Showing papers on "Thermal radiation published in 1998"

Thermal Diffusivity of Semitransparent Materials Determined by the Laser-Flash Method Applying a New Analytical Model

Abstract: We have developed an analytical model to determine the thermal diffusivity of nonscattering materials from samples with low optical thickness and opaque boundaries with arbitrary emissivities. The paper outlines the new analytical model and describes measurements on two samples: a microscope slide glass and a high-grade fused quartz plate. Results show that the new model applied to measurements on gold- or graphite-coated samples leads to the same results as if a conventional model is used on gold-coated samples.

Effects of radiative emission and absorption on the propagation and extinction of premixed gas flames

Abstract: Premixed gas flames in mixtures of CH4, O2, N2, and CO2 were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically thin radiation models, particularly when CO2, a strong absorber, is present in the unburned gas, Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H2O and CO2, most of the radiation from product H2O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum Of CO2 is broader at flame temperatures than ambient temperature: thus, some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO2 Via both mechanisms, some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison with experiment demonstrates the importance of reabsorption in CO2 diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emission-absorption spectra of the reactant and product -gases and their temperature dependence and cannot be predicted using gray-gas or optically thin model parameters. Applications to practical flames at high pressure, in large combustion chambers, and with exhaust-gas or flue-gas recirculation are discussed.

Analysis of thermal radiation effects on temperatures in turbine engine thermal barrier coatings

Abstract: Thermal barrier coatings on combustor liners and on turbine vanes and rotating blades are important for reducing metal temperatures in current and advanced turbine engines. Some coating materials such as zirconia are partially transparent to thermal radiation, and radiation within a coating will increase as temperatures are raised for higher efficiency engines. Hence, it is necessary to determine if radiation effects in a coating are a design consideration. For this purpose, the engine thermal environment is first summarized with regard to factors affecting radiative heat transfer. Radiative and thermal properties of zirconia are then considered, and methods of radiative analysis are briefly discussed. Typical temperature distributions and heat fluxes are given from the analysis of zirconia thermal barrier coatings on vanes and rotating blades, and on a combustor liner where the coating surface is expected to be covered with soot. The effects of various thermal conditions and heat transfer parameters are examined to indicate when radiation effects might be significant within a coating in a turbine engine. The largest effects were found in the combustor where coatings are subjected to large incident radiation. For coatings on turbine blades away from the combustor, and hence without large incident radiation, effects of radiation were found to be very small.

Low emissivity window films

Abstract: A solar control window film having low emissivity minimizes the transfer of thermal energy through window glass by thermal radiation between a temperature controlled interior environment and a uncontrolled exterior environment. The film is comprised of a transparent flexible polymeric substrate bearing various transparent thin film layers of coating materials, including a highly reflective metal. The film is devoid of laminations and laminating adhesives and provides high visual light transmission of up to 50% or more while maintaining a very low emissivity of 0.30 or less. The film is especially suited for retrofitting existing plain glass windows to convert the same to solar control windows.

Thermal radiation and amplified spontaneous emission from a random medium

Abstract: We compute the statistics of thermal emission from systems in which the radiation is scattered chaotically, by relating the photocount distribution to the scattering matrix---whose statistical properties are known from random-matrix theory. We find that the super-Poissonian noise is that of a blackbody with a reduced number of degrees of freedom. The general theory is applied to a disordered slab and to a chaotic cavity, and is extended to include amplifying as well as absorbing systems. We predict an excess noise of amplified spontaneous emission in a random laser below the laser threshold.

Magnetohydrodynamic flow past a plate by the presence of radiation

Abstract: An analysis of the unsteady magnetohydrodynamic flow of a viscous and electrically conducting fluid past to a plate by the presence of radiation is considered. The fluid is a gray, absorbing-emitting but nonscattering medium and the Rosseland approximation is used to describe the radiative heat flux in the energy equation. Analytical solutions for the mean temperature, velocity and the magnetic field have been derived and the effect of the radiation on the temperature is discussed.

