Showing papers on "Thermal radiation published in 1972"

Thermal conductivity of Earth materials at high temperatures

Abstract: The total thermal conductivity (lattice plus radiative) of several important earth materials is measured in the temperature range from 500 to 1900 K. A new technique is used in which a CO2 laser generates a low-frequency temperature wave at one face of a small disk-shaped sample, and an infrared detector views the opposite face to detect the phase of the emerging radiation. Phase data at several frequencies yield the simultaneous determination of the thermal diffusivity and the mean extinction coefficient of the material. The lattice, radiative, and total thermal conductivities are then calculated. Results for single-crystal and polycrystalline forsterite-rich olivines and an enstatite indicate that, even in relatively pure large-grained material, the radiative conductivity does not increase rapidly with temperature. The predicted maximum total thermal conductivity at a depth of 400 km in an olivine mantle is 0.020 cal/cm/sec/deg C, which is less than twice the surface value.

Thermal properties of granulated materials.

Abstract: Review of the thermophysical properties of granular materials or silicates believed to simulate the lunar surface layer. Emphasis is placed on thermal conductivity data and the effects of material and environmental variables on the thermal conductivity. There are three basic mechanisms of heat transfer in particulate materials: conduction by the gas contained in the void spaces between the particles; conduction within the solid particles and across the interparticle contacts; and thermal radiation within the particles, across the void spaces between particle surfaces, and between void spaces themselves. Gas and solid conduction, thermal radiation, and the interaction between conduction and radiation are considered.

Thermal history of the moon.

Abstract: The thermal history of the lunar interior has been investigated for many sets of parameters and initial conditions by the construction of mathematical models. These models have been extended to include the effects of melting and redistribution of radioactive heat sources with time. The models considered include the possibility of heat transfer by lattice conduction, radiative transfer, removal of radioactive heat sources and, in a molten zone, fluid convection. The energy sources are divided into initial temperature sources that operate during the formation of the moon or shortly thereafter, and long-lived radioactive heat sources.

Infrared Radiative Energy Transfer in Gases

Abstract: Publisher Summary This chapter illustrates the incorporation of spectroscopic information into the radiative transfer equations and presents a reasonable means of treating radiative energy transfer within gases. Specific restriction is made to infrared gaseous radiation that results from molecular transitions involving both vibrational and rotational energies, and emphasis is placed upon the application of the molecular band models to radiative transfer analyses. The chapter also reviews the infrared band spectra, introduces simple band models, and discusses the formulation of total band absorptance information with the aid of these models. The basic equations describing the radiative transfer within an infrared absorbing–emitting gas are formulated and these allow for the radiatively induced departures from local thermodynamic equilibrium. The chapter also presents some illustrative radiative transfer analyses, with emphasis upon physical interpretations and the relative importance of thermal radiation versus molecular conduction as energy transport mechanisms.

Method of simultaneous measurement of radiative and lattice thermal conductivity.

Abstract: A new technique of high-temperature thermal-conductivity measurement is described. A CO2 gas laser is used to generate a low-frequency temperature wave at one face of a small disk-shaped sample, and an infrared detector views the opposite face to detect the phase of the emerging radiation. A mathematical expression is derived which enables phase data at several frequencies to be used for the simultaneous determination of thermal diffusivity and mean extinction coefficient. Lattice and radiative thermal conductivities are then calculated. Test results for sintered aluminum oxide at temperatures from 530 to 1924 K are within the range of error of previously existing data.

The Population of High Atomic Levels at Low Temperatures

Abstract: Calculated coefficients relating the population in level n to its value in equilibrium are presented for various atomic levels under conditions of low temperature, density, and thermal radiation fields that may be expected in the interstellar medium. A thermal continuum such as that produced by an H II region is found to substantially change both the departure coefficients and their slope under conditions of low density.

An experimental and analytical study of natural convection with appreciable thermal radiation effects

Abstract: An experimental and theoretical investigation has been carried out to determine the effect of thermal radiation on a natural convection boundary layer formed adjacent to a vertical flat surface with uniform heat flux input. In the experiment, the gases air, argon and ammonia were used as the fluid medium, thus permitting the observation of radiation effects in non-abosrbing and absorbing media. Experimental results were obtained for three different wall emittances at ambient pressures ranging from 2 to 8 atmospheres in air and argon and from 2 to 7 atmospheres in ammonia. An interferometer was used to measure the temperature distributions in the boundary layer and to evaluate the conductive (convective) heat flux from the surface into the fluid medium. The experimental temperature distributions and heat-transfer results obtained in ammonia gas are compared to the predictions of a perturbation analysis developed by the present writers. General agreement between theory and experiment is found. The presence of a radiating gas is seen to increase the convective heat transfer by as much as 40 % for the conditions of the present experiments. The results further indicate that the temperature distributions and wall-temperature gradients are strongly affected by both variations in the surface emittance and variations in gaseous emission and absorption. For non-absorbing gases, the experimental results are found to be in general agreement with existing theory. It is also shown that the experimental temperature distributions agree very well with theoretical predictions obtained by treating the convection and radiation processes as independent and superimposed.

