Radiation/Absorption temperature

The effect of radiation-source temperature on the low-temperature absorptivity of a number of additional materials is presented in Fig. 5-12. It will be noted that polished aluminum (cui ve 15) and anodized (surface-oxidized) aluminum (cui ve 13), representative of metals and nonmetals respectively, respond oppositely to a change in the temperature of the radiation source. The absorptance of surfaces for solar... 

The total emissivity of concrete a( 330 K is 0.89, whilst the total absorptivity of solar radiation (sun temperature --- 5500 K) at this temperature is 0.60. Use the data of Problem 9.31 for aluminium. 

There are numerous properties of materials which can be used as measures of composition, e.g. preferential adsorption of components (as in chromatography), absorption of electromagnetic waves (infra-red, ultra-violet, etc.), refractive index, pH, density, etc. In many cases, however, the property will not give a unique result if there are more than two components, e.g. there may be a number of different compositions of a particular ternary liquid mixture which will have the same refractive index or will exhibit the same infra-red radiation absorption characteristics. Other difficulties can make a particular physical property unsuitable as a measure of composition for a particular system, e.g. the dielectric constant cannot be used if water is present as the dielectric constant of water is very much greater than that of most other liquids. Instruments containing optical systems (e.g. refractometers) and/or electromechanical feedback systems (e.g. some infra-red analysers) can be sensitive to mechanical vibration. In cases where it is not practicable to measure composition directly, then indirect or inferential means of obtaining a measurement which itself is a function of composition may be employed (e.g. the use of boiling temperature in a distillation column as a measure of the liquid composition—see Section 7.3.1). 

Calculate the heat transferred by solar radiation on the flat concrete roof of a building, 8 m by 9 m, if the surface temperature of the roof is 330 K. What would be the effect of covering the roof with a highly reflecting surface, such as polished aluminium, separated from the concrete by an efficient layer of insulation The emissivity of concrete at 330 K is 0.89, whilst the total absorptivity of solar radiation (sun temperature = 5500 K) at this temperature is 0.60. 

The FDS5 pyrolysis model is used here to qualitatively illustrate the complexity associated with material property estimation. Each condensed-phase species (i.e., virgin wood, char, ash, etc.) must be characterized in terms of its bulk density, thermal properties (thermal conductivity and specific heat capacity, both of which are usually temperature-dependent), emissivity, and in-depth radiation absorption coefficient. Similarly, each condensed-phase reaction must be quantified through specification of its kinetic triplet (preexponential factor, activation energy, reaction order), heat of reaction, and the reactant/product species. For a simple charring material with temperature-invariant thermal properties that degrades by a single-step first order reaction, this amounts to -11 parameters that must be specified (two kinetic parameters, one heat of reaction, two thermal conductivities, two specific heat capacities, two emissivities, and two in-depth radiation absorption coefficients). 

If an electrode has an irregular surface, it is sometimes more profitable to detect absorbed radiation rather than transmitted or reflected radiation. Radiation absorption causes an increase in electrode temperature that can be detected directly with a thermocouple or thermistor (photother-mal spectroscopy61). 

Equation (11.29) is KirchhofPs law of radiation. The law states that the spectral emissivity for the emission of radiation at temperature T is equal to the spectral absorptivity for radiation from a blackbody at the same temperature T. The relation... 

If the effective temperature of our defined system is less than the universal radiation background temperature of 2.7 K, transitions between the two levels can be observed in absorption. This is the case with interstellar formaldehyde. Alternatively absorption can be observed against the continuum radiation from a nearby bright source. Spontaneous emission will always occur provided the upper of the two levels is populated, and can be observed if the populations are different. There are, in addition, examples of the exceptional situation in which N2 > N the result of this population inversion is that stimulated emission dominates, and maser emission is observed. Interstellar OH and SiO provide diatomic examples of this unusual situation, as also does interstellar H2O we shall describe the results for OH later in this chapter. Departures from local thermodynamic equilibrium are very common, and the concept of temperature in interstellar gas clouds is not simple this is a major part of astrophysics which is, however, beyond the scope of this book. 

I Reconsider Prob. 7-24. Using HHS (or other) software, investigate the effects of the train velocity and the rate of ab.sorplion of solar radiation on the equilibrium temperature of the top surface of the car. Let the train velocity vary from 10 ktn/h to 120 km/h and the nite of solar absorption from 100 V/in to 500 W/m". Plot the equilibrium temperature as functions of train velocity and. solar radiation absorption rale, and discuss the results. 

Unlike emissivity, the absorptivity of a material is practically independent of surface temperature. However, the absorptivity depends strongly on the temperature of the source at which the incident radiation is originating. This is also evident from Fig. 12 33, which shows the absorplivilies of various materials at room temperature as functions of the temperature of the radiation source. For example, the absorptivity of the concrete roof of a house is about 0.6 for solar radiation (source temperature 5780 K) and 0.9 for radiation originating from the surrounding trees and buildings (source leriiperaiurc 300 K), as illustrated in F ig. 12-34. 

Here, a (T, T2) is the hemispherical total absorptivity of body 1 for black radiation at temperature T2. This gives... 

Alessi et al. (2005) examined the y-irradiation and electron-beam processing of an epoxy-resin system in the presence of a photoinitiator. They showed that increasing the irradiation dose frequency and photoinitiator concentration greatly increases the temperature reached by the samples. The increase in temperature of the system during irradiation is related to the balance between the heat-evolution rate, due to both polymerization reaction and radiation absorption, and the rate of heat release towards the environment. High dose rates and high photoinitiator concentmtions increase the reaction rate and the difference between the heat produced and the heat released to the environment (Alessi et al. (2005). 

First of all, volcanic activity must be mentioned it introduces both gases (see Section 2.3 and Subsection 3.6.2) and particles into the atmosphere. The particles play an important temporary role in the control of atmospheric optical properties and radiation balance. Thus, after the eruption of Krakatoa in 1883 unusual darkness was observed over Batavia and the height of the volcanic cloud reached the altitude of nearly 30 km (18 miles). After the violent eruption of the Agung volcano in 1963 the optical effect of ash particles was identified at several points of the Earth and a temperature increase of 2 C was measured in the stratosphere (see Cadle, 1973)due to the radiation absorption of particles. While an important part of volcanic particulate matter consists of dispersed lava, sulfuric acid also was detected in volcanic fume (Cadle, 1973). 

Apart for exothermic curing reactions, another thermal effect has to be considered, due to the interaction of ionizing radiation with matter. The temperature profile depends on the balance among (on one hand) the rate of heat production, due both to curing reactions and radiation absorption, and (on the other hand) the heat released, in unit of time, by the system toward the environment. Taking constant the geometry of the reacting system, the heat released toward the environment is constant, while the heat production increases with the pulse frequency. 

The effect of radiation source temperature on low-temperature absorptivity for a number of representative materials is shown in Fig. 

In a thermocouple—used for measuring infrared radiation—a junction of two dissimilar metals is blackened to increase absorption of incident radiation. The temperature rise at the junction relative to a cold junction on which infrared radiation does not fall increases the potential across the junction this potential is amplified to a usable voltage. Thermocouples have a relatively slow response (thermal lag) and if the infrared radiation is time-varying, it must not vary too rapidly. 

---