Galileo probe measurements of thermal and solar radiation fluxes in the Jovian atmosphere

Abstract: The Galileo probe net flux radiometer (NFR) measured radiation fluxes in Jupiter's atmosphere from about 0.44 to 14 bars, using five spectral channels to separate solar and thermal components. Onboard calibration results confirm that the NFR responded to radiation approximately as expected. NFR channels also responded to a superimposed thermal perturbation, which can be approximately removed using blind channel measurements and physical constraints. Evidence for the expected NH3 cloud was seen in the spectral character of spin-induced modulations of the direct solar beam signals. These results are consistent with an overlying cloud of small NH3 ice particles (0.5-0.75 microns in radius) of optical depth 1.5-2 at 0.5 microns. Such a cloud would have so little effect on thermal fluxes that NFR thermal channels provide no additional constraints on its properties. However, evidence for heating near 0.45 bar in the NFR thermal channels would seem to require either an additional opacity source beyond this small-particle cloud, implying a heterogeneous cloud structure to avoid conflicts with solar modulation results, or a change in temperature lapse rate just above the probe measurements. The large thermal flux levels imply water vapor mixing ratios that are only 6% of solar at 10 bars, but possibly increasing with depth, and significantly subsaturated ammonia at pressures less than 3 bars. If deep NH3 mixing ratios at the probe entry site are 3-4 times ground-based inferences, as suggested by probe radio signal attenuation, then only half as much water is needed to match NFR observations. No evidence of a water cloud was seen near the 5-bar level. The 5-microns thermal channel detected the presumed NH4SH cloud base near 1.35 bars. Effects of this cloud were also seen in the solar channel upflux measurements but not in the solar net fluxes, implying that the cloud is a conservative scatterer of sunlight. The minor thermal signature of this cloud is compatible with particle radii near 3 gm, but it cannot rule out smaller particles. Deeper than about 3 bars, solar channels indicate unexpectedly large absorption of sunlight at wavelengths longer than 0.6 microns, which might be due to unaccounted-for absorption by NH3 between 0.65 and 1.5 microns.

Active thermal control of surfaces by steering heating beam in response to sensed thermal radiation

Abstract: An apparatus and method for controlling the temperature profile of the surface of a medium such as a solid or a liquid. The surface is imaged onto an array of detectors which generate an electronic output indicative of the temperature profile of the surface. A heating beam provided by a laser is scanned across the surface, providing radiation which heats the surface locally in a pattern based upon the detected temperature profile and the desired temperature profile.

The coherence length of black-body radiation

Abstract: We consider two-beam interference with black-body radiation. It is shown that the coherence length of thermal radiation is given by the formula . The coherence length corresponds to the mean wavelength of the thermal radiation.

Calculation of thermal emissivity for thin films by a direct method

Abstract: The emissivity variation of a body, according to the modifications of its surface, has been described by two kinds of arguments. A direct argument consists in adding the energy, leaving each element of volume $dV,$ considered as independent and incoherent Planckian radiators, weighted by its transmissions and its possible reflections. An indirect argument consists in assuming the validity of Kirchhoff's law. The emissivity is then deduced from the absorption coefficient calculated by using a huge collection of theoretical means. However, in the case of very thin films deposited on a substrate, the emissivity calculated according to their thickness does not give the same results, depending on the argument used. As a matter of fact, up to now the direct argument did not allow a description of interferential phenomena. Such phenomena are still observed when the film thickness is lower than, or of the same order of magnitude as the wavelength of the radiation concerned. On the other hand, the use of Kirchhoff's law requires delicate handling in the case of mesoscopical structure materials. Besides, the indirect method leads to an argument by default, which occults a part of the physics implied. Here, a direct model is proposed, only based on emission phenomena. This direct theory allows a description of the interferential behavior in thermal radiation, by taking into account the self-coherence of the emitted waves, in contrast to the previous direct approach. It is shown that this approach accounts for the experimental behavior of growing thin films.

