Showing papers on "Thermal radiation published in 1995"

Generalized Planck's radiation law for luminescence via indirect transitions

Abstract: Based on microscopic transition probabilities and on the principle of detailed balance, absorption and emission rates are calculated for indirect transitions. It is found that the emission rate at a photon energy ħω can be expressed by the absorption coefficient for the same photon energy in the same way as for direct transitions. This relation is quite generally valid including cases where the electrons in the exited state (conduction band) and the electrons in the ground state (valence band) are distributed according to two different quasi-Fermi distributions. A generalized Planck's law is formulated for luminescence which contains a nonzero chemical potential of the emitted photons as the only difference to the description of thermal radiation.

Radiative heat transfer by the Monte Carlo method

Abstract: Part 1 Principles of radiation: thermal radiation radiation heat transfer. Part 2 Principles of Monte Carlo methods: formulation methods of solution special treatises. Part 3 Applications of the Monte Carlo method: two-dimensional systems some industrial applications references applications on disk list of variables in computer programs.

Theoretical modeling of carbon content to minimize heat transfer in silica aerogel

Abstract: Silica aerogel has a small absorption coefficient over the range 3–8 μm where significant thermal energy is transferred by radiation. Adding carbon to silica aerogel reduces thermal radiation but increases solid conduction. Whether the total energy transfer increases or decreases depends on the carbon content. This paper presents a theoretical method for determining the optimal carbon-loading level in silica aerogel to minimize the energy transfer. This method includes calculation of heat transport by coupled conduction and radiation through aerogel which is optically thin in some spectral ranges and thick in others, and the calculation of solid conductivity and spectral absorption coefficient, both of which vary with the carbon content. At ambient temperature, about 8% carbon in silica aerogel can lower the total energy transfer by about 1 3 . At temperatures as high as 600 K, non-opacified aerogel has a total energy transfer that is 10 times bigger than that of opacified aerogel with optimal carbon content. The optimal carbon content that minimizes total energy transfer increases linearly with temperature.

Profiled substrate heating utilizing a support temperature and a substrate temperature

Abstract: An apparatus and method for thermal processing, and more particularly for rapid thermal processing wherein a first thermal radiator generates and projects a first pattern of thermal radiation onto a first surface of a substrate, and wherein a second thermal radiator generates and projects a second pattern of thermal radiation onto a second surface of the substrate. The temperatures on the first and second surfaces are sensed by temperature sensors. A mechanism for selectively controlling the first and second thermal radiators in response to the temperature sensors causes a prescribed temperature profile to be produced within the substrate.

Theoretical studies of sonoluminescence radiation: Radiative transfer and parametric dependence.

Abstract: We present results for sonoluminescent (SL) radiation from a strongly modulated air bubble in water. The SL pulse is due to high temperatures in the bubble that are produced by the collapse of a shock wave. The dependence of SL radiation on acoustic pressure and initial bubble size is discussed, as well as the inclusion of various mechanical energy loss mechanisms. We find that the inclusion of loss terms, especially due to radiation, is necessary in order to understand the bubble dynamics, the resulting SL pulse, and some recent experiments.

Method and apparatus for wavevector selective pyrometry in rapid thermal processing systems

Abstract: A method and apparatus for optical pyrometry in a Rapid Thermal Processing (RTP) System, whereby the radiation used to heat the object to be processed in the RTP system is in part specularly reflected from specularly reflecting surfaces and is incident on the object with a particular angular distribution, and the thermal radiation from the object is measured at an angles different from the angle where the incident radiation specularly reflected from the surface of the object is a maximum.

Profiled substrate heating

Abstract: An apparatus and method for thermal processing, and more particularly for rapid thermal processing wherein a first thermal radiator generates and projects a first pattern of thermal radiation onto a first surface of a substrate, and wherein a second thermal radiator generates and projects a second pattern of thermal radiation onto a second surface of the substrate. The temperatures on the first and second surfaces are sensed by sensing means. Means for controlling the first and second thermal radiator means in response to the sensing means cause a prescribed temperature profile to be produced within the substrate.

Modeling Effects of Soot and Turbulence-Radiation Coupling on Radiative Transfer in Turbulent Gaseous Combustion

Abstract: A three-dimensional combustion model which couples turbulent flow statistics with chemical reactions and radiative heat transfer is used to evaluate the effect of soot and turbulence-radiation coupling on radiative transfer in an industrial-scale furnace. Radiation and soot formation models are presented which include the nonlinear dependencies between fluid turbulence effects, soot formation and radiative absorption and emission in the participating medium. The discrete-ordinates method is used to solve the radiation intensity field. Soot formation is predicted with a formation-destruction model based on local volatile content and stoichiometry. The impact of soot and turbulence-radiation coupling was evaluated based on incident surface fluxes and centerline temperature and emission. Result trends were as expected, but suggest that the magnitude of soot effects on radiative transfer may be less than originally thought for the type of furnace and operating conditions modeled in this research. Pre...

