Showing papers on "Thermal radiation published in 1990"

A model for lava flows with two thermal components

Abstract: This paper presents a mathematical model for the cooling of an active flow with two separate thermal components, one of which is a crust that cools by radiation and thickens with time and the other an inner core that is vertically isothermal and partially exposed at the top surface, where heat is lost by radiation. This model provides a more realistic description of active lava flows than the existing models that assume thermal homogeneity in each vertical cross section perpendicular to the direction of flow advance. The results of the model were found to compare favorably with existing field data. The effects of different crustal thickening rates, fractional areas of crust, and eruption temperatures on the cooling of the two thermal components were examined.

Radiative and free-convection effects on the oscillatory flow past a vertical plate

Abstract: The interaction of free convection with thermal radiation of the oscillatory flow past a vertical plate is studied. The Rosseland approximation is used to describe the radiative heat flux in the energy equation.

Dependent absorption and extinction of radiation by small particles

Abstract: This study presents an analytical model to evaluate the dependent absorption and extinction characteristics of dense particulate systems. Simple expressions are obtained by considering statistical averages based on the random character of the particulate medium. Departure from the assumption of independent scattering and absorption of radiation originates from two mechanisms; perturbation of the internal field of each particle by the presence of other particles, and coherent addition (i.e., taking into account the constructive/destructive interference) of the far-field scattered radiation. Dependent scattering has been previously studied analytically by considering the second mechanism only. Dependent absorption, which is due to the former, has not been considered in the literature even though absorption is more important than scattering in the extinction of radiation by small (Rayleigh) absorbing particles.

Experimental observation of laser-induced radiation heat waves.

Abstract: The propagation of a radiation heat wave through a thin foil of solid gold was investigated experimentally. The wave is driven by the intense thermal radiation in 1--3-mm-diam gold cavities heated by an intense laser pulse (duration 0.8--0.9 ns, wavelength 0.35 \ensuremath{\mu}m) to temperatures of more than 200 eV. Evidence of the propagating wave was obtained from the delayed onset of intense thermal emission from the outer side of the foil. The results agree with theoretical predictions for a self-similar ablative heat wave and with numerical simulations.

Single-beam interferometry of a thermal bump.

Abstract: A novel single-beam interferometric measurement of a thermal bump produced by an intensity modulated and focused laser beam is described. The effects of the ac change in optical reflectivity and the displacement of the surface are determined independently by fitting data to a phenomenological theory. This technique is insensitive to sample vibrations and variations in optical path resulting from thermal fluctuations.

Nonequilibrium and equilibrium shock front radiation measurements

Abstract: The intensities of the radiation emitted behind a normal shock wave in N were measured in an electric-arc driven shock tube at a shock velocity of 6.2 km/sec. Both a time-resolved broad-band radiation intensity measurement and a time-frozen spectral measurement were conducted. The rotational and vibrational temperatures are determined in both the equilibrium and the nonequilibrium regions. The results are compared with the similar data obtained by Allen et al. (1961). The measured rotational temperature seems to be in nonequilibrium, contradicting the two-temperature assumption of Park (1988), but the measured vibrational temperature agrees with Park's model.

Radiative transfer in stratified atmospheres: Development and verification of a unified model

Abstract: A reliable and efficient discrete method is verified for multiple scattering, radiative- transfer calculations in vertically inhomogeneous, non-isothermal atmospheres in local thermodynamic equilibrium. The linear-in-optical-depth approximation to the Planck function is used to obtain accurate solutions for thermal radiation. We show that this approximation significantly improves computing efficiency while still maintaining adequate accuracy. Using this multiple-scattering scheme, we have constructed and validated a radiation model for stratified atmospheres, taking into account molecular (Rayleigh) scattering and absorp- tion/emission. The exponential sum fitting of transmissions technique is utilized to parameter- ize gaseous absorption, thereby achieving a unified treatment of shortwave and longwave radiative transfer. To validate the model, we present computed fluxes and heating/cooling rate profiles for the five McClatchey atmospheres. The results are compared with other models having different spectral resolution (As) and gaseous scattering and absorption/ emission structure. Specifically, we compare broad-band (As > 100 cm-'), narrow-band (As < 100 cm-') and line-by-line computations and find that at the expense of accuracy by a few W-m-* for flux or a few tenth "C/day for heating/cooling rate computations, the broad-band models are very fast and may be suitable for many applications. We also find that in spite of good agreement between fluxes (at the top and bottom boundaries) computed by different methods, the heating/cooling rate profiles may differ substantially due to compen- sating errors.