Heating of the solar flare plasma by high energy electrons

Abstract: Heating of the ambient plasma by high energy electrons in solar flares is discussed. It is shown that for large flares the heating is enough to produce a thermal plasma of a temperature up to a few times of 107K rapidly in the initial phase of the flares. Thus thermal bremsstrahlung in addition to non-thermal bremsstrahlung should be considered for the X-ray emission of solar flares in the initial phase.

Closed Time as an Explanation of the Black Body Background Radiation

Abstract: 3 K isotropic black body radiation is explained in terms of the accumulated starlight from a subsequent time reversed cycle of the universe, thermalized in the adjoining dense state.

Apollo 12 thermal radiation properties

Abstract: The spectral and total thermal radiation properties as a function of bulk density are presented for lunar fines from the Apollo 12 mission. The total emittance is presented as a function of temperature from 90 to 400°K and the solar reflectance (albedo) for near normal incidence.

Mechanism for the X-ray Pulsations in Cyg X-1

Abstract: The mechanism proposed involves the occurrence of a flare in a region of high magnetic field on the surface of a star. The flare produces oscillations in the magnetic field structure. The oscillations lead to heating of the plasma in a flux tube which emits thermal radiation. The heating may be caused by magnetic pumping or particles accelerated in periodic bursts by the flare. Synchrotron radiation from high energy particles accounts for the hard X-ray emission.

Guide and Data for Greenhouse Air Conditioning

Abstract: Thermal radiation cooling during the night is one of the most significant phenomena for the greenhouse and the long wave net radiation exchange between the cold sky and the greenhouse surface has been analysed under the assumption that there is no significant temperature difference betwen the greenhouse surface and the surrounding ground surface.In order tlo calculate the air-conditioning load, especially the heating load of the greenhouse, it is indispensable to analyse the radiative heat transfer mechanism of the whole system which includes the green house and to calculate the amount of the heat flow of each section involved. The diagrammatic representation each radiation exhange path is given in Fig. 3.In the present study, the theoretical analysis is expanded to the thermal radiation cooling of the heated greenhouse as well as experimental verification. The view factor to the sky of the finite glass plane whose inclination angle is α is expressed as (1+cosα)/2 under the assumption that the long wave radiation from the sky is uniform and this factor is introduced as one of the important factor of the heat loss from the unheated greenhouse (Takakura 1967 a). Using this relationship, the ratio of the net radiation between the greenhouse surface and the sky to that between the sky and the horizontal surface whose area is the same as the greenhouse floor is expressed as (1+β)/2, where β is the ratio of the greenhouse floor area to the greenhouse surface area (Tachibana 1971).The net radiation between the greenhouse surface and the sky is a function of the water vapor pressure in the air and the angular height of the net radiation intensity. Therfore, the numerical integration of Kondrat'yev's equation has been conducted and one of the results, which is the curve for 6mmHg is given in Fig. 2. The deviation from this curve due to the variation of the water vapor pressure is given as vertical deviation bands at three inclination angles in the same figure instead of giving the family of curves. This curve is expressed by the function (1+cosα-0.23 sin2α)/2 under 2% error.Using this function, the ratio of the net radiation loss from the greenhouse surface to the net radiation of the horizontal surface is expressed as (0.77+β+0.23β2)+γ(1-β)/2, where γ is the ratio of the net radiation between the greenhouse wall and the surrounding ground surface to the net radiation between the greenhouse wall and the cold sky. Suppose that there is no temperature difference between the greenhouse wall and the surrounding ground surface, γ is equal to zero. On the other hand, if there is a significant temperature difference between them, e. g., 10 deg C, γ is approximately 0.5 and the value is increased 60% more than the value at γ is zero in the smaller range of β. The results are given in Fig. 4. Therefore, it is concluded that the thermal radiation cooling of the heated greenhouse should be calculated by Eqs. 13 and 14.The calculation results are verified experimentally.

