Showing papers on "Thermal radiation published in 1978"

Temperature rise induced by a laser beam II. The nonlinear case

Abstract: The steady‐state solution of the nonlinear heat‐conduction equation when the thermal conductivity is strongly temperature dependent is expressed (for arbitrary geometry and heat source) in terms of the corresponding solution for the linear heat‐conduction case. This permits a closed‐form expression for the maximum temperature rise when a Gaussian laser beam hits the surface of a crystal, as well as integral representations for the spatial distribution of the temperature rise. This solution is essential in understanding laser annealing.

Detection of Optical and Infrared Radiation

Abstract: 1. Thermal Radiation and Electromagnetic Modes.- 2. The Ideal Photon Detector.- 3. Coherent or Heterodyne Detection.- 4. Amplifier Noise and Its Effect on Detector Performance.- 5. Vacuum Photodetectors.- 6. Noise and Efficiency of Semiconductor Devices.- 7. Thermal Detection.- 8. Laser Preamplification.- 9. The Effects of Atmospheric Turbulence.- 10. Detection Statistics.- 11. Selected Applications.- References.

Thermal radiation from metal surfaces

Abstract: The thermal radiation from metals is calculated by including the Holstein processes of phonon-assisted and surface-assisted scattering in the free-electron model. This approach provides a fairly accurate description of the experimentally measured total emissivity of good conductors in terms of the dc resistivity and the radius of the free-electron sphere. These calculations do not support the Foote and Davisson–Weeks-type equations which have been used previously.

Description and validation of a three-dimensional procedure for combustion chamber flows

Abstract: A prediction method for three-dimensional reacting flows is described. It comprises a numerical solution technique for the time-averaged governing partial differential equations and physical modeling for the turbulence, combustion, and thermal radiation. The requirement of computational economy is strongly emphasized by the method, which employs an implicit numerical technique of the finite-difference kind to solve the governing equations iteratively. The turbulence model is of the "two-equation" variety, while the combustion model is based on a "fast kinetics" statistical approach. A newly developed flux model is employed for the thermal radiation. Comparisons of predictions and data are presented for industrial furnaces, but the method is applicable to all forms of combustion chambers including gas turbine cans. Nomenclature apyan — coefficient in finite-differ ence equation A b = area of cell boundary A i, A 2, A 3 = coefficients of the Taylor series for radiation intensity BlfB2,B3 = coefficients of the Taylor series for radiation intensity B* = defined by (B2 + B22 + B/) i/2

Thermal flux reduction by electromagnetic instabilities

Abstract: From a dispersion relation, linear growth rates of electromagnetic instabilities are obtained in an electron plasma whose velocity distribution function has a high-energy tail. Theory is developed to derive the reduction in the thermal flux caused by these electromagnetic instabilities. Nonlinear theory leads to the saturation level of instabilities. Numerical simulations are carried out using a particle-in-cell method. The reduction in thermal conduction predicted by the theory is found to be in good agreement with computer simulations.

The structure and spectrum of a colliding-cloud system and its possible relationship to QSOs

Abstract: A collision between two gas clouds with initial densities of approximately 10 million per cu cm, velocities of about 1000 km/s, and radii of approximately 1 pc is investigated quantitatively by coupling a calculation of the radiation spectrum resulting from the anticipated shock fronts with a computation for the conversion of this high-energy radiation into optical emission in adjacent photoionized regions. The detailed structure of the colliding clouds is discussed, and the effects of an ambient magnetic field are considered. The combined emission-line spectrum is presented along with continuum emission estimates for thermal, synchrotron, and very-high-energy bremsstrahlung mechanisms. It is shown that significant continua can be produced over the range from 300 microns to 3 keV, including a blackbody contribution from a high-density neutral region between the shock fronts, free-free and free-bound radiation from the cooling zones directly behind the shocks, and free-free, free-bound, and two-photon radiation from the photoionized regions immediately ahead of and behind the cooling zones. The theoretical spectrum of the structure resulting from the collision is found to be similar in general and in some details to those observed for typical quasars.

An exact similarity solution for a spherical shock wave in a magneto-radiative gas

Abstract: An exact similarity solution for a spherical magneto-gas dynamic shock wave is obtained in the case when the loss of energy due to radiation escape is significant. The total energy of the shock wave is not constant but decreases with time. We have shown that due to the magnetic field, the radiation flux changes considerably.

