Radiation, blackbody diffuse

A planar polished surface reflects heat radiation in a similar manner with which it reflects light. Rough surfaces reflect energy in a diffuse manner hence radiation is reflected in all directions. A blackbody absorbs all incoming radiation and therefore has no reflection. A perfect blackbody does not exist a near perfect blackbody surface such as soot reflects 5% of the radiation, making it the standard for an ideal radiator. 

The interior of the furnace may be treated as a blackbody. Calculate the radiation lost through the quartz window to a room at 30°C. Diffuse surface behavior is assumed. 

A blackbody is defined as a perfect emitler and absorber of radiation. At a specified temperature and wavelength, no surface c.in emit more energy than a blackbody. A blackbody absorbs all incident radiation, regardless of wavelength and direction. Also, a blackbody emits radiation energy unifomily in all directions per um t area normal to direction of emission (Fig. 12-7). That is, a blackbody is a diffuse emitter. The tenti diffitse means independent of direction. The radiation energy emitted by a blackbody per unit time and per unit surface area was determined experimentally by Joseph Stefan in 1879 and expressed as... 

A blackbody is said to be a diffuse emitler since it emits radiation energy uniformly in all directions. 

This value of intensity is the same in ail directions since a blackbody i.s a diffuse emitter. Intensity represents the rale of radiation emission per unit area normal to the direction of emission per unit solid angle. Therefore, the rale of radiation energy emitted by A, in the direction of di through the solid angle is determined by multiplying /] by the area of Aj normal to 0i and the solid angle oij.,. That is,... 

A few comments about the validity of tlie diffuse approximation are in order. Although real surfaces do not emit radiation in a perfectly diffuse manner as a blackbody does, they often come close. The variation of emissivity with direction for both electrical conductors and nonconductors is given in Fig. 12 26. Here 0 is tlie angle measured from the normal of the surface, and thus 0 = 0 for radiation emitted in a direction normal to the surface. Note that Sg remains nearly constant for about 0 < d0° for conductors such as metals and for 6 < 70° for nonconductors such as plastics. Therefore, the directional emissivity of a sur face in the normal direction is representative of the hemispherical emissivity of the surface. In radiatioit analysis, it is common practice to assume the surfaces to be diffuse emitters with an emissivity equal to the value in the normal (6 = 0) direction. 

The emissivity of the gas media is a function of many parameters including gas pressure, temperature, partial pressures of radiatively participating species, and optical path length or characteristic dimension. Thus, if the concentration of the absorb-ing/emitting species is increased, the emissivity of the media increases as well. If the optical thickness of a medium tends to infinity, then the emissivity of such a medium tends to 1, which corresponds to the blackbody limit. At this limit, radiation becomes a totally diffusive process. 

The blackbody is an ideal surface that absorbs all incident radiation regardless of wavelength and direction. Furthermore, a blackbody is a diffuse emitter and, at any prescribed wavelength and temperature, no surface can emit more energy than a blackbody. The total emissive power of a blackbody depends only on its temperature and is given by the Stefan-Boltzmann law. 

Blackbody radiation is achieved in an isothermal enclosure or cavity under thermodynamic equilibrium, as shown in Figure 7.4a. A uniform and isotropic radiation field is formed inside the enclosure. The total or spectral irradiation on any surface inside the enclosure is diffuse and identical to that of the blackbody emissive power. The spectral intensity is the same in all directions and is a function of X and T given by Planck s law. If there is an aperture with an area much smaller compared with that of the cavity (see Figure 7.4b), X the radiation field may be assumed unchanged and the outgoing radiation approximates that of blackbody emission. All radiation incident on the aperture is completely absorbed as a consequence of reflection within the enclosure. Blackbody cavities are used for measurements of radiant power and radiative properties, and for calibration of radiation thermometers (RTs) traceable to the International Temperature Scale of 1990 (ITS-90) [5]. 

Consider a hypothetical stirface that is opaque and diffuse. At 300 K, its spectral emissivity ex may be approximated with the wavelength dependence shown in Figure 7.8. Determine its total emissivity, e, and the absorptivity for radiation from the sun, a, and a blackbody furnace at 2000 K,... 

The total emissivity can be calculated from the spectral emissivity using the Planck distribution see Equation (7.9). Using Equation (7.7), it can be shown that at 300 K nearly 98% of the blackbody radiation is at wavelengths longer than 6 jm therefore, the total emissivity at 300 K is equal to the spectral emissivity beyond 6 pm (see Figure 7.8). Hence, e 3 = 0.3. Because the surface is diffuse, from Kirchhoff s law, The total absorptivity can be calculated from Equation (7.11). The... 

Comment by Author The analogy pointed out by D. J. Santeler is certainly correct and can be used to advantage in the design of space solar simulation systems. A collimated solar radiation beam is analogous to a high gas-speed ratio, w hereas, a diffuse radiation source, such as blackbody radiation from a large test article is equivalent to the random diffuse gas case. 

Boehman [15] made similar observations in his work comparing channels with different aspect ratios and wall emissivities. As the aspect ratio decreases, the effect of radiation increases. For microreactors with smaller aspect ratio, the radiation flux can be computed vhth reasonable accuracy employing a blackbody assumption for the reactor wall. This assumption reduces the computational complexity in obtaining the radiation flux the net radiation equation for gray diffuse emitters involves solving an integro-differential equation, which can be avoided with the blackbody radiation flux assumption [15]. Fu et al. [16] reported that the radiation effect becomes significant... 

It has become apparent that the preceding discussion of synchrotron radiation gives a false impression that multipoles do not offer a significant reduction in synchrotron losses,when in fact the point I was trying to make was that the improvement factor involves a difficult calculation and cannot be approximated by a simple estimate. Neither the single-particle nor the blackbody formula can give a reasonable answer, and classical diffusion is probably not as relevant as maximum B and 3 in comparing different devices. 

In everyday experience, energy is diffuse and matter is chunky, but certain phenomena—blackbody radiation (the light emitted by hot objects), the photoelectric effect (the flow of current when light strikes a metal), and atomic spectra (the specific colors emitted from a substance that is excited)—can only be explained if energy consists of "packets" (quanta) that occur in, and thus change by, fixed amounts. The energy of a quantum is related to its frequency. 

To see how ASa depends on the intensity of the radiation, suppose an ensemble of absorbers is exposed to continuous light with an irradiance of I(y) = Ib(v) + Ir v), where Ib v) is the diffuse blackbody radiation fi om the surroundings at the ambient temperature (T) and /r(v) is the radiation from any additional source. Molecules that absorb at frequency v will be excited with a... 

The concept of blackbody is determining the basis for describing the radiation properties of real surfaces. The black body denotes an ideal radiative surface which absorb all incident radiation, being a diffuse emitter and emit a maximum amount of energy as thermal radiation for a given wavelength and temperature. The black body can be considered as a perfect absorber and emitter. 

Fig. 4.1.7 Scattering model in thermodynamic equilibrium. An opaque slab is placed over an isothermal semi-infinite partially scattering medium. Both the slab and fte medium are held at the same temperature 7a. Radiation from the slab is incident on the medium in all downward directions, and a component of intensity h is diffusely reflected by the medium into the direction jx. The intensity of radiation thermally emitted by the medium into the direction /u. is /r. Because the space between the slab and medium is equivalent to a blackbody cavity (see Section 1.7), the sum of h and /r is the Planck intensity B Ta).

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