Radiation opaque surfaces

The network method which we have used to analyze radiation problems is an effective artifice for visualizing radiant exchange between surfaces. For simple problems which do not involve too many surfaces the network method affords a solution that can be obtained quite easily. When many heat-transfer surfaces are involved, it is to our advantage to formalize the procedure for writing the nodal equations. For this procedure we consider only opaque, gray, diffuse surfaces. The reader should consult Ref. 10 for information on transmitting and specular surfaces. The radiant-energy balance on a particular opaque surface can be written... 

The absorption of radiation incident on an opaque surface of absorptivity a. 

It is very tempting to use Kirchhoff s law in radiation analysis since the relation c - a together with p = 1 - a enables us to determine all three properties of an opaque surface from a knowledge of only one property. Although Eq. 12-47 gives acceptable results in most cases, in practice, carp should be exercised when there is considerable dilference between the surface temperature and the temperature of the source of incident radiation. 

Tile spectral absorptivity of an opaque surface is as shown on the graph. Determine the absorptivity of the surface for radiation emitted by a source at (o) 1000 K and b) 3000 K. 

Solar radiations is incident on an opaque surface at a rate o17400 W/m . The emissivity of the surface is 0.65 and the... 

Consider two diffuse, gray, and opaque surfaces of arbitrary shape maintained at uniform temperatures, as shown in I ig. ] 3-22. Recognizing that the radios-ity J represents the rate of radiation leaving a surface per unit surface area and dial the view factor, y represents the fraction of radiation leaving surface i that strikes surface j, the net rate of radiation heat transfer from surface i to surface j can be expressed as... 

Consider an enclosure consisting of two opaque surfaces at specified temperatures r, and T2, as shown in Fig. 13-24, and try to determine llie net rate of radiation heat transfer between the two surfaces with the network method. Surfaces 1 and 2 have emissivities c, and and surface areas /1 and A2 and are maintained at uniform temperatures T, and T, respectively. There are only two surfaces in the enclosure, and thus we can write... 

The net rate of radiation transfer between any two gray, diffuse, opaque surfaces that form an enclosure is given by... 

When radiation strikes an opaque surface, only a fraction is absorbed. The remainder is reflected. The fraction absorbed is indicated by the emissivity e of the surface. The emissivity depends upon surface composition. As stated above, the emissivity... 

The radiation exitent from an opaque surface can include emitted and reflected components. The radiosity is the sum of the emissive power and the portion of irradiation that is reflected by the... 

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. 

In furnaces and other high-temperature equipment, where radiation is particularly important, the usual objective is to obtain a controlled rate of net heat exchange between one or more hot surfaces, called sources, and one or more cold surfaces, called sinks. In many cases the hot surface is a flame, but exchange of energy between surfaces is common, and a flame can be considered to be a special form of translucent surface. The following treatment is limited to the radiant-energy transfer between opaque surfaces in the absence of any absorbing medium between them. 

Also, Fourier transform infrared absorption spectroscopy provides relevant information regarding the specific interactions of different probes within substrates [17], especially in the diffuse-reflectance mode when applied to the study of powdered opaque surfaces that disperse the incident radiation. The extension of this technique to obtain time resolved transient absorption spectra in the IR wavelength range (laser flash-photolysis with IR detection) will certainly play in the near future an important role in terms of clarifying different reaction mechanisms in the surface photochemistry field [17c, 18]. 

Radiation falUng on an opaque surface is either reflected or absoibed. The proportions reflected and ab-soibed are measured by the surface quahties reflectance p and absorptance a. The sum of the two must be unity pa =. For a surface exposed to radiation, the rate of eneigy input is Ga. As the surface is heated and becomes warmer than its surroundings, it will lose heat at a rate depending on the surface conductance ho) and the temperature difference between the surface Ts) and the air To). ho Ts-Ta). The surface will reach an equilibriiun temperature when the heat input rate equals the heat loss rate, that is, when Ga = ho Ts-Ta), from which the surface temperature can be expressed as Ts = Ta + Gajh. This is the maximum temperature the surface could reach in the absence of any heat flow into the body of material behind that surface, and it is often referred to as the sol-air temperature. (More precise definitions would include a radiant emission term, so that the numerator would become the net irradiance absorbed by the surface.)... 

Resolution of cracks and pores exposed to the surface can be enhanced using radiation opaque penetrant dies. Opaque dies can also be used to differentiate between low-density inclusions and surface-connected pores. Suppose a surface-connected feature is observed that has lower X-ray absorption than the rest of the material. After it is soaked in dye, a pore will have a higher X-ray absorption than the background, while the absorption of a low-density inclusion will not change. 

On quiescent planets or satellites without substantial atmospheres, the surface temperature is determined by a balance between incident solar flux, thermally emitted radiation, and conductive heat transport into or out of the opaque surface. By measuring the surface temperature and the bolometric albedo the absorbed and emitted radiation can be found and the conductive flux into the solid body derived. After sunset or dining a solar eclipse the cooling rate of the surface depends on the thermal inertia of the subsurface layers. A study of such cooling rates provides a sensitive means of discriminating between powdery, sandy, or solid rock surfaces. We now review the theory behind such an analysis, and discuss examples of thermal inertia measurements. 

Fig. 1. The hthographic process. A substrate is coated with a photosensitive polymer film called a resist. A mask with transparent and opaque areas directs radiation to preselected regions of the resist film. Depending on resist characteristics, exposed or unexposed portions of the film are removed using a developer solvent. The resulting pattern is then transferred to the substrate surface and the resist is stripped.

For opaque materials, the reflectance p is the complement of the absorptance. The directional distribution of the reflected radiation depends on the material, its degree of roughness or grain size, and, if a metal, its state of oxidation. Polished surfaces of homogeneous materials reflect speciilarly. In contrast, the intensity of the radiation reflected from a perfectly diffuse, or Lambert, surface is independent of direction. The directional distribution of reflectance of many oxidized metals, refractoiy materials, and natural products approximates that of a perfectly diffuse reflector. A better model, adequate for many calculational purposes, is achieved by assuming that the total reflectance p is the sum of diffuse and specular components p i and p. ... 

In the previous Section we noted that the typical temperature, above which the star becomes opaque to neutrinos is Topac 0.4 4- 3 MeV, where we ignore here the differences in the absorption/production properties of different neutrino flavors [45], Saying neutrino we actually will not distinguish neutrino and antineutrino, although their absorption/production could be different. If we assume an initial temperature of To < T%pac, the star radiates neutrinos directly from the interior region. For To > T"po/P the neutrino transport to the surface is operative and leads to a delay of the cooling evolution. 

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. 

The coalescence of atoms into clusters may also be restricted by generating the atoms inside confined volumes of microorganized systems [87] or in porous materials [88]. The ionic precursors are included prior to irradiation. The penetration in depth of ionizing radiation permits the ion reduction in situ, even for opaque materials. The surface of solid supports, adsorbing metal ions, is a strong limit to the diffusion of the nascent atoms formed by irradiation at room temperature, so that quite small clusters can survive. 

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