Transient Thermal Effects of Radiant Energy in Translucent Materials

Abstract: When a solid or stationary fluid is translucent, energy can be transferred internally by radiation in addition to heat conduction. Since radiant propagation is very rapid, it can provide energy within a material more quickly than diffusion by heat conduction. Radiation emitted in a hot material can also be distributed rapidly in the interior. The result is that transient temperature responses including radiation can be significantly different from those by conduction alone. This is important for evaluating the thermal performance of translucent materials that are at elevated temperatures, are in high temperature surroundings, or are subjected to large incident radiation. Detailed transient solutions are necessary to examine heat transfer for forming and tempering of glass windows, evaluating ceramic components and thermal protection coatings, studying highly backscattering heat shields for atmospheric reentry, porous ceramic insulation systems, ignition and flame spread for translucent plastics, removal of ice layers, and other scientific and engineering applications involving heating and forming of optical materials. Radiation effects have been studied less for transients than for steady state because of the additional mathematical and computational complexities, but an appreciable literature has gradually developed. This paper will review the applications, types of conditions, and geometries that have been studied. Results from the literature are used to illustrate typical radiation effects on transient temperatures, and comparisons are made of transient measurements with numerical solutions.

Thermal Transport and Flow in High-Speed Optical Fiber Drawing

Abstract: The thermal transport associated with optical fiber drawing at relatively high drawing speeds, ranging up to around 15 m/s, has been numerically investigated. A conjugate problem involving the glass and the purge gas regions is solved. The transport in the preform/fiber is coupled, through the boundary conditions, with that in the purge gas, which is used to provide an inert environment in the furnace. The zonal method, which models radiative transport between finite zones in a participating medium, has been employed to compute the radiative heat transfer in the glass. The flow of glass due to the drawing process is modeled with a prescribed free-surface neck-down profile. The numerical results are compared with the few that are available in the literature. The effects of important physical variables such as draw speed, purge gas velocity and properties, furnace temperature, and preform diameter on the flow and the thermal field are investigated. It is found that the fiber drawing speed, the furnace temperature, and the preform diameter have significant effects on the temperature field in the preform/fiber, while the effects of the purge gas velocity and properties are relatively minor. The overall heating of the preform/fiber is largely due to radiative transport in the furnace and the changes needed in the furnace temperature distribution in order to heat the glass to its softening point at high speeds are determined.

Thermal radiation from small particles

Abstract: We derive an expression for the emission of electromagnetic radiation from small, highly excited, and isolated particles in a cold environment. The result is a generalization of Planck’s blackbody formula on two counts. One is due to the finite level densities of small particles. It is most pronounced for very small particles and for high photon energies. The other effect is the absence of stimulated emission which influences the low-energy part of the spectrum. We discuss some consequences for the interpretation of experimental emission spectra. @S1063-651X~98!03211-5#

Heat Transfer in Fibrous Insulations: Comparison of Theory and Experiment

Abstract: The validity of an improved theoretical model for radiative heat transfer through high-porosity e ber insulation is examined by comparison of experimental measurements with theoretical predictions of heat transfer. Radiative thermal conductivity of an optically thick medium is modeled by a diffusion approximation in which the spectral extinction properties are calculated by utilizing a rigorous treatment of the e ber medium scattering phase function and the composition of the e ber material. A semiempirical model is used to calculate the e ber-matrix conduction. The accuracy of the model is tested by comparison with experimental heat transfer data measured in vacuum for temperatures from 400 to 1500 K for three types of bonded silica e ber thermal insulation materials having different e ber size distributions. The validity of the modeling approach is demonstrated by the excellent agreement between the theoretical predictions and the experimental results.

Apparatus and method for determining the temperature of objects in thermal processing chambers

Abstract: The present invention is generally directed to a system and process for accurately determining the temperature of an object, such as a semiconductive wafer, by sensing and measuring the object radiation being emitted at a particular wavelength. In particular, a reflective device is placed adjacent to the radiating object, which causes thermal radiation being emitted by the wafer to be reflected multiple times. The reflected thermal radiation is then monitored using a light detector. Additionally, a reflectometer is contained within the system which independently measures the reflectivity of the object. The temperature of the object is then calculated using not only the thermal radiation information but also the information received from the reflectometer.