On the Validity of Kirchhoff's Law in a Nonequilibrium Environment

Abstract: In the heat radiation literature, the words absorptivity and emissivity have both been used in different senses. This can lead to confusion, and in one case it was argued that Kirchhoffs law of equality between emissivity and absorptivity does not apply to the radiation from a material for which the stimulated emission depends on the temperature of the environment. We clarify the proper thermodynamic definition of absorptivity (and emissivity) and show that Kirchhoff's law stated in terms of those definitions correctly predicts the results of experiments. This follows even when there are large deviations from equilibrium, or when there is nonlinear absorption or emission.

The Thermal Radiation Spectra of Supermassive Stars and X-Ray Sources

Abstract: In the external layers of supermassive stars and thermal sources of X-ray radiation electron scattering contributes more to the opacity than free-free process. Therefore, the thermal radiation spectrum of supermassive stars must greatly differ from Planckian. The approximate formulae obtained for the radiation spectra are applicable to any objects with a predominant role of electron scattering in the opacity and power dependence of plasma temperature and density on geometrical depth of the layers.

A mathematical simulation of radiant heat transfer in a cylindrical furnace

Abstract: The radiative heat transfer in a cylindrical furnace was investigated both experimentally and theoretically. The flow pattern within the combustion chamber was measured and found to differ from that of an identical cold model for which extensive theoretical and experimental work had been previously carried out. The Monte Carlo Method was used to solve for the radiative transfer part of the non-linear integro-differential equations describing the combustion and heat transfer in a cylindrical furnace. The radiative heat flux distribution obtained from the simulation agreed well with data obtained from a cylindrical test furnace.

Thermal Imaging with Pyroelectric Television Tubes

Abstract: Publisher Summary This chapter describes thermal imaging with pyroelectric television tubes. In operation of the pyroelectric vidicon tube, the incident thermal radiation is modulated by a chopper and gives rise to a variation of the target temperature, which is transformed by the pyroelectric effect into an electric-charge signal. This is sensed by the scanning electron beam, which thus provides the video-signal current. Unlike the photonsensitive and other bolometric detectors of infrared radiation, which give signals proportional to the scene temperature, the pyroelectric target produces a charge signal that is proportional to its change of temperature between scans, and this is proportional to the small-scene temperature increments (∆T) at ambient temperature. The signal-to-noise ratio is enhanced or correspondingly, the noise-equivalent temperature is reduced, by the use of te low bandwidth. The fall off in performance with the increasing spatial frequency is described by the overall modulation-transfer function. This chapter relates detailed calculations on the possible modes of operation to the choice of pyroelectric material. The performance recently achieved in such tubes is also reported.

Effect of External Radiation on the Burning Rates of Solid Propellants

Abstract: A means for improving the performance of a solid fuel pulsed plasma thruster has been described and analyzed. The key to improvement is control over the mass consumed per discharge. Figures 3 and 4 show that the mass consumption can be chosen by adjusting the distance between the two side-fed Teflon fuel bars. For high mass consumption, the thruster operates in the gas-dynamic regime and provides high thrust. At low mass consumption, thruster operation is primarily electromagnetic with high specific impulse. Efficiency increases as the thrust mechanism is predominately in one regime or the other. By using side feed, the performance has been significantly increased over the conventional rear-feed design. Carbonization of the Teflon side walls remains the one stumbling block to the full utilization of this design to flight thrusters. Geometry changes may reduce or eliminate this problem. References

Thermal radiation in finite cavities

Abstract: The mode density D(v) of the electromagnetic resonances of a lossless closed cavity plays a dominant role in the theory of far-infrared radiation Standards, the Einstein coefficients of spontaneous and stimulated emission, and radiation corrections as e. g. the Lamb-shift. Refinements of the Planck-Weyl asymptotic density D0(v) for cavities of finite size are studied by computational methods involving the first IO6 eigenvalues. Averaging over the modes for a large bandwidth Av, second order corrections of the type D/Do = 1-Cr2 F"2/3 with V = volume of the cavity are obtained for a variety of cavity geometries including the parallelepiped, the circular cylinder, the sphere, the hemisphere,and cylindrical as well as spherical sectors (wedges and cones). The relation of the constantC to the edge lengths and curvatures of the cavity is investigated. The secondorder corrections resulting for the Wien displacementlaw and for the Lamb-shift are determined. The implications for the Stefan-Boltzmann radiation law as well as for the thermodynamical properties of the radiation field are studied. A rigorous connectionbetween the asymptotic spectral and total radiation formulae is established in terms of Abelian and Tauberian theorems. For narrow bandwitdh Av, fluctuations of the mode density are much stronger than the • second order average correction. The relative mean fluctuation is found to be proportional to v-HAv)-1. A new proof for the wellknown vanishing of the average first order or surface correction term is given in terms of the scalar Eand H-type wave potentials. Surface terms appear only if the JEand H-type densities DE and DH are determined separately. The surface corrections of DE and DH are equal, but have opposite signs. The correspondingfirst order refinementof the temporal autocorrelation functions of the black-body radiation field is studied. «Es ist wohl nicht überflüssig,hier zu bemerken, dass es bis jetzt noch nicht gelungen ist, eine exakte Definition eines .vollkommen schwar¬ zen' Körpers, welche sich auf beliebig lange Wellen anwendenlässt, aufzufinden.» Max Planck 1898 [136]

