Radiation between surfaces

It is difficult to obtain the correct temperature boundary conditions in a model. Radiation between surfaces in a room and conduction throu the surfaces are important for the level of the surface temperature T, x,y,z). It is difficult to establish the similarity principles based on radiation and conduction. A practical method is to estimate the influence of radiation and conduction and include this level in the boundary values of the model. In this way it... 

The processes of scattering and absorption of radiation in the atmosphere so significantly alter the spectral distribution that any similarity to extra terrestrial radiation is almost coincidental. Experiments with radiation between surfaces have shown that blackbody radiation theory can be extended successfully to many radiation heat transfer situations. In these situations the strict equilibrium requirements of the initial model have so far not proved to be necessary for practical designs. Most importantly the concept of temperature has proved useful in non-equilibrium radiation flux situations(3). 

The complex subject of thermal radiation transfer has received much study in recent years and is covered in a number of texts. The following introductory treatment discusses the following topics emission of radiation, absorption by opaque solids, radiation between surfaces, radiation to and from semitransparent materials, and combined heat transfer by conduction-convection and radiation. 

The absorption of radiation between surfaces for most earth-bound electronic systems is relatively small. 

For radiation between surfaces through a non-absorbing gas (e.g. He-Xe), surface area, rather than separating distance, impacts heat transfer... 

Thickness. The traditional definition of thermal conductivity as an intrinsic property of a material where conduction is the only mode of heat transmission is not appHcable to low density materials. Although radiation between parallel surfaces is independent of distance, the measurement of X where radiation is significant requires the introduction of an additional variable, thickness. The thickness effect is observed in materials of low density at ambient temperatures and in materials of higher density at elevated temperatures. It depends on the radiation permeance of the materials, which in turn is influenced by the absorption coefficient and the density. For a cellular plastic material having a density on the order of 10 kg/m, the difference between a 25 and 100 mm thick specimen ranges from 12—15%. This reduces to less than 4% for a density of 48 kg/m. References 23—27 discuss the issue of thickness in more detail. 

As an example, consider radiation between two surfaces Ai and Ao, which together form a complete enclosure. Equation (5-130) takes the form... 

The radiation heat transfer (cf> ) from the heat loads such as machines, lamps, persons, and sun has to be determined separately for the lower zone ( ./ ) and upper zone (4>nn The radiation between zone wall surfaces ( 4 u uJ has to be determined as well. 

Most room models are nongeometric, with the radiation exchange between surfaces being calculated solely on an area-weighted basis. 

Angle factor The geometrical shape factor used in calculating radiation exchange between surfaces / and /. 

Radiative heat exchange The heat exchange by radiation between the clothing surface, including uncovered skin, and the environment, in W m -. 

The resistances of the Sheetrock, siding, and insulation may be viewed as conductive resistances, while the resistance at the two surfaces combine the effects of convection and radiation between the surface and its surroundings. Typical values for these resistances in units of (W/m — W/m"-°C)" ((Btu/h-ft -°F) ) are as follows ... 

This is the rate of heat transfer from a surface to the surrounding air (or fluid) due to conduction convection and radiation. It is generally used only in still-air conditions and when the temperature difference between surface and ambient is of the order of 30 K. It is obtained by dividing the thermal transmission per unit area in watts per square meter by the temperature difference between the surface and the surrounding air. It is expressed as W/nf K. 

The availability of high-intensity, tunable X-rays produced by synchrotron radiation has resulted in the development of new techniques to study both bulk and surface materials properties. XAS methods have been applied both in situ and ex situ to determine electronic and structural characteristics of electrodes and electrode materials [58, 59], XAS combined with electron-yield techniques can be used to distinguish between surface and bulk properties, In the latter procedure X-rays are used to produce high energy Auger electrons [60] which, because of their limited escape depth ( 150-200 A), can provide information regarding near surface composition. 

The rate of heat transfer by radiation between two surfaces may be reduced by inserting a shield, so that radiation from surface 1 does not fall directly on surface 2, but instead is intercepted by the shield at a temperature Tsh (where 7, > T,h > T2) which then reradiates to surface 2. An important application of this principle is in a furnace where it is necessary to protect the walls from high-temperature radiation. 

In practice, as a result of introducing the radiation shield, the temperature T2 will fall because a heat balance must hold for surface 2, and the heat transfer rate from it to the surroundings will have been reduced to q h. The extent to which 72 is reduced depends on the heat transfer coefficient between surface 2 and the surroundings. 

In Table 11.4, PR was calculated considering the thermal exchange for radiation between two cylindrical coaxial surfaces with the following hypotheses ... 

The bulk of stellar radiation comes from the surface layers or atmosphere of a star, more particularly the photosphere , which is defined as the region having optical depths for continuum radiation between about 0.01 and a few. The optical depth ti is measured inwards from the surface and represents the number of mean free paths of radiation travelling vertically outwards before it escapes from the star. It is related to the geometrical height z above some arbitrary layer by... 

Emission infrared spectroscopy is used for thin films and opaque polymers. The sample is heated so that energy is emitted. The sample acts as the radiation source and the emitted radiation is recorded giving spectra similar to those of classical FTIR. In some cases, IR frequencies vary because of differences in the structures at different depths and interactions between surface and interior emissions. 

What are the advantages of the radiochemical method compared with other in situ techniques It offers a direct relationship between surface radiation (N ) and surface concentration, which allows a direct measurement of the amount of adsorbed molecules on the electrode, a condition difficult to determine with other in situ techniques. The main limitation of the technique is the availability of radioactive forms of the compound the experimenter wants to study. In this respect, the type of radiation preferred is of the P-type, mainly because of the ease of detection and minimal safety hazards. Typical P-emitters used are H, C, S, Cl, and P, which as constituents of molecules, open a great variability of compounds for study. Figure 6.21 shows some experimental results obtained for the measurement of adsorption on single crystals using this radiochemical method. 

Figure 19. Sample isotherms and interface shapes computed for the QSSM for CZ crystal growth by Atherton et al. (153). The model includes detailed radiation between the surfaces of the melt, crystal, and crucible. Isotherms are spaced at 10 K increments in the melt and 30 K increments in the other...

Atherton et al. (153) have extended the calculations to include diffuse-gray radiation between components of the enclosure and reached essentially the same conclusions regarding the stability of the process. However, they discovered a new mechanism for the damped oscillation of the crystal radius caused by the radiative interaction between the crystal surface just above the melt level and the hot crucible wall. These oscillations are especially apparent when the vertical temperature gradient in the crystal is low, so that radiative heat transport has a dominating influence. 

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