Computational analysis of coupled radiation-convection dissipative non-gray gas flow in a non-Darcy porous medium using the Keller-box implicit difference scheme

Abstract: The effects of thermal radiation parameter (F), transpiration (\gamma), Eckert number (Ec), Prandtl number (Pr), buoyancy (Grashof number Gr), a Darcy parameter (Re/Gr Da) and a Forcheimmer inertial parameter $(Fs Re^2/Gr Da)$ on two-dimensional free convective flow of an optically thin, near-equilibrium, non-gray gas past a vertical surface in a non-Darcy porous medium, are studied using the robust Keller finite-difference technique incorporating Newtonian quasilinearization and block-tridiagonal elimination. The Darcy-Brinkman-Forcheimmer inertial-viscous flow model is used for the momentum equation and the Cogley-Vincenti-Giles formulation is adopted to simulate the radiation component of heat transfer. The one-dimensional thermal radiation model works successfully for gases in the optically thin limit. Pseudo-similarity transformations are employed to simplify the highly non-linear partial differential equations for momentum and heat transfer into numerically manageable pseudosimilar ordinary differential equations which are solved with Keller's box method. Effectively, the radiation contribution is seen to take the form of a linear temperature term F\theta coupled with the streamwise pseudo-similar variable \xi. Local wall shear stress and local heat transfer rates are systematically computed for a wide selection of radiation parameter F values. The results are presented graphically for different gases.

A new way of solving transient radiative-conductive heat transfer problems

Abstract: One-dimensional transient energy transfer by conduction and radiation is solved for a finite medium. The semitransparent layer emits, absorbs, and scatters radiation (participating medium). The coupled transfer is solved analytically by considering the well-known two-flux approximation, assuming linear transfer and using the Laplace transform. The semitransparent layer can then be modeled by a matrix transfer function. The accuracy of the solution is verified in the case of sharp thermal excitation by a heat pulse on the front face. It is shown that this general model is very accurate for simulating both the limiting cases of purely scattering and purely absorbing media. In the latter case, the same modeling is derived using the kernel substitution technique, and very good agreement is achieved compared with numerical simulations. The resulting computation times are very small, and suggest that such a model can be used in the inverse approach of thermal problems involving semitransparent materials.

Radiative and Free Convective Effects of a MHD Flow Through a Porous Medium Between Infinite Parallel Plates with Time-dependent Suction

Abstract: The problem of magnetohydrodynamic free-convection flow, with radiative heat transfer in porour media subject to time-dependent suction of an incompressible and optically transparent medium has been solved making fairly realistic assumption. For a small-time-dependent perturbation of the fluid velocity and temperatures, the nonlinear problem is tackled by asymptotic approximation, giving solutions for steady-flow on which a first-order transient component is superimposed. The effect of heat radiation and free convection on the flow of the fluid is demonstrated analytically and quantitatively. The flow field is seen to be affected mainly by radiation and convection parameters, in addition to the porosity and magnetic factors.

Blackbody emission under laser excitation of silicon nanopowder produced by plasma-enhanced chemical-vapor deposition

Abstract: Previous results concerning radiative emission under laser irradiation of silicon nanopowder are reinterpreted in terms of thermal emission A model is developed that considers the particles in the powder as independent, so under vacuum the only dissipation mechanism is thermal radiation The supralinear dependence observed between the intensity of the emitted radiation and laser power is predicted by the model, as is the exponential quenching when the gas pressure around the sample increases The analysis allows us to determine the sample temperature The local heating of the sample has been assessed independently by the position of the transverse optical Raman mode Finally, it is suggested that the photoluminescence observed in porous silicon and similar materials could, in some cases, be blackbody radiation

Effect of thermal radiation on natural convection over cylinders of elliptic cross section

Abstract: The effect of conduction-radiation on natural convection flow of an optically dense viscous incompressible fluid along an isothermal cylinder of elliptic cross section has been investigated. The boundary layer equations governing the flow are shown to be nonsimilar. Full numerical solutions of the governing equations are obtained using the implicit finite difference method. The solutions are expressed in terms of the Nusselt number Nu against the eccentric angle α in the range [0, π]. The working fluid is taken to have unit value of the Prandtl number, Pr, and the effects of varying the Planck number,Rd, the surface temperature parameter, θw, and the parameterAO representing the ratio of the major and minor axes of the cylinder are investigated. From the present analysis it is found that the rate of heat transfer from the slender body is higher than from the blunt body and that these higher values become even higher due to an increase in the effect of radiation in the flow field.

