Blackbody emissivity

It should be noted that this technique is not without some disadvantages. The blackbody emission background in the near IR limits the upper temperature of the sample to about 200°C [43]. Then there is the dependence of the Raman cross-section ( equation (B 1.3.16) and equation ( B1.3.20)-equation ( B 1.3.21)) which calls for an order of magnitude greater excitation intensity when exciting in the near-IR rather than in the visible to produce the same signal intensity [39]. 

The iaterpretation of the spectroscopy of SBSL is much less clear. At this writing, SBSL has been observed primarily ia aqueous fluids, and the spectra obtained are surprisiagly featureless. Some very interesting effects are observed when the gas contents of the bubble are changed (39,42). Furthermore, the spectra show practically no evidence of OH emissions, and when He and Ar bubbles are considered, continue to iacrease ia iatensity even iato the deep ultraviolet. These spectra are reminiscent of blackbody emission with temperatures considerably ia excess of 5000 K and lend some support to the concept of an imploding shock wave (41). Several other alternative explanations for SBSL have been presented, and there exists considerable theoretical activity ia this particular aspect of SBSL. 

An additional surface arrangement of importance is a single-zone surface enclosing gas. With the gas assumed gray, the simplest derivation of GSi is to note that the emission from surface Ai per unit of its blackbody emissive power is Ai i, of which the fractions g and (1 - G)ei are absorbed Dy the gas and the surface, respectively, and the surface-reflected residue always repeats this distribution. Therefore,... 

Figure 22. Schematic overlay of the most intense IR absorptions for neutral acetone and acetone-dg with a 300-K blackbody emission spectrum. The intensity axis refers to the blackbody radiation only.

Figure 24. Variation of ionic intensities as a function of time after isolation and thermalization in the FTICR cell of the externally generated cluster ion, (H20)4H at a pressure of CH4 of 2.0 x 10 torr in the FTICR cell and irradiation by (a) 300-K blackbody emission, and (b) 1.8 W cm" CW CO2 laser.

Another consideration in flames is radiatioiL The light that one sees in a flame is mostly fluorescence from the radiation of particular radical species formed in electronically excited states. (The blue color from CH4 flames is CH emission.) Gases also radiate blackbody radiation, primarily in the infiared. The glow from burning wood or coal is blackbody emission radiated from the surface. 

Outside the atmosphere, the solar flux approximates blackbody emission at 5770 K. However, light absorption or scattering by atmospheric constituents modifies the spectral distribution. The attenuation due to the presence of various naturally occurring atmospheric constituents is shown by the hatched areas in Fig. 3.12. 

Clearly, 254 K is much colder than the typical temperatures around 288 K (15°C) found at the earth s surface. This difference between the calculated effective temperature and the true surface temperature is dramatically illustrated in Fig. 14.4, which shows the spectra of infrared radiation from earth measured from the Nimbus 4 satellite in three different locations, North Africa, Greenland, and Antarctica (Hanel et al., 1972). Also shown by the dotted lines are the calculated emissions from blackbodies at various temperatures. Over North Africa (Fig. 14.3a), in the window between 850 and 950 cm-1, where C02, O-, HzO, and other gases are not absorbing significantly, the temperature corresponds to blackbody emission at 320 K due to the infrared emissions from hot soil and vegetation. 

The blackbody emission spectrum at right changes with temperature, as shown in the graph. Near 300 K, maximum emission occurs at infrared wavelengths. The outer region of the sun behaves like a blackbody with a temperature near 5 800 K, emitting mainly visible light. 

Wien displacement law Approximate formula for the wavelength, X, of maximum blackbody emission Xmax - T hc/5k — 2.878 X 10" 3 m K, where T is temperature in kelvins, h is Planck s constant, c is the speed of light, and k is Boltzmann s constant. Valid for T > 100 K. working electrode One at which the reaction of interest occurs. 

Using essentially the same method, but assuming blackbody emission (in disagreement with our calculated spectra), Branch (1987) obtained a distance of 55 5 kpc. This is another example of effects such as flux dilution and distortion produced by a scattering dominated photosphere (Wagoner 1981). 

A serious problem of using an Nd YAG laser to excite FT-Raman is the difficulty of attempting to study samples at temperatures > 150°C. The thermal blackbody emission from the sample becomes more intense (broad background) than the Raman signal. The S/N ratio is lowered, and the detector becomes saturated. 

For engineering practice, the spectral blackbody emissive flux qxb(T) at a surface is defined as... 

Figure 11.3 is a plot of the spectral blackbody emissive flux as a function of wavelength at various temperatures. From this figure, it is clear that at any given wavelength, the radiative energy emitted by a blackbody increases as the absolute temperature of the body increases. Each curve displays a peak, and the peaks shift toward smaller wavelengths as the temperature rises. The locus of the peaks calculated analytically by Wien s displacement rule is... 

The blackbody emissive flux qb(T) at an absolute temperature T is gained by integrating qxb(T) over all wavelengths,... 

Since the body is in radiative equilibrium, qx(T) also expresses the spectral radiative flux emitted by the body at the wavelength X. The incident radiation q (T) comes from the black walls of the enclosure at temperature T, and the emission by the walls is not influenced by the body regardless if it is a blackbody or not. Let qxb(T) be the spectral blackbody emissive flux at temperature T. Then,... 

The spectral emissivity e (T) of the body for radiation at temperature T is defined as the ratio of the spectral emissive flux qx(T) of the body to the spectral blackbody emissive flux q h(T) at the same temperature. Expressed mathematically,... 

Fig. 8-5 (a) Blackbody emissive power as a function of wavelength and temperature, (b) comparison of emissive power of ideal blackbodies and gray bodies with that of a real surface. 

With no windows at all, the heat transfer would have been the difference in blackbody emissive powers,... 

Blackbody Radiation Engineering calculations involving thermal radiation normally employ the hemispherical blackbody emissive power as the thermal driving force analogous to temperature in the cases of conduction and convection. A blackbody is a theoretical idealization for a perfect theoretical radiator i.e., it absorbs all incident radiation without reflection and emits isotropically. In practice, soot-covered surfaces sometimes approximate blackbody behavior. Let /.V, = /. A... 

Integration of Eq. (5-102) over all wavelengths yields the Stefan-Boltzman law for tne hemispherical blackbody emissive power... 

Heat capacity per unit mass, J kg 1-K 1 Shorthand notation for direct exchange area Area of enclosure or zone t, m2 Speed of light in vacuum, m/s Planck s first and second constants, W-m2 and m-K Particle diameter and radius, Jim Monochromatic, blackbody emissive power, W/(m2 lm)... 

Exponential integral of order n, where n = 1, 2,3,.. . Hemispherical emissive power, W/m2 Hemispherical blackbody emissive power, W/m2 Volumetric fraction of soot Blackbody fractional energy distribution Direct view factor from surface zone i to surface zonej Refractory augmented black view factor F-bar Total view factor from surface zone i to surface zonej Planck s constant, J s Heat-transfer coefficient, W/(m2 K)... 

Conversion of gray-gas total exchange areas GS and SS to their nongray form depends on the fact that the relation between radiative transfer and blackbody emissive power dT is linear and proportional. The gray-gas-equivalent emissivity is applicable only to the... 

---