Effects of aerosol optical properties and size distributions on heating rates induced by stratospheric aerosols

Abstract: A previous analysis of the energy balance of small particles in the earth's atmosphere is extended to substances that have special significance in the stratospheric aerosol layer, such as sulphuric acid and volcanic ash. The analysis is based on establishing a balance between the power absorbed by the particles from solar and planetary radiation fields, the power emitted by the particles as thermal radiation and the power exchanged through collisions with the ambient gas. The planetary radiation field is calculated as a function of altitude and includes radiation from the surface as well as emission and absorption by the infrared bands of carbon dioxide, ozone and water vapour. The various terms in the energy balance equation change as a function of the composition, radius and altitude of the aerosol particles, and of the latitude, season, time of day and albedo of the underlying earth-atmosphere system; aerosols may heat or cool the atmosphere and their temperature may differ from the ambient gas temperature. Average values for rates of heating induced in the ambient gas by monodisperse and polydisperse aerosol particles are computed for various substances. The rates of heating induced by volcanic ash particles and sulphuric acid droplets are calculated for equatorial, mid-latitude summer and mid-latitude winter conditions for altitudes to 40 km. Our results illustrate the effects on atmospheric heating rates of changing the size distribution of the particles and the particle composition. We also show that heating rates of at least 0.25 K day−1 were induced by the stratospheric aerosol layer after the 1963 eruption of the Gunung Agung volcano in Bali.

Climatological effects of atmospheric particulate pollution

Abstract: THE large amount of dust thrown into the atmosphere by volcanoes has long been recognised as an important factor in initiating climatic change that generally leads to cooler surface temperatures1–4. Another source of atmospheric dust is man's agricultural and industrial activity. It has been suggested that this anthropogenic source is a cause of cooling in a manner similar to that of volcanoes5,6. The primary mechanism thought to be responsible for cooling in both these situations is the backscatter to space of a portion of the incoming solar radiation. This reduction in heat input to the planet would cool its surface. Idso7–10 has argued for another effect of atmospheric dust—warming due to ‘thermal blanketing’. This mechanism would absorb some of the Earth's thermal radiation that would otherwise escape to space and then reradiate a portion of this radiation back to the surface, hence a rise in surface temperature with increasing atmospheric dust would be expected. We have completed an experiment recently in which the relative magnitudes of the solar and thermal radiation interactions with atmospheric dust were compared11. The results reported there indicate that there is a general climatological superiority for the thermal radiation interaction, predicting a net warming trend with increasing atmospheric dust for all but the most extreme cases of dust-loading of the atmosphere.

Emittance and absorptance of NASA ceramic thermal barrier coating system

Abstract: Spectral emittance measurements were made on a two-layer ceramic thermal barrier coating system consisting of a metal substrate, a NiCrAly bond coating and a yttria-stabilized zirconia ceramic coating. Spectral emittance data were obtained for the coating system at temperatures of 300 to 1590 K, ceramic thickness of zero to 0.076 centimeter, and wavelengths of 0.4 to 14.6 micrometers. The data were transformed into total hemispherical emittance values and correlated with respect to ceramic coating thickness and temperature using multiple regression curve fitting techniques. The results show that the ceramic thermal barrier coating system is highly reflective and significantly reduces radiation heat loads on cooled gas turbine engine components. Calculation of the radiant heat transfer within the nonisothermal, translucent ceramic coating material shows that the gas-side ceramic coating surface temperature can be used in heat transfer analysis of radiation heat loads on the coating system.

Parameters of a plasma formed by the action of microsecond laser pulses on an aluminum target in vacuum

Abstract: The results are given of a numerical solution of the one-dimensional (plane) time-dependent radiative–gasdynamic problem of the action of laser radiation of wave lengths in the range 0.1–1 μ on an aluminum target in a vacuum. It is shown that the emission of thermal radiation originating in the ablation plasma lowers its temperature substantially. A considerable fraction of the incident laser energy (up to 30–40 %) is emitted into vacuum from a plasma jet in a spectrum which extends into the UV and vacuum UV regions.

Superconducting generator thermal radiation shield having substantially uniform temperature

Abstract: A superconducting generator having rotor including a field winding operated at superconducting temperature and a cylindrical electromagnetic shield surrounding and shielding said winding. A cylindrical thermal radiation shield disposed between said electromagnetic shield and said field winding to intercept radiation from said shield and re-radiate to the field winding at a lower temperature with a heat transfer mechanism for rapid cooldown of the thermal radiation shield and serving to maintain the thermal radiation shield at substantially uniform temperature throughout its length.

