Showing papers on "Thermal radiation published in 2004"

Thermal radiation from photonic crystals: a direct calculation.

Abstract: A classical simulation of equilibrium thermal emissivity from dispersive, lossy photonic crystals is presented. Normal emission results consistent with those assuming Kirchoff's law are obtained; i.e., a photonic crystal does not emit more than what a blackbody does. Significant enhancement, however, can be achieved over the radiation intensity from a uniform slab, indicating the potential usefulness of photonic crystals in incandescent lighting and thermal photovoltaic applications.

The temperature dependence of effective thermal conductivity of open-celled steel alloy foams

Abstract: The effective thermal conductivity of steel alloy FeCrAlY (Fe—20 wt.% Cr—5 wt.% Al—2 wt.% Y—20 wt.%) foams with a range of pore sizes and porosities was measured between 300 and 800 K, under both vacuum and atmospheric conditions. The results show that the effective thermal conductivity increases rapidly as temperature is increased, particularly in the higher temperature range (500–800 K) where the transport of heat is dominated by thermal radiation. The effective conductivity at temperature 800 K can be three times higher than that at room temperature (300 K). Results obtained under vacuum conditions reveal that the effective conductivity increases with increasing pore size or decreasing porosity. The contribution of natural convection to heat conduction was found to be significant, with the effective thermal conductivity at ambient pressure twice the value of vacuum condition. The results also show that natural convection in metal foams is strongly dependent upon porosity.

Coherent spontaneous emission of light by thermal sources

Abstract: The emission of light by a material at temperature T has been shown recently to be coherent in the near field These properties were attributed to the thermal excitation of surface polaritons We review the origin of this phenomenon We analyze the influence of the microstructure and temperature on the coherence properties and show how to engineer thermoradiative properties of surfaces We report the design of a quasi-isotropic source and a very directional source of thermal light We also report a measurement of the transverse coherence length of a thermal source of light

Heat and mass transfer effects on moving vertical plate in the presence of thermal radiation

Abstract: Thermal radiation effects on moving infinite vertical plate in the presence variable temperature and mass diffusion is considered. The fluid considered here is a gray, absorbing-emitting radiation but a non-scattering medium. The plate temperature and the concentration level near the plate are raised linearly with time. The dimensionless governing equations are solved using the Laplace-transform technique. The velocity and skin-friction are studied for different parameters like thermal Grashof number, mass Grashof number, time and radiation parameter. It is observed that the velocity slightly decreases with increasing value of the radiation parameter.

Light-emitting apparatus

Abstract: A reflection-type light emitting apparatus has an excellent heat radiation property and allows the use of a high-power light emitting element, the minimization of a reduction in radiation efficiency of reflected light, and the focused radiation of high-output light at high efficiency. The apparatus has: a case 10 of metallic material and with an excellent heat radiation property; a reflection mirror section 11 formed fitted to the lower portion of case 10; a transparent plate 12 to cover the upper surface of case 10; heat radiation plates 13, 14 of metallic material with excellent heat conductivity and inserted inside the case 10; an LED element 2 mounted on the heat radiation plate 13; lead sections 15 A, 15 B fixed through an insulating layer 15 a to the heat radiation plate 13 to serve as a power supply member to supply power to the LED element 2; and a spacer 16 of an insulating material to insulate the lead sections 15 A, 15 B from the case 10.

Solar Carbothermal Reduction of ZnO: Shrinking Packed-Bed Reactor Modeling and Experimental Validation

Abstract: The thermodynamics and kinetics of the carbothermic reduction of ZnO are examined over the temperature range 400−1600 K. Above 1340 K, the equilibrium composition of the stoichiometric chemical system consists of an equimolar gas mixture of Zn (vapor) and CO. Assuming a first-order rate constant for the surface reaction kinetics between ZnO(s) and CO and further applying a shrinking spherical particle model with an unreacted core, the apparent activation energy obtained by linear regression of the thermogravimetric data is EA = 201.5 kJ/mol. A numerical model is formulated for a solar chemical reactor that uses concentrated solar radiation as the energy source of process heat. The model involves solving, by the finite-volume technique, a 1D unsteady-state energy equation that couples heat transfer to the chemical kinetics for a shrinking packed bed exposed to thermal radiation. Validation is accomplished by comparison with experimentally measured temperature profiles and Zn production rates as a function ...

A deterministic method to characterize canopy radiative transfer properties

Abstract: Investigations of snowcover dynamics beneath vegetation canopies require either measured or estimated solar and thermal radiation values at the snow surface. A deterministic method is presented that uses portable arrays of pyranometers and pyrgeometers to quantify the amount of incoming radiation at the snow surface. Example solar and thermal radiation datasets are presented from boreal deciduous, boreal coniferous and temperate coniferous forest stands. The data indicate that the canopies transmitted 33% (4-8 March), 15% (6-10 February), and 3% (22-24 September) of the above-canopy radiation. In the boreal deciduous and temperate conifer stands, thermal radiation is increased by 25% and 34% respectively. Thermal gains partially offset solar reduction, such that incoming all-wave radiation is decreased by 22% and 25% respectively for each of these stands. When recorded at a high temporal resolution, array data can estimate below-canopy diffuse solar radiation values for estimation techniques that treat direct and diffuse transmission independently. We provide examples of how radiometer array data are used to derive simple canopy radiation transmissivity parameters for global, beam and diffuse radiation. Radiometer arrays also provide data for detailed investigations to assess within-stand radiation variability, or to investigate radiation variations across land cover discontinuities, to advance our understanding of snowcover energetics in complex environments.