Issues concerning shock temperature measurements of iron and other metals

Abstract: Determining the melting temperature of iron at the Earth's inner and outer boundary near 3 Mbar is an important problem in geophysics. A knowledge of the pressure, volume, temperature, and internal energy of solid and molten iron at megabar pressures and several thousand degrees provides a critical constraint for modeling the Earth's core. An optical method measuring the thermal radiation from the thin metal film deposited on an optically transparent window material is described. Issues which need to be addressed to obtain accurate thermodynamic shock temperatures of metals from the thermal radiation measured are discussed. These issues include characteristics of metal films, thermal conduction occurring at the metal/window interface, optical and thermal properties of metal and window material, and characteristics of the radiation measured.

Finite element/Newton method for the analysis of Czochralski crystal growth with diffuse‐grey radiative heat transfer

Abstract: A quasi-steady-state, integrated system model describing high temperature heat transfer, solidification and the action of capillarity in the Czochralski crystal growth process is solved by a finite element/Newton method. The numerical analysis couples the calculation of the temperature field in all phases and the determination of the melt/crystal and melt/gas interfaces and the crystal radius free boundaries. The analysis includes conductive heat transfer in the melt, crystal, crucible, pedestal, heater and the surrounding insulation and diffuse-grey radiation, which couples the heat transfer between surfaces, the crystal radius and the melt/gas free boundary through the view factors. Finite element approximations are used to reduce the entire problem to a coupled set of non-linear algebraic equations. These are solved simultaneously by Newton's method with the Jacobian matrix computed by a combination of closed form expressions and finite difference approximations. Quadratic convergence of the Newton iteration is demonstrated along with a factor of four increase in computational efficiency over a successive iteration procedure that decouples the calculation of radiation from the rest of the heat transfer model.

Characterizing windows for shock wave radiation studies

Abstract: Sapphire and lithium fluoride have been used as windows in shock wave experiments to transmit reflected light from lasers to perform velocity measurements and to measure thermal radiation for temperature measurements, while maintaining substantial pressure at the interface. In the latter case there is a fundamental problem in that the radiation coming through the window is not that of the shocked sample but is the radiation from the interface, which in the ideal case is at some temperature between the temperature of the sample and the relatively cold windows. The interface temperature can be used to calculate the temperature of the sample, but values for the thermal diffusivities of both the window and the sample must be used in the heat flow calculation. Experiments are described that may resolve that difficulty. Two other problems thought to exist in these measurements are due to interface imperfections, such as gaps and the transparency of the windows under shock loading. Preliminary experiments are reported, which showed that in the pressure regime studied, the sapphire remained transparent. These experiments also showed that imperfections at the interface are a major problem, but it is a recognizable one and can be minimized or eliminated so that meaningful temperature measurements can be made.

