Showing papers on "Thermal radiation published in 1968"

Radiation Heat Transfer

Abstract: Thermal radiation is the dominant mode of heat transfer in flames with characteristic lengths exceeding approximately 0.2 m. It is for this reason that quantitative analysis of fire dynamics requires a working knowledge of thermal radiation. This chapter will introduce the fundamentals of thermal radiation and offer several methods for calculating radiant heat transfer in fires. Basic thermal radiation concepts are presented with an emphasis on application to fire phenomena; the reader is referred the literature for specialized topics [1–4].

Radiative heat transfer from a cylindrical cloud of particles.

Abstract: In this work the Monte Carlo method is applied to investigate thermal radiation to the exterior base region of a finite cylindrical dispersion of absorbing, emitting, and anisotropically scattering particles. The analysis assumes a scattering function characteristic of aluminum oxide dispersions. Searchlight effect, chamber emitted radiation scattered by the particles to the surroundings, is introduced by permitting the radiation to originate from a black circular surface at the base of the cloud, as well as from within the cloud. The results obtained show that anisotropic scattering and searchlight effect play important roles in heat transfer in this geometry.

Visible and near-infrared emittance of ablation chars and carbon.

Abstract: Initial results of an experimental study of the visible and near-infrared spectral reflectance and emittance of ablation chars, carbon, and graphite at temperatures ranging from 2200° to 3500° K are presented Reflectance measurements were made with a double-ellipso idalmirror reflectometer that utilizes a carbon arc to heat and to irradiate the materials The ablation materials, from which the chars were formed by thermal degradation in atmosphericentry-simulation facilities, included high-density and low-density phenolic-nylon, carbonfiber-reinforced phenolic, and honeycomb-reinforced, low-density filled epoxy Measurements were made on a dense high-purity and a porous grade of carbon and on high-purity, structural, and pyrolytic grades of graphite A photomultiplier and a lead sulfide cell were used as detectors for the visible and infrared measurements, respectively The results indicate that emittance for all the materials is maximum in the visible spectrum and decreases with wavelength in the near-infrared spectrum The angular distributions of radiation fluxes incident on, reflected from, and radiated from, each material, were measured with an array of solar cells The distribution measurements showed that the materials whose surfaces are porous or have been roughened by oxidation obey Lambert's cosine law approximately in the emission of radiation, and also in reflecting the arc incident radiation, although the incident radiation is nonuniform in distribution

Thermal Radiation from Rough Tungsten Surfaces in Normal and Off‐Normal Directions

Abstract: The effect of surface roughness on the normal spectral emittance and on the spectral angular dependence of thermal radiation was investigated for tungsten at 2800°K over a wavelength range of 0.3–6 μm. The normal emittance of rough surfaces is higher than of flat surfaces for all wavelengths, the largest increase occurring in the infrared. In the angular dependence of thermal radiation a minimum was found which depends on the surface geometry and on the wavelength. The values are interpreted in terms of emission from flat facets, which do not radiate according to Lambert's cosine law.

Optical solar reflector - A highly stable, low alpha sub S/epsilon spacecraft thermal control surface.

Abstract: Stable optical solar reflector for spacecraft thermal control, summarizing simulated exposures results

Radiative heat exchange and equilibrium surface temperature in a space environment.

Abstract: An analysis is presented for evaluating radiant heat transfer and temperature of surfaces in a typical space environment. A semigray spectral subdivision of surface properties is employed to account for differences in property values for solar and surface radiation. In each spectral interval, a reflectance model with both diffuse and specular components is used to approximate the nondiffuse character of surface reflectance. An engineering formulation is given for a system of surfaces, some of which have prescribed temperature and the remainder prescribed radiant heat flux. The solution for flux or temperature, whichever is unknown, is given in terms of known temperatures, known fluxes, and certain ratios of determinants involving surface properties and geometry. The formulation is extended to include variation of flux and temperature on the surfaces. The analysis is applied to the adjoint plate system and results are presented for heat transfer and equilibrium temperature. Significant differences in heat transfer and equilibrium temperature occur between semigray analysis with the specular-diffuse reflectance model and that determined on the basis of gray diffuse theory.

Acoustic Wave Propagation in a Non-Gray Radiating Atmosphere.

Abstract: An analysis based on radiative decay times allows the quantitative calculation of the effects of thermal radiation on acoustic wave propagation in an infinite non-gray atmosphere. Computations for the earth's troposphere, Mars, Venus, and ammonia in the laboratory show that in no case is measurable alteration of the propagation velocity predicted, but radiation is the main damping mechanism at low frequencies. Calculated damping is consistent with atmospheric observations. Laboratory demonstration of this effect is probably not possible in air, but appears feasible in low pressure ammonia. Acoustic wave propagation at high altitudes (low pressures) is shown to be adiabatic, contrary to a suggestion by Golitsyn. An approximate treatment to include the effects of the finite depth of the atmosphere shows that the results are somewhat altered in detail. An important finding is that for this problem a non-gray atmosphere cannot be usefully approximated by one or more gray coefficients. A simple approx...

