Grey-body

Accuracy of Pyrometers Most of the temperature estimation methods for pyrometers assume that the objec t is either a grey body or has known emissivity values. The emissivity of the nonblack body depends on the internal state or the surface geometry of the objects. Also, the medium through which the therm radiation passes is not always transparent. These inherent uncertainties of the emissivity values make the accurate estimation of the temperature of the target objects difficult. Proper selection of the pyrometer and accurate emissivity values can provide a high level of accuracy. 

The intensity of a flare is largely determined by its temp, which in turn depends on the stability of the reaction products. In order to generate grey body radiation which encompasses the spectral sensitivity of the human eye (0.4— 0.74pm), 3000°K should be exceeded. Whereas this is possible using nitrates and perchlorates with alkaline earth metals as well as Zr, Ti and Hf (Ref 34) (H, C, B, Si and P form oxides which dissociate at high temps), in practice Mg and A1 are found to be best in terms of heat output, cost, and transparency to visible radiation... 

Figure 9.35. Comparison of black body, grey body and real surface radiation at 2000 K.<45 ...

In this way, ihe emissive power of a grey body is a constant proportion of the power-emitted by the black body, resulting in the curve shown in Figure 9.35 where, for example, e = 0.6. The assumption that the surface behaves as a grey body is valid for most engineering calculations if the value of emissivity is taken as that for the dominant temperature of the radiation. 

The absorptivity of a grey body is therefore less than unity. 

For a grey body, the emissivity and the absorptivity are, by definition, independent of temperature and hence equation 9.115 may be applied more generally showing that, where one radiation property (a, r or e) is specified for an opaque body, the other two may be obtained from equations 9.115 and 9.124. KirchofPs Law explains why a cavity with a small aperture approximates to a black body in that radiation entering is subjected to repeated internal absorption and reflection so that only a negligible amount of the incident radiation escapes through the aperture. In this way, a - e = 1 and, at T K, the emissive power of the aperture is aT4. 

What do you understand by the terms black body and grey body when applied to radiant heat transfer Two large, parallel plates with grey surfaces are situated 75 mm apart one has an emissivity of 0.8 and is at a... 

On the other hand, for molecules, the electronic transitions result in bands lO SOnm in width due to the changes in vibrational energy levels which also occur. A third type of radiation emitted by stars in the near-UV visible near-IR region is a continuum emission originating from hot particles e.g. hot AI2O3 particles) but this is considered to be grey body radiation and does not contribute to the colour of the star. 

For grey bodies, the emissivity, e, must be included and the net radiation per unit area is then... 

E Total energy emitted by grey body per unit area per unit time W/m2 MT 3... 

Many pyrot reactions take place by the formation of metallic oxides whose emissivity varies strongly with wavelength (Ref 9). As long as the emissivity is reasonably constant over the temp range investigated (ie, as long as the grey body... 

Fig. 2. Photographs of MCF-7 cells recovered from the enriched fraction after immunomagnetic sorting and stained with anti-HEA FITC as primary antibody anti-FITC phosphatase alkaline as secondary antibody and enzyme substrate (FastRed ). In this image, cells appear as medium-grey bodies. In specimens viewed in the laboratory, the cell bodies would appear stained red as opposed to grey. The dark, defined circles are pores in the membrane.

To calibrate the pixel sensitivities black body radiation is usually measured at different temperatures. Since a black body has an emissivity of 1 at every position, variations in detector pixel sensitivities are eliminated by a calibration function. As this IRT-method should be used here to quantify very small heat signals on combinatorial libraries with diverse materials, differences in emissivities have to be considered. Most materials are grey bodies with individual emissivities less than 1. Therefore, the calibration was not performed with a black body but with the library, as described before, a procedure that corrects not only for pixel sensitivity but also for emissivity differences across the library plate [5]. For additional temperature calibration, the IR-emission of the library is recorded at several temperatures in a narrow temperature window around the planned reaction temperature. By this procedure, emissivity changes, temperature dependence and individual sensitivities of the detector pixels can be calibrated in one step. After this... 

The energy transfer of photons through a grey body (i.e. one in which absorption by photons is finite, non-zero and independent of wavelength) is given by (Stacey, 1969, p. 248)... 

The absorptivity, a, is the ratio of the absorbed energy to the incident energy from the source. A perfect absorber is called a black body. Nonperfect absorbers, 1 > a > 0, are known as grey bodies. 

The emissivity, s, is the ratio of the emitted energy to that of a perfect emitter. A perfect emitter is also a perfect absorber, a black body. For grey bodies, e = a = constant is often assumed. Values of s for many materials are given in the literature, e.g. ref. 7. 

What do you understand by the terms black body and grey body when applied to radiant heat transfer ... 

The terms black body and grey body are discussed in Sections 9.5.2 and 9.5.3. 

The radiative component hrad can be estimated by treating the suspension as a grey body ... 

The most common radiation sources are thermal ones. In the NIR region quartz-halogen lamps are mainly used. These emit the radiation of a tungsten wire, which is a grey body at about 3000 K, the emissivity of which has been studied by De Vos, 1954, Rutgers and De Vos, 1954, 1973. 

For an independent of wavelength (grey body), the value of the integral in relation (21) is equal to 4 so that in this case one obtains... 

In the following sections we will look at the radiation properties of real bodies, which, with respect to the directional dependence and the spectral distribution of the radiated energy, are vastly different from the properties of the black body. In order to record these deviations the emissivity of a real radiator is defined. Kirch-hoff s law links the emissivity with the absorptivity and suggests the introduction of a semi-ideal radiator, the diffuse radiating grey body, that is frequently used as an approximation in radiative transfer calculations. In the treatment of the emissivities of real radiators we will use the results from the classical electromagnetic theory of radiation. In the last section the properties of transparent bodies, (e.g. glass) will be dealt with. 

The directional spectral emissivity is independent of the wavelength A sA = s x(f3,cp,T). A body with this property is called a grey body or a grey radiator. 

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