Transient combined conduction and radiation with anisotropic scattering

Abstract: The analysis of transient combined radiation and conduction heat transfer in an absorbing, emitting, and anisotropically scattering planar material is investigated theoretically The medium boundaries are assumed with specified temperatures Both specular and diffuse reflectivities are included The Crank-Nicolson method is used to solve the transient energy equation, and the nodal approximation provides the solution for coupled radiative transfer This solution method may be described as a finite-difference/nodal approximation method and would replace the governing energy and radiative transfer equations by a set of algebraic equations Using the method, the present study examines the effect of scattering anisotropy on combined conduction and radiation heat transfer Results are presented for temperature and heat-flux distributions, and results for isotropic scattering are compared to existing data The agreement is excellent 21 refs

The relative timing of microwaves and X-rays from solar flares

Abstract: The delay of microwaves relative to hard X-rays from solar flares is investigated For short-time scale delays, it was found that for reasonable thick-target model parameters and for injected electron distributions which are separable in time, energy, pitch angle, and position, the observed delay cannot be explained by magnetic trapping of electrons in the corona It can be accounted for if higher energy microwave producing electrons are accelerated later than lower energy hard X-ray producing electrons For larger time-scale delays, it is found that the flux during the rising phase of the flares can be explained well in terms of the thick-target model, but during the decay phase this model predicts too little microwave flux A number of possibilities for this excess microwave flux are explored including spectral hardening, magnetic trapping, and thermal synchrotron and free-free emission

Design principle for simultaneous emissivity and temperature measurements

Abstract: One of the critical questions regarding radiation thermometers is how to obtain the true temperature of an emitting opaque body with an unknown emissivity and with unknown circumferential radiations. We suggest that two enclosed chambers with given temperatures and emissivities be used to cover part of the area of the emitting body. By use of two simultaneous radiothermometer readings, both the true temperature and the emissivity of the graybody can be calculated.

Monte Carlo simulation of indoor radiant environment

Abstract: This paper describes a numerical method of radiant environment analysis in a complicated enclosure containing human bodies and furniture. An arbitrary configuration expressed in a three-dimensional co-ordinate system can be treated by this method. A unique space index and another surface index are proposed to specify the geometry of solid body and surface in the enclosure. The Monte Carlo method is used to determine direct interchange areas, which are then transformed into total exchange areas using the zone method. Energy balance equations accounting for thermal radiation, convection, wall conduction and air ventilation are then formulated for each solid surface segment in the enclosure. A human body model with skin temperatures and clothing is also proposed to simulate its heat release and to predict local thermal sensation around the body. As a practical application, this method is applied to the analysis of the radiant environment in a floor-heated meeting room with eight people seated around a table.

Effect of radiation shield angle on temperature and stress profiles during rapid thermal annealing

Abstract: The effects of radiation shield angle and oven-temperature ramping rates on the temperature and thermal stress profiles in a gallium arsenide wafer undergoing rapid thermal annealing are studied. The numerical model of the heat transfer in a cylindrical oven considers conduction in the wafer radiative heat transfer from all oven and shield surfaces to the wafer. All simulations show that at some location in the wafer the induced thermal stress exceeds the critical stress. These results indicate that at high temperatures (T>750 degrees C) it is very difficult to maintain a sufficiently flat temperature profile such that the induced thermal stress is maintained below the critical stress throughout the wafer. Possible ways to minimize the induced thermal stress during the annealing process using a radiation shield and specific oven-temperature ramping rates are discussed. >

Two-dimensional convection and radiation with scattering from a Poiseuille flow

Abstract: Two-dimensional combined convection and radiation heat transfer from a gray scattering fluid in a reflecting channel is considered. The model, represented by a set of simultaneous nonlinear integro-partial differential equations, is solved numerically. The effects of aspect ratio, conduction-radiation parameter, scattering albedo, and wall emissivity, are systematically investigated. It is found that these parameters have a significant influence on the temperature field and alter the radiative and convective fluxes at the hot and cold walls. In particular, when radiation effects are considerable, the heat-transfer characteristics of the fluid at the hot and cold walls are very different.

The polarization of thermal radiation from supermassive accretion disks

Abstract: Thermal radiation from an accretion disk is a promising explanation for the optical-ultraviolet 'bump' noted in the spectra of active galactic nuclei. However, the expected polarization of radiation from a standard thin disk conflicts with observation. The polarization of light emitted by a thin disk with a rough surface and by a thick disk (radiation torus) is computed. Improved agreement with observations is found, particularly in the case of a thick disk.