Radiation studies of arc heated nitrogen, oxygen, and argon plasmas.

Abstract: : This report presents continuum radiation and line radiation measurements for nitrogen, oxygen, and argon covering wavelength ranges from the vacuum ultraviolet to the infrared for a selection of pressures and temperatures. A mechanically constricted dc arc was used as the radiation source. Nitrogen continuum data at 1 and 2 atmospheres is reported for wavelengths from 700 to 37,000 A and temperatures from 9,000 to 13,500 K. The spectral data have been treated analytically to separate the radiation from electron-neutral atom and electron-positive ion collisions, and comparison is made with the theory of Burgess and Seaton for the positive ion recombination radiation. Values of the ratio, R, by which this theory differs from our experimental results are given as a function of wavelength. A subtraction of the positive ion radiation from the total continuum yields the contribution from the neutral atom, from which was extracted the approximate absorption edges of the nitrogen negative ion and an estimate of the photo-detachment cross section. Earlier data reported for the argon continuum at atmospheric pressure has been improved by a reinterpretation of the temperature measurement. New argon continuum intensities at 5 atmospheres pressure and 12,300 K are reported. (Author)

An exact general solution for the temperature distribution and the composite radiation convection heat exchange along a constant cross-sectional area fin

Abstract: Differential equations for composite radiation and convection heat exchange along constant area fins

An apparatus for the measurement of the normal and off-normal spectral emissivity of incandescent materials

Abstract: An apparatus for the measurement of the normal spectral emissivity and the angular dependence of thermal radiation of electric conducting solid materials is described. The method of measurement is the direct comparison of radiation from the material and from a black body by means of a rotating mirror and a prism monochromator. The radiating surface and the black body are part of one sample which is heated by electron bombardment. The temperature of the samples is between 2000 and 3000?K. The wavelength range is 0?35 to 6 ?m. Results of measurements of the emissivity and the angular dependence of the thermal radiation of flat and rough tungsten are given.

Radiative Terms in the Thermal Conduction Equation for Planetary Atmospheres

Abstract: Terms in the thermal conduction equation arising from infrared emissions and absorptions by atomic oxygen and carbon monoxide are investigated. The purpose of the investigation is to develop general expressions for the net emission by O and CO taking into account absorption of planetary radiation from below as well as radiation from regions of the mesosphere and thermosphere. These expressions are valid at all optical depths in the thermosphere. An expression for the net emission from a molecular band is also given. The radiative terms are developed under the assumption of local thermodynamic equilibrium. The expressions developed for the radiative terms permit the evaluation of net heating as well as cooling in the O and CO emission lines. The results are compared to the Bates' approximation to the radiative loss terms for a partly dissociated CO2 atmosphere which is optically thick in both O and CO. For this model it is found that, at high altitudes where the atmosphere is optically thin in O a...

Evaluation of direct electrical heating methods for the determination of thermal conductivity at elevated temperatures.

Abstract: : The report discusses the progress on a program for evaluating direct electrical heating methods for high temperature thermal conductivity determinations. A multi-purpose apparatus used in the evaluation of these methods, has generated data simultaneously on thermal conductivity, electrical resistivity, total hemispherical emittance and spectral emittance (0.65 microns) to 1800 K under high vacuum. Problems associated with emittance are extremely important in thermal conductivity determinations at high temperatures using direct heating methods. Great care must be taken to insure that the emittance remains constant over the central portion of the test specimen. The mathematical techniques used to compute thermal conductivity from the experimental data were explored and considerably advanced. These techniques eliminated the need for mathematical approximations which severely limit the region of applicability. One of these techniques was easily extended to the general case which includes three conductivity methods as special cases. Therefore, the need to match certain experimental conditions was eliminated. The inclusion of temperature-dependent physical properties was also readily included in this technique. The mathematical techniques also yielded temperature profiles which could be compared to the experimental profiles. (Author)

The Significance of the Polarization of Solar Short-Wavelength X-rays

Abstract: For the interpretation of the physical events occurring at the beginning of a flare it seems to be significant not only to observe the spectra of the short-wavelength X-ray radiation but also its polarization. Solar X-ray spectra below 10 A during flares observed by Bowen et al (1964) show in the region of long wavelength a slow fall in their intensity and below 5 A approximately a rapid fall (Figure 1). This spectral variation cannot be understood on the basis of thermal radiation only. However, it is possible to explain the observed spectra by assuming a superposition of a thermal radiation as well as a non-thermal one. Indeed, one should assume that the fast electrons, during their acceleration, obtain a preferred direction and that only afterwards their velocity distribution becomes a Maxwellian one. The assumption of the existence of anon-Maxwelhan distribution of electrons in the beginning of a flare is also supported by the fact that the rise in intensity by more than one order of magnitude takes place in a few minutes. This point has already been emphasised by Bowen et al.

