Hemispherical emittance

FIG. 5" 14 Hemispherical emittance and the ratio of hemispherical to normal emittance for a semi-infinite absorhing-scattering medium. 

Table 5-4 illustrates values of emittance for materials encountered in engineering practice. It is based on a critical evaluation of early emissivity data. Table 5-4 demonstrates the wide variation possible in the emissivity of a particular material due to variations in surface roughness and thermal pretreatment. With few exceptions the data in Table 5-4 refer to emittances e normal to the surface. The hemispherical emittance eh is usually slightly smaller, as demonstrated by the ratio Ct/eri depicted in Fig. 5-12. More recent data support the range of emittance values given in Table 5-4 and their dependence on surface conditions. An extensive compilation is provided by Goldsmith, Waterman, and Hirschom (Thermophysicat Properties of Matter, Purdue University, Touloukian, ed., Plenum, 1970-1979). 

R. D. Skocypec and R. O. Buckius, Total Hemispherical Emittances for C02 or H20 Including Particulate Scattering, International Journal of Heat Mass Transfer, 27, p. 1,1984. 

As indicated in the hard-sphere model in Fig. 33 a hemispherical emitter surface is made up of a variety of crystal planes. The closer packed of these, such as the 110, 211, and 100 in the body centered cubic lattice, have a higher work function they therefore appear as dark spots in the more brightly emitting areas corresponding to stepped regions. With the aid of a standard orthographic projection (Fig. 34). the orientation of the emitter and the identity of the planes can thus be deduced from the symmetry of the pattern. 

In the list of thermophysical properties, some properties are of greater importance for use in industrial applications, while oAers are of more scientific interest for different applications. The properties most relevant to casting simulations are heat of fusion, heat capacity, electrical resistivity, density, thermal conductivity and diffiisivity, thermal expansion, hemispherical emittance, viscosity and surface tension [5]. 

Besides measurements of thermophysical properties, millisecond experiments are the tools of choice for metrological investigations. They have been used by NIST, USA INRiM, Italy NRLM, Japan and lately by HIT, China, for measurements dealing with radiance temperatures, radiometry, and total hemispherical emittance measurements [10,11]. 

Independent of all the efforts to implement ellipsometry to pulse-heating systems, a second branch of measurements has been jointly developed at NRLM-NMIJ [17] and NIST [101] to obtain the total hemispherical emittance. The principle of this technique is to interrupt the heating process of a pulse calorimeter and to create a short steady-state temperature condition by using the pyrometer as a feedback device. The energy input to maintain this steady-state equals the total radiative losses and thus can be related to the total hemispherical emittance, assuming that no convection occurs during the short steady-state time. 

TABLE 2.4. Total hemispheric emittances (and absorptances) of metals and their oxides, selected from references 42,51, and 70. Emittances of refractories and miscellaneous nonmetals are listed in chapter 4 of reference 51. 

A specimen is heated by electrical self-resistance in vacuum. Total hemispherical emittance is determined from the Stefan-Boltzmann law of radiation, knowing the power input, the total surface area, and the temperature. By using a solar simulator, as in method 3, the difference in electric power required to maintain a given temperature with the solar simulator on and off determines the solar absorptance. 

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