Entropy and other thermodynamic properties of classical electromagnetic thermal radiation.

Abstract: Building upon previous work, several new thermodynamic properties are found for classical electromagnetic random radiation in thermal equilibrium with classical electric dipole harmonic oscillators. Entropy is calculated as a function of temperature and as a function of the positions of the dipole oscillators. In the process, a new derivation is obtained for what is often called Wien's displacement law. The original derivation of this law makes a number of implicit assumptions not found in the present derivation, which prevents the original analysis from being sufficiently general to address an important class of thermal radiation spectrum candidates: namely, those that are nonzero at T=0. While leading up to the entropy calculation, a number of other thermodynamic properties are deduced. For example, a natural development is presented for reformulating the St\'efan-Boltzmann law to correspond to experimental observations about changes in thermal radiation energy. Also, the Rayleigh-Jeans spectrum is shown to conflict with basic concepts of thermodynamic processes, and asymptotic limits are found for the spectrum of classical electromagnetic thermal radiation. One asymptotic restriction arises from the demand of finite specific heat for thermal radiation. This restriction is sufficient to ensure that the classical electrodynamic system of dipole oscillators and thermal radiation must obey the third law of thermodynamics. The calculations described here include full nonperturbative evaluations of retarded van der Waals thermodynamic functions.

Quantum electrodynamics based on self-fields: On the origin of thermal radiation detected by an accelerating observer.

Abstract: We continue with our series of papers concerning a self-field approach to quantum electrodynamics that is not second quantized. We use the theory here to show that a detector with a uniform acceleration {ital a} will respond to its own self-field as if immersed in a thermal photon bath at temperature {ital T}{sub {ital a}}={h bar}a/2{pi}kc. This is the celebrated Unruh effect, and it is closely related to the emission of Hawking radiation from the event horizon of a black hole. Our approach is novel in that the radiation field is classical and not quantized; the vacuum field being identically zero with no zero-point energy. From our point of view, all radiative effects are accounted for when the self-field of the detector, and not the hypothetical zero-point field of the vacuum, acts back on the detector in a quantum-electrodynamic analog of the classical phenomenon of radiation reaction. When the detector is accelerating, its transformed self-field induces a different back reaction than when it is moving inertially. This process gives rise to the appearance of a photon bath, but the photons are not real in the sense that the space surrounding the accelerating detector is truly empty of radiation, a factmore » that is verified by the null response of an inertially moving detector in the same vicinity. The thermal photons are in this sense fictitious, and they have no independent existence outside the detector.« less

Superconductive magnet with thermal diode

Abstract: A superconductive magnet having at least one superconductive coil is provided. A thermal radiation shield is situated inside a vacuum vessel and the thermal radiation shield encloses the superconductive coil. A thermal diode is provided for thermally linking the superconductive coil and the thermal radiation shield when the thermal radiation shield is colder than the superconductive coil.

Interaction of surface radiation with convection in crystal growth by physical vapor transport

Abstract: Growth of single crystals from vapor in closed ampoules is governed by an intricate interplay between mass, momentum and heat transfer processes. The objective of this study is to examine and isolate the effects of surface radiation heat transfer on the vapor transport process using a mathematical model. The model consists of a set of coupled nonlinear partial differential equations for conservation of mass, momentum, energy and species, and the integrodifferential equations which represent radiative exchange. It depends on five important physical parameters. These are Grashof number, Prandtl number, Schmidt number, aspect ratio and the radiation-conduction number. The effects of these dimensionless groupings are systematically investigated. From the cases examined, it is concluded that surface radiation can change the flow structure appreciably. This is especially true in microgravity environment where radiation competes primarily with conduction in modifying the thermal profiles. The numerical results also show that in the presence of radiation, the top heating configuration (source on top) is no longer stable and that near the growing crystal, radiation-induced vortices can introduce significant nonuniformities in the growth flux.