Thermal conductivity of silica aerogel powders at temperatures from 10 to 275 K

Abstract: Thermal conductivities of physical mixtures of evacuated silica aerogel powders mixed with carbon black were measured as a function of temperature and external load. The measured thermal conductivity results from solid conduction and infrared radiative transport. The latter was quantified with infrared-optical transmission and reflection measurements in the wavelength range 2.3–200 μm and by solving the radiative heat transfer equation. Radiative contributions to the total thermal conductivity are found to be negligible at temperatures g′ = λ mono λ pow is between 3 and 10, depending on the powder type and external pressure load. The quantity g = λ vs λ pow , which compares the measured thermal conductivity of aerogel powders with that of vitreous silica, is temperature-independent only for temperatures > 50 K and is of the order of 500–1500. For lower temperatures it increases strongly due to the limitation of the phonon mean free path caused by the nanoporosity of the aerogel. Silica aerogel powders provide a thermal conductivity considerably lower than that of perlite, which is often used for low-temperature thermal insulation.

Measurement of radiation-driven shock-induced mixing from nonlinear initial perturbations

Abstract: We present the first experimental measurement of the growth of a mixing region at a density interface from a high-amplitude, initially nonlinear perturbation. Thermal radiation from a gold {ital Hohlraum} irradiated by the Nova laser was used to drive a shock across a perturbed plastic-foam interface in a cylindrical shock tube attached to the {ital Hohlraum}. When the effects of target decompression are removed, the width of the mix region was found to grow logarithmically with time---in agreement with recent theories for the nonlinear phase of the Richtmyer-Meshkov instability.

In-Depth Absorption of Externally Incident Radiation in Nongray Media

Abstract: During flame spread along a surface, the thermal radiation emitted by high-temperature combustion products supports the advancement of the flame front. To model the response of the solid to the externally incident radiation, it is necessary to consider the spectral variation of radiative properties of the solid. For highly absorbent solids, such as wood or particle board, almost all of the externally incident radiation is absorbed at or very near the surface. However, for highly semitransparent materials, such as a plastic material whose surface is not clean, the externally incident radiation is absorbed both at the surface and within the material. In this work, the objective is to study both theoretically and experimentally the importance of in-depth radiation. A transient, one-dimensional model is formulated and solved numerically. The spectral radiative properties employed in the radiation model have been obtained from separate experiments on polymethylmethacrylate (PMMA), a clear plastic. The model demonstrates the importance of in-depth absorption. Model results exhibit the same trend as those revealed in experiments for the rise in surface temperature of the sample.

Effect of radiative and convective heat transfer on thermal transients in power cables

Abstract: A model to study the thermal behaviour of a system of directly buried electrical power cables is presented. The model completely describes the conditions of heat transfer at the soil-air interface. In particular, the different incidences that the various radiative components have on the evolution of temperature in the system examined are evaluated. The method is applied to the case of a three-phase line placed under a daily cyclic load, based on experimental observations made on a line with cable of similar characteristics. Different evaluations are carried out assuming different characteristics for the soil and the climatic conditions.

Characteristics of Radiation Absorption in Metallic Particles

Abstract: Thermal radiation absorption in metallic particles is an important phenomenon in many contemporary laser-processing techniques, including laser cladding of coating materials and laser cleaning of particulate contaminations. In this work, the Drude free-electron theory and electromagnetic wave theory are utilized to characterize the internal absorption of CO{sub 2} laser radiation in aluminum, chromium, and nickel particles. The results show that metallic particles have unique radiation properties. Radiation absorption in large particles occurs only in a very narrow region of the front particle surface, which results in inefficient radiation absorption. On the other hand, micron and submicron particles can absorb radiation very efficiently due to the strong diffraction effect at the particle surface. For extremely small particles (e.g., nanometer particles), radiation absorption becomes less efficient. The particle absorption efficiency is found to increase with temperature, and this temperature dependence can be determined from those of flat metal surfaces at the normal incidence. 28 refs., 8 figs., 1 tab.

Atomic species radiation from air modeled with direct simulation Monte Carlo method

Abstract: This article deals with the evaluation of thermal radiation from atmospheric atomic species with the direct simulation Monte Carlo method under conditions of nonequilibrium. Models to calculate bound-bound, freebound, and free-free radiation are introduced. This new scheme attempts to improve and extend an earlier approach towards a more detailed modeling of the mechanism of radiation. The results are compared against experimental results and the differences are discussed and evaluated.

Measurement of thermal diffusivity by optical excitation and infrared detection of a transient thermal grating

Abstract: A contactless technique is described for measuring the thermal diffusivity in a direction parallel to the surface of a thin sample by pulsed‐laser excitation and thermal‐radiation detection of a thermal grating on the surface. The transient thermal grating (TTG) with periodicity Λ in the range 20–1000 μm is created with a novel interferometer which does not require a long‐coherence‐length laser. Fast thermometry is achieved with an infrared detector focused on a spot ∼30 μm in diameter to examine in detail the shape and the time dependence of the TTG. The performance of the system is demonstrated on a thin Ti foil with Λ ranging over an order of magnitude. Data are also presented for an 11 μm thick diamond film with a factor‐of‐20 higher diffusivity than for Ti. The results indicate that the system is well suited to measuring local diffusivities in regions as small as ∼100 μm in thin, free‐standing films.

