movements). Furthermore, measurements in reality, as well as in the laboratory, contain various methodological problems. In this chapter some important aspects of dynamic water vapour and heat transport through fabrics are discussed. 

VIP and VG offer outstanding thermal resistance because evacuation of the porous core material or the glazing cavity, respectively, results in a drastic reduction of heat transport by gas molecules. Aerogels on the contrary are nonevacuated superinsulators. Their low thermal conductivity is correlated with the pore structure of these materials. Before we briefly touch on the effect of the unique structural features of aerogels on heat transport, let us recapitulate the basics. Generally, heat is transferred by conduction, convection, and radiation. In porous materials there are five possible contributions to the total heat transfer, namely ... 

Other energy terms encountered with particular flow conditions are work of expansion or viscous dissipation terms, primarily important in high-speed flow external field effects, mechanical or electrical, can also occur. Since they usually are of much less importance, they will not be considered here. Heat radiation in the reactor is often neglected, except in the case of fixed bed catalytic reactors operating at high temperatures, but then it is generally lumped with the heat conduction and a few more heat transport mechanisms into an "effective"... 

The radial heat transport is complex, involving conduction, convection, and radiation between voids and solid and between solid particles. Possible modes of heat transfer in the radial direction are shown in Figure 14.3. Different physical models result depending on whether various resistances to the heat transport are in series, parallel, or a combination of both. Here an additive model is considered, which assumes that the radial effective thermal conductivity consists of static (conduction and radiation) and dynamic contributions, the latter caused by fluid motion. These two contributions are considered to be additive ... 

If assume that the excess heat from the hot leg piping, the turbine and the piping from the turbine to the recuperator (total of about 14 kW) is rejected via radiation to a 600 K sink, and then transported across a 100 K temperature delta to 500 K radiator surfaces, then the required radiator area to reject the excess heat is approximately 5 square meters, which is about 25% of the assumed plant outer envelope area. A more effective heat transport and distribution system that increases the heat rejection temperature would decrease radiator area. For example, an increase from 500 to 560 K would reduce area to almost 3 square meters. 

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