Radiation bodies

Kirchhoff s law The relationship that exists between the absorptivity and emissiv-ity of radiating bodies. It is the capacity of a body to absorb radiation, w hich varies with the wavelength of the incident radiation and the angle of incidence. 

Strahl-keil, m. belemnite. -kies, m. marcasite. -kondensator, m. jet condenser, -korper, m. radiating body, radiator, -ptimpe, /. jet pump, injector, -punkt, m. radiating point, radiant point, -quarz, m. fibrous quartz, -schdrl, m. radiated tourmaline, -stein, m. actinolite amianthus. 

Figure 2.10 Examples of the intensity versus wavelength or frequency distribution of black-body radiation according to the Planck equation. Each distribution corresponds to a temperature T of the radiating body...

There is an upper limit for the emission of heat radiation, which only depends on the thermodynamic temperature T of the radiating body. The maximum heat flux from the surface of a radiating body is given by... 

We consider an element of the surface of a radiating body, that has a size of dA The energy flow (heat flow) d, emitted into the hemisphere above the surface element, is called radiative power or radiative flow, Fig. 5.2. Its Si-unit is the Watt. The radiative power divided by the size of the surface element... 

This is the defining equation for the fundamental material function Lx, the spectral intensity, it describes the directional and wavelength dependence of the energy radiated by a body and has the character of a distribution function. The (thermodynamic) temperature T in the argument of Lx points out that the spectral intensity depends on the temperature of the radiating body and its material properties, in particular on the nature of its surface. The adjective spectral and the index A show that the spectral intensity depends on the wavelength A and is a quantity per wavelength interval. The Si-units of Lx are W/(m2/um sr). The units pm and sr refer to the relationship with dA and dec. 

In radiative exchange calculations, it is preferable to use the model, described in the previous section, of a grey, diffuse radiating body as a simple approximation for the radiative behaviour of real bodies. As Lambert s cosine law is valid for this model, we denote these bodies as grey Lambert radiators. The energy radiated from them is distributed like that from a black body over the directions in... 

The differential form of the energy balance for a multicomponent mixture can be written In a variety of forms.1 6 It would contain terms reprenenting heat conduction and radiation, body forces, viscous dissipation, reversible work, kinetic energy, and the substantial derivative of the enthalpy of die mixture. Its formulation is beyond the scope of this chapter. Certain simplifled forms will be used in later chapters in problems such as simultaneous heal and mass transfer in air-water operations or thermal effects in gas absorbent. 

FUNDAMENTAL FACTS CONCERNING RADIATION. Radiation moves through space in straight lines, or beams, and only substances in sight of a radiating body can intercept radiation from that body. The fraction of the radiation falling on a body that is reflected is called the reflectivity p. The fraction that is absorbed is called the absorptivity a. The fraction that is transmitted is called the transmissivity... 

Determination of the time to ignition would be unreliable unless the transferred thermal energy can be clearly defined. Accurately controlled and measured thermal energy can be obtained from radiating bodies, so that radiant heat is applied as the igniting source for measuring the time to ignition. 

In the Flooring-Radiant-Panel-Test of ASTM D 648-1978, a horizontally placed specimen of 230 mm x 1050 mm is exposed to a radiating body oriented at 30 deg. to the horizontal and an igniting flame. The radiation intensity at the surface of the specimen is 1.0 to 0.1 W/cm. Both heat sources operate for the first ten minutes of the test procedure until the specimen has inflamed. If not, the test is continued for another ten minutes with the radiator only. The lowest radiation intensity necessary for inflaming the specimen is determined. 

We can explain the visible solar spectrum qualitatively by considering two characteristics of atmospheres (1) their absorption opacity r(v) depends upon frequency, and (2) their temperature varies with atmospheric depth, and one basic rule— that a radiating body emits its energy to space most efiiciently at wavelengths where the opacity is approximately unity. This rule is explained in terms of the competing effects of absorption and emission. In spectral regions where the atmosphere is transparent (t(v) 1), it neither emits nor absorbs efiiciently. In con-... 

In the calculation of heat flow from the radiant flame or heating element to the tube wall, a geometry factor must be given. The geometry factor is dependent on the shape and relative orientation of the radiating bodies and implies that each furnace is unique. 

The difficulty in applying this formula lies in the determination of the value of the coefficient Ku- The latter depends on the geometry of the radiating body, the temperatures Ti and T2, and the directional characteristics of the radiating surfaces, as well as their spectral emittance, absorptance, and reflectance (i.e., their color ). Obviously, the heat flux depends to a major extent on the empirical coefficient K, which at best can be given only approximately even when the geometry is known and the surface properties are deflned. Heat radiation increases sharply with temperature because the absolute temperature in the formula appears to the fourth power. 

According to Wien s law, the wavelength of maximum intensity in black-body radiation, is inversely proportional to the temperature of the radiating body. Mathematically,... 

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