In situ high P-T Raman spectroscopy and laser heating of carbon dioxide

Abstract: In situ high P-T Raman spectra of solid CO2 up to 67 GPa and 1660 K have been measured, using a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system in diamond anvil cells. A metallic foil was employed to efficiently absorb the incoming Nd:YLF laser and heat the sample. The average sample temperature was accurately determined by detailed balance from the anti-Stokes/Stokes ratio, and was compared to the temperature of the absorber determined by fitting the thermal radiation spectrum to the Planck radiation law. The transformation temperature threshold and the transformation dynamics from the molecular phases III and II to the polymeric phase V, previously investigated only by means of temperature quench experiments, was determined at different pressures. The P-T range of the transformation, between 640 and 1100 K in the 33–65 GPa pressure interval, was assessed to be a kinetic barrier rather than a phase boundary. These findings lead to a new interpretation of the high P-T phase di...

Mars pathfinder: New data and new model simulations

Abstract: The diurnal cycle in the temperature and wind speed of the Mars Pathfinder lander (MPF) is compared with the results from a one-dimensional high-solution boundary-layer model. The radiation scheme of the model has been validated against line-by-line calculations in an average Martian case. The agreement with MPF observations is good. Strong daytime turbulent variability and steep vertical temperature gradients characterize the observations. A detailed look at the model physics reveals that the thin atmosphere responds strongly to thermal radiation. During the daytime, absorption of surface-emitted thermal radiation by CO2, water vapour and dust is the main heating agent below 200 m despite the fact that the surface layer is strongly convective and dust absorbs solar radiation. The night-time inversion is also transferred to the air mainly by long-wave processes, although turbulent cooling dominates very near the surface (below 40 m). The near-surface net heating/cooling displays a sensitive balance between several large and opposite physical processes, wherefore accurate algorithms and high resolution are needed for detailed model results. The modelled hydrologic cycle reveals a well-mixed layer of up to 5 km with thin icy fog and frost forming in the night-time. These sublimate back to the air soon after sunrise. Copyright © 2004 Royal Meteorological Society

Narrow-Band Thermal Radiation with Low Directivity by Resonant Modes inside Tungsten Microcavities

Abstract: Narrow-band thermal radiation with low directivity is presented using a periodic array of microcavities on tungsten surfaces. Thermally excited resonant modes inside the microcavities are found to enhance thermal radiation at specific wavelengths. Compared with a flat tungsten surface, thermal radiation from the microcavities has been enhanced by ~200% at the most enhanced wavelength of λ=5.5 µm. This strong enhancement yields narrow-band thermal radiation with a full width at half maximum of Δλ=0.59 µm. The enhancement peak positions are well explained using a simple cavity resonator model. By measuring thermal radiation at oblique directions, it is also found that this narrow-band radiation has low directivity.

Absolute temperature measurement in a laser‐heated diamond anvil cell

Abstract: [1] The laser-heated diamond anvil cell has been widely used to study mineral physics under high pressure and temperature, and these studies have provided valuable information in understanding planetary interiors; however, use of the spectroradiometric method in the studies has raised concerns about the accuracy of obtained temperature values. We have built a laser-heating system coupled with nuclear resonant inelastic x-ray scattering to explore particular physical properties of deep-Earth materials. Energy spectra of iron were measured up to 58 GPa and 1700 K. The detailed balance principle applied to the inelastic x-ray scattering spectra provides absolute temperatures of the laser-heated sample. These temperatures are in very good agreement with values determined from the thermal radiation spectra fitted to the Planck radiation function up to 1700 K. Our data provide, for the first time, independent confirmation of the validity of temperatures determined from spectroradiometric method in the laser-heated diamond cell experiments.

Nuclear resonant scattering at high pressure and high temperature

Abstract: We introduce the combination of nuclear resonant inelastic X-ray scattering and synchrotron Mossbauer spectroscopy with the laser-heated diamond anvil cell technique for studying magnetic, elastic, thermodynamic, and vibrational properties of materials under high pressures and high temperatures. An Nd:YLF laser, operating in continuous donut mode (TEM01), has been used to heat samples inside a diamond anvil cell from both sides. Temperatures of the laser-heated sample are measured by means of spectral radiometry and by the detailed balance principle of the energy spectra. The temperature measured by the detailed balance principle is in very good agreement with values determined from the thermal radiation spectra fitted to the Planck radiation function up to 1700 K. Nuclear resonant scattering on 57 Fe-containing materials (i.e., Fe, FeO, Fe2O3) has been studied up to 2500 K and 100 GPa. A detailed description of the laser-heating optics, temperature determination, the X-ray monochromatization, and the X-ray focusing optics is given in this article.

