Radiation Between Solids

Radiant-absorption Rate. Radiation between solid surfaces is dependent upon the fourth power of the temperature difference and upon a constant, the value of which is dependent on the kind of material and the condition of the surface. Radiation from a flame, as in pipestill or boiler furnaces, is governed by the same laws except that the size of the flame... 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

Hlavacek (1970) has shown that radiation between the solid catalyst and gas can significantly affect the temperature dynamics in packed bed systems operating in excess of 673 K. Since most packed bed systems usually operate well below these conditions, radiation terms are not explicitly included in the model. However, their effect can to some degree be accounted for in the overall heat transfer coefficients.4... 

No heat exchange by radiation between gases and solid ... 

The preceding analysis is based on Tien s model of heat transfer by a gas-solid suspension in turbulent pipe flow [Tien, 1961]. However, nonuniform distribution of solids, slip between solids and gas, and effect of thermal radiation were excluded in Tien s work. 

The optical attenuation resulting from the solid was more clearly shown by monitoring some runs with radiation between 3000-4200 A (principally 3660 A), where the O3 absorption is unimportant. Some results are shown in Figure 3. Initially the optical attenuation increases to a maximum as the aerosol is formed and then drops as the aerosol deposits... 

Thermal equilibrium between solid and gas locally. Calculations including energy transferred between char and gas by radiation and convection support this statement. The equations are illustrated in the section Temperature differences between the gas and solid phases ... 

Due to the Langrangian formulation applied to the solid phase, the use of an effective thermal conductivity as usually applied to porous media is not necessary. In a packed bed heat is transported between solid particles by radiation and conduction. For materials with low thermal conductivity, such as wood, conduction contributes only to a minor extent to the overall heat transport. Furthermore, heat transfer due to convection between the primary air flow through the porous bed and the solid has to be taken into account. Heal transfer due to radiation and conduction between the particles is modelled by the exchange of heat between a particle and its neighbours. The definition of the neighbours depends on the assembly of the particles on the flow field mesh. 

Photographic emulsions and photoelectric detection devices can be used as detectors for electromagnetic radiation between 150 and 800 nm. Among the photoelectric devices, photomultipliers are the most important but new solid state devices have become a useful alternative. 

The convection coefficient can be predicted from the data on heat transfer between solids and fluids flowing in packed beds. Such data were given in Fig. 10-2. The radiation and conduction coefficients, and hp, depend on the value of (AT) defined by Eq. (13-34). A derivation based on the same assumptions employed in obtaining Eq. (13-33) leads to the results... 

Fourier s law states that k is independent of the temperature gradient but not necessarily of temperature itself. Experiment does confirm the independence of k for a wide range of temperature gradients, except for porous solids, where radiation between particles, which does not follow a linear temperature law, becomes an important part of the total heat flow. On the other hand, fc is a function of temperature, but not a strong one. For small ranges of temperature, k may be considered constant. For larger temperature ranges, the thermal conductivity can usually be approximated by an equation of the form... 

The complex subject of thermal radiation transfer has received much study in recent years and is covered in a number of texts. The following introductory treatment discusses the following topics emission of radiation, absorption by opaque solids, radiation between surfaces, radiation to and from semitransparent materials, and combined heat transfer by conduction-convection and radiation. 

Figure 16.2 Various interactions between radiation and solids. The monochromatic ray with frequency v and intensity Iq can be transmitted (with the same frequency but a reduced intensity), scattered (Rayleigh and/or Raman), absorbed, or reflected.

To convert the radiant heat transfer coefficient to a contribution to the effective bed conductivity, the particle diameter must be included, because this affects the path length for radiant energy transfer. Radiation between particles and conduction through the solid are accounted for in the model of Schotte [32] ... 

This method was applied to produce carbides of titanium, zirconium, lead, and bismuth (Barcicki Myrdzik, 1974). Applications ofthe method are limited becanse of insufficient contact between solid particles in the presence of plasma flow if the solid mixtnre is not bricked or sintered. The efficiency of the process arranged in bricks or sintered solid mixture is limited by radiation heat losses and insufficient heat transfer inside of the brick, especially taking into account the decrease of density dne to CO formation (7-92). The most effective condensed-phase synthesis of carbides (7-92) is that from melt containing carbon compounds (Tumanov, 1981). A relevant example is the synthesis of carbides of uraninm and plutonium from a melt containing their nitrites and carbon compoimds (Coppinger Johnson, 1969). 

Activated phosphors are used in fluorescent lights, older TV screens, and other light sources because they emit light when they are hit with radiation. They are often made of a metal oxide with trace amounts of another substance, or dopant. They appear to exist on the border between solid solutions and unique chemical substances. When the EPA was compiling the initial Inventory, it accepted some submittals for activated phosphors, but rejected others as mixtures that cannot be listed on the Inventory. The EPA also issued informal guidance letters concluding that activated phosphors are solid solutions. However it later concluded that activated phosphors cannot be manufactured without chemical reactions and therefore they are unique substances and are not solid solutions. Emthermore, the EPA pointed out that the ratio of starting materials is closely controlled and that supports the conclusion that activated phosphors are substances with chemical formulas. 

Spectroscopic techniques that employ electromagnetic radiation between vacuum UV energy ( 10-40 eV, 125-31 nm) and soft X-rays (40-1500 eV, 31-0.8 nm) and going on to hard X-rays (1500-105 eV, 0.8-0.01 nm) have also been of great importance in elucidating the electronic structure of complex materials [34]. It is well known that an electromagnetic wave can be scattered and/or absorbed when interacted with the surface of material. In the case of absorption, the X-ray beam intensity (/) is attenuated by the solid... 

Luminosity is generated by the cracking of fossil fuels into micron-sized solids and gaseous hydrocarbon compounds. The heaviest of those compounds, perhaps with some solid carbon, is called soot. When the soot particles become very hot and begin to burn, they radiate like other solids. Since solids radiate in all wavelengths and follow the rules of heat transfer between solids, luminous flames transfer more heat... 

Heat is involved in most real-life processes. This permits heat—into or out of a system—to serve as a universal detector. In many cases, the heat into or out of a system can be measured nondestruc-tively. Heat transfer occurs in three ways conduction, convection, and radiation. Conduction occurs between solid materials when placed in contact with each other. Convection occurs when a hot material and a cold material are separated by a fluid (gas or liquid). Radiative heat transfer involves the emission and consequent absorption of electromagnetic radiation between a hot and cold material. 

Before we can derive a general relationship for the view factor between two finite bodies we must consider and discuss two quantities, a solid angle and the intensity of radiation. A solid angle cu is a dimensionless quantity which is a measure of an angle in solid geometry. In Fig. 4.11-3a the differential solid angle dm, is equal to the normal projection of d/lj divided by the square of the distance between the point P and area dAj. 

Electrons and ion beams, as well as photons with different energies, are usually employed as probe in characterization techniques for the study of porous silicon (PS) properties. The interaction between the different beams and PS is also present in a variety of sensing devices and filters. These devices usually exploit the energy exchange between the incident beam and the porous silicon. As a result, some structural or electronic changes occur, which may be reversible, metastable, or irreversible. So it is important to know these effects in order to properly design the devices and also to properly interpret the results of the analysis techniques. A general textbook on the effects of radiation in solids can be found in reference (Sickafus et al. 2007). 

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