Heat and Mass Transfer

Abstract: troduction: Technical applications Heat conduction and mass diffusion Convective heat and mass transfer. Single phase flow Convective heat and mass transfer Flow with phase change Thermal radiation. - Appendices (supplements, property data, solutions to the exercises) Literature Index.

Free-Free Radiation from Dense Interstellar Shock Waves

Abstract: We present detailed calculations of the free-free emission behind shock waves in media where the postshock gas may be optically thick at radio frequencies. Using a numerical shock code, we solve the free-free radiative transfer equation at frequencies of 1.5, 5, and 15 GHz for a family of planar shock models that cover a wide range of densities (1000 cm-3 ≤ n0 ≤ 109 cm-3) and shock velocities (30 km s-1 ≤ V0 ≤ 300 km s-1). These models also predict how different preshock magnetic fields and viewing angles affect the overall free-free emission. As the shock velocities and preshock densities increase, the free-free spectral indices generally rise from optically thin values (~-0.1) and approach optically thick ones (~2.0). We find that for V ≥ 120 km s-1, the ionized precursor produced by the emergent postshock UV radiation can itself become optically thick to free-free radiation when the preshock density exceeds 106 cm-3. We also find that for some combinations of shock parameters, the superposition of radiation from the precursor onto that of the postshock region can produce spectral indices exceeding 2.0. In addition, we find that magnetic fields can suppress the free-free radiation only if they are strong enough to lower the magnetosonic Mach number below 4 for V ≥ 100 km s-1. An observer who knows the resolved angular sizes, fluxes, and spectral indices of a thermal radio source can use our grids to determine the preshock density, shock velocity, and viewing angle to the source. Possible applications include HH objects, shocked cloudlets, and accretion shocks deep within molecular clouds. As an example, we use our results to predict shock parameters for HH 1-2 and several thermal radio sources in Cep A east. We find that the predicted shock parameters for HH 1 and 2 agree well with those inferred from optical line ratios and line profiles. Radio emission from sources in Cep A are explained reasonably well as arising from thermal radiation behind a dense, fast shock like one might expect from a stellar jet deeply embedded within the molecular cloud core.

Free convection-radiation interaction from an isothermal plate inclined at a small angle to the horizontal

Abstract: The interaction of free convection with thermal radiation in boundary layer flow from an inclined isothermal plate is studied numerically. Introducing appropriate transformations the equations governing the flow are expressed in the form of local nonsimilarity equations valid near the leading edge as well as in the downstream region. A group of transformations is also introduced such that the flow near the leading edge and far downstream can be described. Heated upward facing plates with positive and negative inclination angles are investigated. When the inclination is negative the boundary layer separates from the surface and the numerical solutions can be extended downstream past the point of separation. From the present investigation it may be concluded that the position of the separation point moves away from the leading edge with the increase of either of the thermal radiation parameter or the surface temperature parameter of the heated surface.

Process and apparatus for splitting flat pieces of brittle material, particularly of glass

Abstract: The method involves a relative motion of the workpiece material (1) and a symmetric heat radiation spot (3) along a separation line (2), with subsequent cooling of the heated separation line section. The heat radiation spot has a higher radiation intensity on its edge zone and a maximum temperature at its back end. The spot with higher heat radiation on a V or U shaped curve (4) has a maximum temperature which is below the melting point of the workpiece material at the vertex (16). The apparatus includes a heat radiation source, particularly a laser, and an optical system for producing a radiation spot with the required characteristics. It further includes means for producing a relative motion of the workpiece and the heat radiation spot, as well as means for subsequent cooling the radiated separation line.

Radiative divertor plasmas with convection in DIII-D

Abstract: The radiation of divertor heat flux on DIII-D is shown to greatly exceed the limits imposed by assumptions of energy transport dominated by electron thermal conduction parallel to the magnetic field. Approximately 90% of the power flowing into the divertor is dissipated through low Z radiation and plasma recombination. The dissipation is made possible by an extended region of low electron temperature in the divertor. A one-dimensional analysis of the parallel heat flux finds that the electron temperature profile is incompatible with conduction dominated parallel transport. Plasma flow at up to the ion acoustic speed, produced by upstream ionization, can account for the parallel heat flux. Modeling with the two-dimensional fluid code UEDGE has reproduced many of the observed experimental features.