Coating substance with low emissivity in the heat radiation range

Abstract: A coating substance with a low emissivity or high reflectivity in the heat radiation wavelength range. A binder with high transparency in the heat radiation range, especially in the range of wavelengths from 3 to 50 μm, contains particles having a high transparency in this range and the refractive index of which in the heat radiation wavelength range differs from that of the binder.

Miniaturized fabry-perot spectrometer for optical analysis

Abstract: A miniaturized spectrometer for gas concentration measurement includes a radiation source for admitting electromagnetic radiation onto the gas to be measured, a detector for detecting the radiation transmitted through or emitted from the gas, an electrically tunable Fabry-Perot interferometer placed in the path of the radiation prior to the detector, control electronics circuitry for controlling the radiation source, the interferometer and the detector. The radiation source, the detector, the interferometer and the control electronics are integrated in a miniaturized fashion onto a common, planar substrate and the radiation source is an electrically modulatable micromechanically manufactured thermal radiation emitter.

Transient conductive and radiative heat transfer in a silica window

Abstract: A detailed thermal analysis for transient conduction and nongray radiation of glass subjected to high levels of convection is discussed. Only a small variation of results due to the best available refractive index data was found. A peak value that does not coincide with the peak temperature was created by the radiation heat flux. This effect in turn produced a hump in the results for dimensional conduction heat flux, but has little effect on the temperature distribution. However, the highest temperature always occurred at the first boundary, which allows the prediction of the onset of softening and subsequent distortion optics. 5 refs.

Refractive Index Effects on Transient Cooling of a Semitransparent Radiating Layer

Abstract: Refractive index effects are examined for transient cooling by radiation and conduction of a gray semitransparent layer. The layer is in a vacuum and so its heat loss is only by internal radiation leaving through its boundaries. Emission within the layer and energy reflected internally from its boundaries increase with the material refractive index. The reflected energy and heat conduction act to distribute energy across the layer and partially equalize the transient temperature distributions. For some conditions significant temperature gradients develop near the boundaries. The numerical solution method provides accurate transient temperature distributions in these regions so that the predicted radiative losses are not in error. An implicit finite difference procedure is used with nonuniform space and time increments. The integrals for the local radiative source in the energy equation are evaluated by Gaussian integration. a c D EI, N n q q,

Experimental study of the thermal radiation attenuation of sprays from selected hydraulic nozzles.

Abstract: SUMMARY This paper describes a series of experiments to determine the thermal radiation attenuation of four selected hydraulic nozzles operating over the range of 350 to 7500 dmin water flow. The experiments involved interposing a fine water spray between a gas-fired heat source and a means of recording heat flux at a fixed distance from that source. Each spray is evaluated by the effect volume of water flowing, volume median drop size and mean drop velocity had on the thermal radiation attenuation. The highest water flow resulted in the maximum recorded attenuation, however, sprays having less than half the volume of water flowing could achieve similar attenuation.

Approximate formulation for coupled conduction and radiation through a medium with arbitrary optical thickness

Abstract: In evaluating the thermal properties of some porous media, such as aerogels, difficulties arise due to the following: (1) The media are transparent in some spectral intervals and opaque over other intervals and cannot therefore be treated as optically thin or as optically thick; (2) the conductive and the radiative heat transfer are coupled. In general, the coupled conduction and radiation transport in a nongray medium with nongray walls is described mathematically by a nonlinear, integrodifferential equation, which is difficult to solve. 9 refs., 1 fig.

Thermal analysis of in-situ curing for thermoset, hoop-wound structures using infrared heating: Part I - predictions assuming independent scattering

Abstract: A curing process for unidirectional thermoset prepreg wound composite structures using infrared (IR) in-situ heating is investigated. In this method, the infrared energy is from all incident angles onto the composite structure to initiate the curing during processing. Due to the parallel geometry of filaments in wound composite structures, the radiative scattering coefficient and phase function within the structure depend strongly on both the wavelength and the angle of incidence of the IR incident radiation onto the fibers. A two-dimensional thermochemical and radiative heat transfer model for in-situ curing of thermoset, hoop-wound structures using IR heating is presented. The thermal transport properties that depend on the process state are also incorporated in the analysis. A nongray, anisotropic absorbing, emitting, and scattering unidirectional fibrous medium within a matrix of nonunity refractive index is considered. The temperatures and degrees of cure within the composite during processing are demonstrated numerically as a function of the configuration of IR heat source, nondimensional power input, mandrel winding speed, and size of wound composite. Comparison between the numerical result and experimental data is presented.