Emissivity radiative properties

In comparing the radiative properties of materials to those of a blackbody, fhe terms absorptivity and emissivity are used. Absorptivity is the amount of radiant energy absorbed as a fraction of the total amount that falls on the object. Absorptivity depends on both frequency and temperature for a blackbody if is 1. Emissivity is the ratio of the energy emitted by an object to that of a blackbody at the same temperature. It depends on both the properties of fhe subsfance and the frequency. Kirchhoff s law states that for any substance, its emissivity at a given wavelength and temperature equals its absorptivity. Note that the absorptivity and emissivity of a given substance may be quite variable for different frequencies. 

Radiation is emitted by every point on a plane surface in all directions into the hemisphere above the surface, ITie quantity that describes the magnitude of radiation emitted or incident in a. specified direction in space is the radiation intensity. Various radiation flu.xes such as emissive power, irradiation, and ra-diosity are expressed in terms of intensity. This is followed by a discussion of radiative propertie.s of materials such as emissivity, absoiptivity, reflectivity, and transmissivity and their dependence on wavelength, direction, and lemperatiire. The greenlioiijie effect is presented as an example- of the con.sequenccs of the wavelength dependence of radiation properties. We end tliis chapter with a dis cussion of attno.spheric and solar radiation. 

Some other materials, such as glass and water, allow visible radiation to penetrate to considerable depths before any significant absorption takes place. Radiation through such scmitranspareiu materials obviously cannot be considered to be a surface phenomenon since the entire volume of the material interacts with radiation. On the other hand, both glass and water ace practically opaque to infrared radiation. Therefore, materials can exhibit different behavior at different wavelengths, and the dependence on wavelength is an important consideration in the study of radiative properties such as emissivity, absorptivity, reflectivity, and transmissivity of materials. 

Consider a small body of surface area A, emissivity c. and absorptivity a at temperature T contained in a large isothermal enclosure at the same temperature, as shown in Fig. 12-35. Recall that a huge isothermal enclosure forms a blackbody cavity regardless of the radiative properties of the enclosure surface, and the body in the enclosure is too small to interfere with the blackbody nature of the cavity. Therefore, the radiation incident on any part of the surface of the small body is equal to the radiation emitted by a blackbody at temperature 7. That is, G = Ei, T) - trT", and the radiation absorbed by the small body pet unit of its surface area i.s... 

Table A-2 Boiling and freezing point properties 843 Table A-3 Properties of solid metals 844 846 Table A-4 Properties of solid nonmetals 847 Table A-5 Properties of building materials 848-849 Table A-6 Properties of insulating materials 850 Table A-] Properties of common foods 851-852 Table A-8 Properties of miscellaneous materials 853 TableA-9 Properties of saturated water 854 Table A 10 Properties of saturated refrigerant-134a 855 Table A-11 Properties of saturated ammonia 856 Table A-12 "Properties of saturated propane 857 Table A-13 Properties of liquids 858 Table A-14 Properties of liquid metals 859 Table A- 5 Properties of air at 1 atm pressure 860 TableA-16 Properties of gases at 1 atm pressure 861-862 Table A-17 Properties of the atmosphere at high altitude 863 Table A-18 Emissivities of surfaces 864-865 Table A-19 Solar radiative properties of materials 866 Figure A-20 The Moody chart for friction factor for fully developed flow in circular pipes 867...

The discrete ordinates method in a S4-approximation is used to solve the radiation transport equation. Since the intensity of radiation depends on absorption, emission and scattering characteristics of the medium passed through, a detailed representation of the radiative properties of a gas mixture would be very complex and currently beyond the scope of a 3D-code for the simulation of industrial combustion systems. Thus, contributing to the numerical efficiency, some simplifications are introduced, even at the loss of some accuracy. The absorption coefficient of the gas phase is assumed to have a constant value of 0.2/m. The wall emissivity was set to 0.65 for the ceramic walls and to a value of 0.15 for the glass pane inserted in one side wall for optical access. 

According to section 5.3.2.2, the hemispherical total absorptivity of a body with any radiative properties is equal to its hemispherical total emissivity, if radiation from a black body at the same temperature strikes the body. This is the case here. It therefore follows from (5.160) that A2F21 = A. This corresponds to the reciprocity rule (5.132) with F 2 = 1. Its application to this case was however not assured from the start as the intensity of body 1 is not constant. 

One of the most common manifestations of a deposition problem is reduced heat transfer in the radiant zone of a furnace. Decreased heat transfer due to a reduction in surface absorptivity is a result of the combination of radiative properties of the deposit (emiss-ivity/absorptivity) and thermal resistance (conductivity) of a deposit. Thermal resistance (thermal conductivity and deposit overall thickness) is usually more significant because of its effect on absorbing surface temperature. 

Previous work has indicated that the physical state of the deposit can have a significant effect on the radiative properties, specifically molten deposits show higher emissivities/absorptivities than sintered or powdery deposits (1). Although thin, molten deposits are less troublesome from a heat transfer aspect than thick, sintered deposits, molten deposits are usually more difficult to remove and cause frozen deposits to collect in the lower reaches of the furnace where physical removal then becomes a problem for the wall blowers. 

Surface enhanced fluorescence (SEE) takes place in the proximity of metal structures. The effect of fluorescence enhancement has been intensively studied by several groups [74]. In the proximity of metals, the fluorophore radiative properties are modified and an increase in the spontaneous emission rate is observed. 

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]. 

FIGURE 7.6 Spectral normal emissivity of selected materials. (Adapted from Incropera, F. R, and D. P. DeWitt, Fundamentals of Heat and Mass Transfer, 4th ed., Wiley, New York, 1996 and from Touloukian, Y. S., and D. P. DeWitt, Thermal Radiative Properties, vols. 7, 8, and 9, in Thermophysical Properties of Matter, TPRC Data Series, Y. S. Touloukian and C. Y. Ho, eds., IFI Plenum, New York, 1970-1972.)... 

Kirchhoffs law (Eq. 7.27) may be used to replace the absorptivity in these relations with emissivity if the restrictions of Table 7.1 are observed. Thus, data on one of the radiative properties can often be used to generate the others, although care must be used to avoid violating the restrictions of Table 7.1. 

The effective radiative properties, that is, effective transmissitivity (Tr), effective reflectivity (p,), and effective emissivity (assumed equal to absorptivity) (e,) are determined for various two-dimensional arrangements of spherical particles. Emerging correlations, relating these effective properties to the particle surface emissivity e, and medium porosity e, do not appear to depend significantly on the arrangement. These correlations obtained by Mazza et al. [76] are... 

In the case of metals processing, surface impurities such as thin layers deposited either by adsorption or chemical reaction (such as oxide layers) can increase the surface emissivity dramatically (Fig. 18.31). Because of the extreme sensitivity of the effective radiative properties of metals to minor surface roughness or contamination, it is recommended that measured radiative property values be used when possible. 

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