Approximate temperature distribution for a diffuse, highly reflecting material.

Abstract: The application of a 'two-flux' technique is considered for determining the reflective characteristics and approximate temperature distribution in highly backscattering materials characterized by a very small absorption coefficient. With respect to the transmitted internal radiation fields, the internal radiative emission is assumed to be negligible. A comparison of theory and experiment is presented.

The 1–55 Å X-ray emission from an active limb prominence

Abstract: The solar X-ray emission spectrum between 1 and 55 A has been studied for one solar event, using the two X-ray experiments on-board of the ESRO 2 satellite. Although this event fits the description of the class of X-ray events described as gradual rise and fall events there is evidence to show that it is caused by the superposition of a number of events, of which the largest one was correlated with the appearance of an active loop-like prominence on the East limb of the Sun. By assuming the radiation processes in the source region to be thermal, temperatures and emission measures against time for the source region have been derived using the most recent models for free-free continuum and line emission due to thermal plasmas. The temperatures derived have been used to calculate cooling rates due to radiation losses. It is shown from data in the 2.2–3.0 A band that the cooling rate of the excited corona associated with the prominence is compatible with an electron density of 1 × 1011 cm−3 if a single injection of particles is considered to occur in the corona. From data in the 5.8–8.8 A band one finds a value of 2 × 1010 cm−3. An estimate of the total injected energy is made.

Thermal-Radiation Properties of Thin Metallic Films on Dielectrics

Abstract: The spectral radiation properties of thin metallic films are obtained through the numerical solution of the governing integrodifferential equation for the anomalous skin effect with size effects included. The theory predicts that at low temperatures there will be a film thickness for which the emittance is a minimum. This minimum, which is lower than the emittance for a bulk specimen, is apparently due to interference effects arising from the finite film thickness. It is also found that for certain cases transmission through a film is not negligible compared with the emission from the film. The analysis is extended to include metal-coated dielectric films, and the theoretical results are compared with existing experimental data.

Comparison of the radiation flux profiles and spectral detail from three detailed nongray radiation models at conditions representative of hypervelocity earth entry

Abstract: The radiation models were compared on the basis of the approaches used for the transport calculations and absorption coefficients and of results obtained for the radiation flux profiles and spectral distributions. The calculated results were for shock layer conditions representative of manned earth reentry from an interplanetary mission. The three models are RATRAP, RADICAL, and MDAC. The results show that significant differences exist in the radiation flux computed by the three models. The RADICAL model was found to depend on fewer approximations, to include more detail, and to require less computer time than the other models.

Thermal Radiation from a Microscopically Roughened Dielectric Surface

Abstract: The emission of thermal radiation from a microscopically roughened dielectric surface is treated using a laminar-inhomogeneous layered model for a representation of the effective spatial variation in refractive index associated with a roughened surface layer. The Riccati equation is used to calculate the modification to the spectral angular transmittance of the surface due to the presence of this inhomogeneous surface layer. A consideration of the emission of radiant energy from the bulk dielectric through the surface layer permits the angular emittance to be determined. Total emittance values are obtained using the spectral angular emittance data.

Problems and Design of Black-Body References

Abstract: This contribution covers the following problems involved in the calibration of farinfrared telescopes: (i) thermal radiation in finite resonators and the long-wave corrections of the Planck and Stefan-Boltzmann radiation formulae; (ii) the search for high-temperature black-body wall materials among the refractory oxides; and (iii) a black body for the absolute calibration of a balloon-borne far-infrared lamellar grating interferometer for solar measurements.

An analysis of the radiation field beneath a bank of tubular quartz lamps

Abstract: Equations governing the incident heat flux distribution beneath a lamp-reflector system were developed. Analysis of a particular radiant heating facility showed good agreement between theory and experiment when a lamp power loss correction was used. In addition, the theory was employed to estimate thermal disruption in the radiation field caused by a protruding probe.