The effect of radiation on free convective flow and mass transfer past a vertical isothermal cone surface with chemical reaction in the presence of a transverse magnetic field

Abstract: The effects of radiation and chemical reactions, in the presence of a transverse magnetic field, on free convective flow and mass transfer of an optically dense viscous, incompressible, and electri...

Bulk Formulation of the Surface Heat Flux

Abstract: An interpretive literature survey examines different approachesfor applying the bulk aerodynamic formulato predict the surface heat flux. The surface heat flux is often predicted in terms of the surface radiation temperature, which is also used to predict the upward longwave radiation and the heat flux into the soil. In models, the thermal roughness length based on the surface radiation temperature (radiometric roughness length) is often specified to be smaller than the roughness length for momentum for a number of distinct reasons. The definition of the radiometric roughness length depends on the way that the surface temperature is measured, the choice of stability functions and displacement height and inclusion of any additional resistances.

A transient study of coupled natural convection and radiation in a porous vertical channel using the finite-volume method

Abstract: The present article deals with a numerical study of coupled fluid flow and heat transfer by transient natural convection and thermal radiation in a vertical channel opened at both ends and filled with a fluid-saturated porous medium. The bounding walls of the channel are isothermal and gray. In the present study we suppose the validity of the Darcy law and of the local thermal equilibrium assumption. The radiative transfer equation (RTE) is solved by the finite-volume method (FVM). The net radiative heat flux as well as its divergence is also calculated using the same method. The sensitivity of the fluid flow and the heat transfer to different controlling parameters, namely the conduction-radiation parameter or Planck number or Stark number, N, the optical thickness, τ D , and the wall emissivity, e, are addressed. The results indicate that the controlling parameters of the problems, namely, N , τ D , and e, have significant effects on the flow and thermal fields and on the transient process of heating or...

Effective and Coupled Thermal Conductivities of Isotropic Open-Cellular Foams

Abstract: The effective and the coupled thermal conductivity of the solid microstructure of open-cellular foams and an accompanying saturation fluid are defined in a conceptual representation of conductive heat transfer in a two-phase system of which the phases are in a thermal nonequilibrium state. The effective and coupled thermal conductivities were determined from a close observation of the relationship between microscopic and macroscopic temperature distributions. Temperature distributions were obtained from the numerical solution of the three-dimensional (3-D) conduction equation in a representative geometrical model of the foam solid microstructure and the fluid pores. Empirical correlations are provided for the effective and coupled thermal conductivities in terms of the solid and the fluid thermal conductivity and foam porosity. Thermal radiation was not considered in the energy transfer process. © 2004 American Institute of Chemical Engineers AIChE J, 50: 547–556, 2004

Gravity, radiation, and coflow effects on partially premixed flames

Abstract: Our objective is to characterize gravity effects on the structure of laminar methane–air partially premixed flames through detailed simulations. We examine the heat loss due to radiation from similar flames that are established at various gravitational accelerations and coflow velocities. Radiation is modeled using the optically thin assumption that provides a limiting value for the radiation heat transfer. We have validated the simulations with measurements in a representative 1-g flame. The predictions are in good agreement with the measured reaction zone topologies and temperature distributions. The simulations show that when the gravitational acceleration for a representative 1-g partially premixed double flame is instantaneously decreased to zero, it is possible to establish a nearly steady 0-g flame in roughly 2.2 s. The overall effect of radiation on the structure of the 1-g flame is relatively insignificant in contrast to the corresponding 0-g flame. Due to radiation effects, the heights of both t...

A discontinuous finite-element formulation for internal radiation problems

Abstract: This article presents a discontinuous finite-element formulation for internal thermal radiation problems. In contrast to the conventional finite-element formulation, the discontinuous algorithm permits the discontinuity of field variables across the internal interelement boundaries and is useful for integral-differential equations describing thermal radiation in absorbing/scattering media. Mathematical formulation and numerical implementation are given. The convergence rate and local mesh adaptivity are discussed. Two approaches for coupling of the discontinuous and conventional methods for mixed heat transfer calculations are presented. Numerical results are given for internal radiation and combined conduction/radiation problems and are compared with analytical solutions whenever available.

The Propagation of Infrared Radiation in a Semitransparent Liquid Containing Gas Bubbles

Abstract: A theoretical model is suggested of the propagation of infrared radiation in a semitransparent liquid containing gas bubbles, which includes an approximate description of the radiation characteristics and radiation transfer in a disperse system. Calculations are performed for a layer of water containing vapor bubbles illuminated by the thermal radiation of an external source. It is demonstrated that, for real values of the parameters, the scattering of radiation by bubbles may lead to the absorption of thermal radiation in a much thinner layer of water. The possible application of the obtained results to the solution of a conjugate problem is discussed.