The temperature function for the integral emissivity of certain materials below 300° K

Abstract: We present the method for and the results from an experimental investigation into the temperature function of the integral emissivity for certain materials used in low-temperature multilayer vacuum insulation. We study the effect of fiberglas filler materials on radiative heat transfer.

Means of Measuring the Earth's Velocity Through the 3°K Radiation Field

Abstract: Earth velocity relative to 3 K cosmic background radiation field measured by determining anisotropy with wideband detector sensitive at mm IR wavelengths

Thermal radiative and electrical properties of a cadmium sulfide solar cell at low solar intensities and temperatures

Abstract: Thermal radiative and electrical properties of cadmium sulfide solar cell at low simulated solar intensities and temperatures

Coupled radiation and conduction in free mixing.

Abstract: : Heat transfer by both grey radiation and conduction is examined in the case of Oseen-like free-mixing flow which is formally analogous to a one-dimensional unsteady transfer problem. The equation describing the transport of energy by non-convective means is obtained by linearization of the conservation of energy relation. The geometry of the time-dependent problem may be envisaged as a slab of finite width separating two infinite half-spaces; this configuration is analogous to the steady, free-mixing flow of a two-dimensional jet or wake. The initial temperature profile of the jet with respect to the surrounding gas is taken to be a step function. By means of the transform calculus the determination of the temperature for the entire region of the jet and the fluid adjacent to it may be given as an inversion integral. This approach does not require a kernel substitution in the solution of the integro-differential equation for the temperature and thus avoids some of the limitations imposed thereby. Analytic solutions in the limiting cases of 'thin' and 'thick' gas behavior are readily recovered from the general solution. From the bounded variation of the integrand the temperature distribution may be computed for all possible combinations of the heat conduction and radiation parameters. Exact numerical evaluation of the inversion integral is employed to provide a complete description of the temperature variations in the flow. (Author)

Experimental Measurements of Thermal Radiation Properties by Cyclic Incident Radiation

Abstract: Cyclic incident radiation method for measuring thermal emittance and absorption properties of metals at cryogenic temperatures

Radiation transfer through high-temperature shock layers.

Abstract: Radiation transfer through high temperature shock layers, noting problems in absorption and emission strengths for atomic lines, photoionization and visible continuum

Radiation heat flux from high pressure arcs

Abstract: : An ablation type constrictor was developed in which an electric arc at very high pressure is sufficiently stable for spectrographic measurements. Radial temperature distributions were determined for 250 ampere arcs in Delrin, (COH2)n, plasma at 100 and 150 atmospheres. Voltage gradient is constant along the arc axis and as high as 500 volts/cm. Pressure gradient in the constrictor is negligible. These two results indicate strongly that in spite of the self- induced flow field, the ablation type arc is homogeneous along the column. Temperature measurement was based on non-optically thin Abel inversion of continuum intensity measurements useful to an optical depth of one or more, which related temperature to theoretically computed continuum emission. A previously developed arc radiation model was extended to include a few selected lines, and these contribute significantly to total radiation because they are very much broadened.

A computer program for calculating external thermal-radiation heat loads and temperatures of spacecraft orbiting the planets or the moon

Abstract: Computer program for computation of thermal radiation heat loads and temperatures of spacecraft orbiting planets or moon

Radiation field of the planet earth

Abstract: : In the monograph results of theoretical and experimental investigations of the field of terrestrial radiation are generalized. Methods of calculations and measurements are briefly discussed. Angular spectral characteristics of outgoing emission for shortwave and long-wave radiation are thoroughly examined. A survey is made of works on the problem of radiation balance of the earth-atmosphere system. Considerable attention is given to the problem of outgoing radiation fluxes on surface of different angles. (Author)

Infrared visibility and thermal radiation noise in the atmosphere

Abstract: Infrared contrast transmission was investigated in connection with the passive infrared imaging techniques. Although there is a basic analogy between infrared and light visibility in the atmosphere, the modes of contrast extinction are very different. In the case of a static atmosphere, the infrared visibility was found to be governed by the same transmission function as the radiation flux calculation. Further, in this study, we are concerned with the field of the radiation noise in the atmosphere due to temperature fluctuation. Because of the transfer effect, the scale of the disturbance relative to the mean free path of the photons is important in determining the noise power spectrum. In the simplest case of an isothermal and uniform atmosphere a few examples of the variance evaluation are shown for an atmosphere with absorbing water vapor and carbon dioxide.