Heat Mirrored Solar Energy Receivers

Abstract: RANSPARENT heat mirrors are thin films that transmit visible light but reflect infrared radiation. This paper investigates the potential utilization of this wavelength selective property in the process of trapping incident solar energy and its conversion to heat in a high-temperature (5401650°C) windowed cavity. The heat mirror film permits the solar energy to enter the cavity but reflects part or all of the internally generated blackbody (heat) radiation as it attempts to escape. The characteristics and durability of a number of transparent heat mirror films developed at the MIT Lincoln Laboratory for solar energy applications have been reported. l'3 These transparent heat mirror film types include metal films1 (e.g., Au and Ag), semiconductors2 (e.g., Sn-doped In2O3), composite layer films2 (e.g., TiO2/Ag/TiO2), and thin-film conducting microgrids.3 From a simplistic viewpoint, heat mirror films pass nearly all of the radiation below a given wavelength and reflect nearly all of the radiation above it; the separation point (transmission vs reflection) edge wavelength being determined by the materials and construction methods used to fabricate the heat mirror. Although in the early development stage, heat mirror films offer the potential advantage of reducing the window heat load because of their reflective properties. A simplified model of a heat mirrored cavity receiver is considered herein and used to predict cavity operating efficiency as a function of incident sun level, cavity operating temperature, and heat mirror reflectance edge position. The results are compared with those for a windowed receiver where escape of reradiated energy is prevented by a process of radiation absorption in the window, rather than by reflection back to the cavity interior as in the case of the heat mirror. Experimental data on some existing heat mirrors are summarized, and achievable performance improvements with existing heat mirror films are calculated and compared with idealized predictions. The results reported here should be considered as an initial inquiry into the usefulness of the heat mirror approach in a cavity receiver with the objective of determining whether the use of such films is sufficiently promising to warrant more detailed investigations. Though evaluations are made for three experimental heat mirrors, these heat mirrors have not been optimized for the cavity receiver application, and

Radiative heat transfer in a planar medium with anisotropic scattering and directional boundaries

Abstract: Radiation heat transfer in an absorbing, emitting and scattering medium has been the subject of many previous investigations. Most solutions are numerically complex and the existing analytical solutions are restricted in application by the simplifying assumptions involved. A plane-parallel medium is considered which scatters anisotropically. The boundaries are considered to be specular reflectors, as predicted by Fresnel's relations, while the diffusely incident radiation is refracted according to Snell's law. The emission is restricted to a medium with a uniform temperature distribution. Approximate closed-form solutions for the radiative heat flux and incident intensity are presented for dielectric layers and linear anisotropic scattering. Numerical results are also presented and show that the effects of directional boundaries, anisotropic scattering, scattering albedo and optical depth are accurately predicted by the approximate solution.

Absorption of solar radiation by alkali vapors

Abstract: A theoretical study of the direct absorption of solar radiation by the working fluid of high temperature, high efficiency energy converters has been carried out. Alkali vapors and potassium vapor in particular were found to be very effective solar absorbers and suitable thermodynamically for practical high temperature cycles. Energy loss via reradiation from a solar boiler was shown to reduce the overall efficiency of radiation-heated energy converters, although a simple model of radiation transfer in a potassium vapor solar boiler revealed that self-trapping of the reradiation may reduce this loss considerably. A study was also made of the requirements for a radiation boiler window. It was found that for sapphire, one of the best solar transmitting materials, the severe environment in conjunction with high radiation densities will require some form of window protection. An aerodynamic shield is particularly advantageous in this capacity, separating the window from the absorbing vapor to prevent condensation and window corrosion and to reduce the radiation density at the window.

Apparatus and system for measuring power of heat radiation

Abstract: APPARATUS AND SYSTEM FOR MEASURING POWER OF HEAT RADIATION Abstract of the Disclosure An apparatus for measuring the power of a heat radia-tion, comprising a sensitive element implemented, for exam-ple, as an electrically conductive filament adapted to be displaced in a heat radiation flux at a stabilized speed. Connected to the sensitive element is a meter for register-ing the variations of the temperature of the sensitive ele-ment, resulted from the heat radiation. A system for measuring the power of a heat radiation, comprising at least three such apparatuses. The invention provides for a continuous operation of the proposed apparatus.

Methods and apparatus for examination of surface temperature distribution

Abstract: The invention is concerned with monitoring the temperature distribution of a surface which may not be a good emitter of thermal radiation. In the invention a member which is of relatively low thermal conductivity and high thermal emissivity is placed in contact with the surface the temperature distribution of which it is required to monitor so that a thermal image transfers from the latter to the former. A pyroelectric vidicon camera tube then examines the temperature distribution of the member.