Thermal radiation emissivity

During the selection of the proper materials to be used for protection, several characteristics of each material would be rated to help narrow the decision. The minimum characteristics that should be evaluated for these materials are ability to withstand environmental and plant produced radiation, coefficient of thermal expansion, density, electrical resistivity and conductance to control electrostatic discharge, material chemistry ar d composition, operational temperature range, resilience, specific heat, strength, stiffness, thermal conductivity, thermal radiation absorptivity, thermal radiation emissivity, the ability to fasten the material to the support structure and/or the components themselves, and the compatibility between the protecting material and material to which it would be fastened... 

In order to compute the thermal radiation effects produced by a burning vapor cloud, it is necessary to know the flame s temperature, size, and dynamics during its propagation through the cloud. Thermal radiation intercepted by an object in the vicinity is determined by the emissive power of the flame (determined by the flame temperature), the flame s emissivity, the view factor, and an atmospheric-attenuation factor. The fundamentals of heat-radiation modeling are described in Section 3.5. 

The fundamentals of thermal radiation modeling are treated in Chapter 3. The value for emissive power can be computed from flame temperature and emissivity. Emissivity is primarily determined by the presence of nonluminous soot within the flame. The only value for flash-fire emissive power ever published in the open literature is that observed in the Maplin Sands experiments reported by Blackmore... 

Four parameters often used to determine a fireball s thermal-radiation hazard are the mass of fuel involved and the fireball s diameter, duration, and thermal-emissive power. Radiation hazards can then be calculated from empirical relations. For detailed calculations, additional information is required, including a knowledge of the change in the fireball s diameter with time, its vertical rise, and variations in the fireball s emissive power over its lifetime. Experiments have been performed, mostly on a small scale, to investigate these parameters. The relationships obtained for each of these parameters through experimental investigation are presented in later sections of this chapter. 

Thermal radiation takes place by the emission of electromagnetic waves, at the velocity of light, from all bodies at temperatures above absolute zero. The heat flux from an... 

Thermal radiation becomes important at higher temperatures, especially above 2000°F, when thermal destruction of the monolith substrate probably takes place. Thermal radiation intensities are proportional to the emissivity of the surface multiplied by the absolute temperature raised to the fourth power. The thermal emissivity of the monolith may be close to 1.0 due to the blackened surfaces from deposition of platinum. Each point of the channel is completely visible from any other point of the channel. The... 

The illuminating characteristic of the flare is only partly determined by the thermal radiation from the oxide particles, a second factor being the spectral emission from excited metals. 

Heat energy supplied by the emission of rays. Thermal radiation travels at the speed of light (186,000 miles per second). 

We still need to consider the coherence properties of astronomical sources. The vast majority of sources in the optical spectral regime are thermal radiators. Here, the emission processes are uncorrelated at the atomic level, and the source can be assumed incoherent, i. e., J12 = A /tt T(ri) (r2 — ri), where ()(r) denotes the Dirac distribution. In short, the general source can be decomposed into a set of incoherent point sources, each of which produces a fringe pattern in the Young s interferometer, weighted by its intensity, and shifted to a position according to its position in the sky. Since the sources are incoherent. 

Emissive power E Total thermal radiation energy emitted by a surface per unit time per unit surface area The three terms, Emissive power (E),... 

Emission spectrometry Interaction thermal- radiation energy -species Emitted radiation quanta, hv Photo multiplier tubes, photoplate Spectrum ... 

IR emission spectroscopy makes use of the reciprocal effect of IR absorption spectroscopy. At temperatures above 0 °K, molecules undergo a number of vibrational, vibrational-rotational or purely rotational movements. The relaxation of these excited states leads to the emission of thermal radiation, primarily in the IR region. 

Thermal radiation emitted by an object can be continuous, discontinuous or, in most cases, a mixture. A continuous radiation profile corresponds to an ideal black body, where only the temperature of the emitting object determines the emission profile. Discontinuous thermal emission spectra are caused by photons emitted during the relaxation of excited vibrational states. Since vibrational states are quantised, this results in emission bands at the wavelengths of the corresponding IR absorption bands. 

Since the earth has temperature, it emits radiant energy called thermal radiation or planetary infrared radiation. Measurements by satellites show an average radiant emission from the earth of about 240 watts per square meter. This is equivalent to the radiation that a black body would emit if its temperature is at -19°C (-3°F). This is also the same energy rate as the solar constant averaged over the earth s surface minus the 30% reflected radiation. This shows that the amount of radiation emitted by the earth is closely balanced by the amount of solar energy absorbed and since the earth is in this state of balance, its temperature will change relatively slowly from year to year. 

An important featnre of the thermal radiation field, described by its energy density Pv, is that it is isotropic that is, the emission is the same in all directions. 

When a radiation source is placed inside a closed cavity, its radiation energy is distributed among all of the modes following Equations (2.1) and (2.2), once the system has reached equilibrium. As we have seen in Example 2.1, in spite of the large number of modes in such a closed cavity, the mean number of photons per mode corresponding to the optical region is very small. Specifically, it is very small compared to unity. This is the ultimate reason why, in thermal radiation fields, the spontaneous emission per mode by far exceeds the stimulated emission. (Remember that the stimulated emission process requires the presence of photons to induce the transition, opposite to the case of the spontaneous emission process.)... 

At temperatures above absolute zero, particles can emit as well as absorb and scatter electromagnetic radiation. Emission does not strictly fall within the bounds imposed in the first chapter it is more akin to such phenomena as luminescence than to elastic scattering. However, because of the relation between emission and absorption, and because emission can be an important cooling mechanism for particles, it seems appropriate to discuss, at least briefly, thermal emission by a sphere. 

The total emission of radiant energy from a black body takes place at a rate expressed by the Stefan-Boltzmann (fourth-power) lav/ while its spectral energy distribution is described by Wien slaws, ormore accurately by Planck s equation, as well as by a n umber of oilier empirical laws and formulas, See also Thermal Radiation,... 

As shown in the previous chapter, the spontaneous decay rate of the n state to the lower lying n t state is given by the Einstein A coefficient nf. nf-7 In the presence of thermal radiation the stimulated emission rate Kn,fnt, is simply n times as large as the spontaneous rate. Explicitly,... 

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