Emissive hemispherical total

Integrating the emitted energy over both wavelength and direction and comparing with the similar integrated quantity for a blackbody yields hemispherical total emissivity. Hemispherical Total Emissivity... 

The emissive power M(T) combines the radiation flow emitted at all wavelengths and in the entire hemisphere (hemispherical total quantity). 

In this equation, is the hemispherical total emissivity, calculated according to (5.66). This gives... 

According to 5.3.2.1, the radiation properties of an opaque body are determined by its directional spectral emissivity e x = e x(A, f3,(p,T). In order to determine this material function experimentally numerous measurements are required, as the dependence on the wavelength, direction and temperature all have to be investigated. These extensive measurements have, so far, not been carried out for any substance. Measurements are frequently limited to the determination of the emissivity e x n normal to the surface (/ = 0), the emissivities for a few chosen wavelengths or only the hemispherical total emissivity e is measured. 

Since frequently only the emissivity (, or e x n normal to the surface are determined in radiation experiments, and because the hemispherical total emissivity e is required for radiative exchange calculations, the ratio e/e], is of interest. For... 

Fig. 5.35 Ratio e/e n of the hemispherical total emissivity s to the emissivity s n normal to the surface as a function of s n. Right hand line Dielectrics from Table 5.5, left hand line Metals...

Table 5.6 Hemispherical total emissivity e, ratio e/e and product reT as functions of the (directional) total emissivity s n normal to the surface, for metals, calculated according to the simplified electromagnetic theory (n = k)...

In order to obtain the hemispherical total emissivity e(T) from s, according to Table 5.4 has to be multiplied by M s and integrated over all wavelengths. This yields the numerical value equation (for re in Hem and T in K)... 

If the bodies participating in radiative exchange cannot be assumed to be black bodies, then the reflected radiation flows also have to be considered. In hollow enclosures, multiple reflection combined with partial absorption of the incident radiation takes place. A general solution for radiative exchange problems without simplifying assumptions is only possible in exceptional cases. If the boundary walls of the hollow enclosure are divided into isothermal zones, like in 5.5.2, then a relatively simple solution is obtained, if these zones behave like grey Lambert radiators. Each zone is characterised purely by its hemispherical total emissivity si — whilst at = is valid for its absorptivity, and for the reflectivity... 

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. 

The hemispherical total emissivity co2(T,Pco2sm) of C02 at p = 100kPa is illustrated in Fig. 5.74. cc>2 increases slightly with rising pressure. D. Vortmeyer [5.59] presents a particularly complex pressure correction factor, which can be neglected for pressures below around 200 kPa. 

Fig. 5.74 Hemispherical total emissivity co2 °f carbon dioxide at p = lbar as a function of temperature T with the product of the partial pressure pco2 and the mean beam length sm as a parameter. 1 bar = 100 kPa = 0.1 MPa...

A plate which allows radiation to pass through with a hemispherical total absorptivity a = 0.36, is irradiated equally from both sides, whilst air with = 30 °C flows over both surfaces. They assume a temperature = 75 °C at steady-state. The heat transfer coefficient between the plate and the air is a = 35W/m2K. Using a radiation detector it is ascertained that the plate releases a heat flux qstr = 4800 W/m2 from both sides. Calculate the irradiance E and the hemispherical total emissivity of the plate. 

A smooth, polished platinum surface emits radiation with an emissive power of M = 1.64 kW/m2. Using the simplified electromagnetic theory determine its temperature T, the hemispherical total emissivity s and the total emissivity s n in the direction of the surface normal. The specific electrical resistance of platinum may be calculated according... 

Table B 12 Emissivities of non-metallic surfaces. n Total emissivity in the direction of the surface normal, hemispherical total emissivity.

The hemispherical total emissivity represents an average of e e over all directional and wavelengths and is defined by ... 

This relation is known as Kirchhoff s law. Equation 7.27 may be substituted into the various relationships for the integrated emissivity or absorptivity. However, it does not follow that such quantities as directional total, hemispherical-spectral, or hemispherical total emissivity and absorptivity are necessarily equal. In fact, the integrated properties are only equal if certain restrictions are met. These are given in Table 7.1. 

K. Togano, 2003, Specific heat capacity and hemispherical total emissivity of liquid Si measured in electrostatic levitation , Appl. Phys. Lett. 83, 1122-1124. 

Emittanee and Absorptanee The ratio of the total radiating power of a real surface to that of a black surface at the same temperature is called the emittanee of the surface (for a perfectly plane surface, the emissivity), designated by . Subscripts X, 0, and n may be assigned to differentiate monochromatic, directional, and surface-normal values respectively from the total hemispherical value. If radi-... 

FIG. 5-13 Hemispherical and normal emissivities of metals and their ratio. Dashed lines monochromatic (spectral) values versus r/X. Solid lines total values versus rT To convert ohm-centimeter-kelvins to ohm-meter-kelvins, multiply hy 10"l... 

The spatial distribution of lead anthropogenic emissions in the Northern Hemisphere and particularly in Kazakhstan and Kyrgyzstan in 1990 is shown in Figure 15. As is seen the major emission sources were located in Europe. Some significant emissions were also in Eastern Asia and in North America. The total anthropogenic emission of lead in the Northern Hemisphere was about 146 kt/yr, the emissions of lead in Kazakhstan and Kyrgyzstan were 5.8 kt/yr and 0.7 kt/yr, respectively. 

Little snlfnr is re-emitted from wetlands into the atmosphere. Table 8.7 gives estimates of global emissions of volatile sulfur compounds from different sources. Total emissions are in the range 98 to 120 Tg (S) year 75 % is anthropogenic, mainly from fossil fnel combustion in the northern hemisphere. The main natural sources are the oceans and volcanoes. Wetlands and soils contribnte less than 3 % of the total emission. 

The global natural flux of sulfur compounds to the atmosphere has recently been estimated to be about 2.5 Tmol yr1 (1) which is comparable to the emissions of sulfur dioxide (SO2) from anthropogenic sources (2). A substantial amount of the natural sulfur contribution (0.5-1.2 Tmol yr1) is attributed to the emission of dimethylsulfide (DMS) from the world s oceans to the atmosphere (3.4). One of the major uncertainties in this estimate is due to a scarcity of DMS and other sulfur data from the Southern Hemisphere, particularly the Southern Ocean region between about 40°S and the Antarctic continent, which represents about one fifth of the total world ocean area. 

From the previous chapter, the blackbody total intensity is related to the blackbody total hemispherical emissive power by... 

---