Showing papers on "Thermal radiation published in 1980"

Heat transfer model for cw laser material processing

Abstract: (Received 19 September 1979; accepted for publication 12 October 1979)A three‐dimensional heat transfer model for laser material processing with a moving Gaussian heat source is developed using finite difference numerical techniques. In order to develop the model, the process is physically defined as follows: A laser beam, having a defined power distribution, strikes the surface of an opaque substrate of infinite length but finite width and depth moving with a uniform velocity in the positive x direction (along the length). The incident radiation is partly reflected and partly absorbed according to the value of the reflectivity. The reflectivity is considered to be zero at any surface point where the temperature exceeds the boiling point. This is because a ’’keyhole’’ is considered to have formed which will act as a black body. Some of the absorbed energy is lost by reradiation and convection from both the upper and lower surfaces while the rest is conducted into the substrate. That part of the incident r...

Upper limit for the conversion of solar energy

Abstract: A semiconductor in the solar radiation field acts as a thermal electronic engine. It converts absorbed radiation heat into chemical energy of the excited electron-hole gas. In flow equilibrium, a homogeneous semiconductor gives off this chemical energy by radiative recombination to the surroundings. If provision is made, as by a p-n junction, to divert the excited electrons and holes, before they recombine, from their point of generation, their chemical energy may be converted into electrical energy. The ratio of this chemical energy current, which constitutes an upper limit for the obtainable electrical energy current, to the absorbed heat current is computed as a function of the value of the bandgap of the semiconductor. Under the assumption that the absorptivity of the electron-hole system of the semiconductor is unity for photon energies larger than the bandgap and zero for smaller photon energies, the conversion efficiency for unfocussed sunlight has a maximum of 30 percent for a bandgap of 1:3 eV.

Analytical models for thermal radiation in fibrous insulations

Abstract: The study of thermal radiation in light-weight fi brous insulations is of fundamental importance since a significant portion of the total heat transfer is composed of thermal radia tion. The analyt...

The Thermal Balance of Venus in Light of the Pioneer Venus Mission

Abstract: Instruments flown on the Pioneer Venus orbiter and probes measured many of the properties of the atmosphere of Venus which control its thermal balance and support its high surface temperature. Estimates based on orbiter measurements place the effective radiating temperature of Venus at 228±5 K, corresponding to an emission of 153±13 W/m², and the bolometric Bond albedo at 0.80±0.02, corresponding to a solar energy absorption of 132±13 W/m². Uncertainties in these preliminary values are too large to interpret the flux difference as a true energy imbalance. A mode of submicron particles is suggested as an important source of thermal opacity near the cloud tops to explain the orbiter and probe thermal flux measurements. Comparison of the measured solar flux profile with thermal fluxes computed from the measured temperature structure and composition shows that the greenhouse mechanism explains essentially all of the 500 K difference between the surface and radiating temperatures of Venus. Precise comparison of the observed and computed value of this difference is hindered by uncertainties in the local variability of H_(2)O and in the thermal opacity of CO_2 and H_(2)O at high temperature and pressure. The directly measured thermal flux profiles at the small probe sites are surprisingly large and variable in the lower atmosphere. Observed zonal and meridional circulation are qualitatively as required to produce the observed uniformity of temperature structure. However, the present lack of quantitative estimates of the horizontal and vertical dynamical heat transports implied by these measurements is a significant gap in the understanding of the thermal balance of the atmosphere of Venus.

Thermal models for solar hard X-ray bursts

Abstract: Thermal models for hard X-ray bursts consisting of a one-dimensional flux tube whose central electrons are heated to about 400 million K are examined. It is found that the evolution of a thermal X-ray source is a sensitive function of the electron-ion thermal coupling and the state of the plasma into which the source expands. When this coupling is weak, the heated electrons separate into a region of high temperature of about 400 million K and a region of lower temperature of about 100 million K, a process which leads to a power-law X-ray spectrum. In the case of strong coupling there is only one dominant temperature, about 200 million K, and the X-ray spectrum resembles a true thermal spectrum.

Thermal diffusivity measurements at high temperatures by the radial flash method

Abstract: The flash diffusivity method is extended to measure thermal diffusivity at higher temperatures using a radial heat flow technique. In this technique the energy source of radius R irradiating the front surface of the sample is considerably smaller than the sample’s surface. The resulting temperature rises on the rear surface are measured at two locations (1) directly opposite the center of the energy pulse and (2) at a distance r (which is greater than R). This method, which is called radial flash method is different from the conventional method (in which the entire front surface of the sample is irradiated) in several respects (1) considerable radial heat flow occurs in addition to axial heat flow (2) the radial flash method measures diffusivity in the radial direction (αr) instead of the diffusivity in the axial direction (αx), and (3) the radial flash method is independent of radiation heat losses from the surface. The method was tested with both an isotropic material (POCO AXM-5Q Graphite) and an anisotropic material (reactor grade graphite) at high temperatures. The results showed that thermal diffusivity in the radial direction could be measured without accounting for the very large radiation heat losses.

Dynamic spectral characteristics of thermal models for solar hard X-ray bursts

Abstract: The dynamic spectral characteristics of the thermal model for solar hard X-ray bursts recently proposed by Brown et al. (1979) (BMS) are investigated. It is pointed out that this model, in which a single source is heated impulsively and cooled by anomalous conduction across an ion-acoustic turbulent thermal front, predicts that the total source emission measure should rise as the temperature falls. This prediction, which is common to all conductively cooled single-source models, is contrary to observations of many simple spike bursts. It is proposed, therefore, that the hard X-ray source may consist of a distribution of many small impulsively-heated kernels, each cooled by anomalous conduction, with lifetimes shorter than current burst data temporal resolution. In this case the dynamic spectra of bursts are governed by the dynamic evolution of the kernel production process, such as magnetic-field dissipation in the tearing mode. An integral equation is formulated, the solution of which yields information on this kernel production process, from dynamic burst spectra, for any kernel model. With a BMS-type kernel model in one-dimensional form, the derived instantaneous spectra are limited in hardness to spectral indices γ ≳ 4 for any kernel production process, due to the nature of the conductive cooling. Ion-acoustic conductive cooling in three dimensions, however, increases the limiting spectral hardness to γ ≳ 3. Other forms of anomalous conduction yield similar results but could permit bursts as hard as γ ≳ 2, consistent with the hardest observed. The contribution to the X-ray spectrum from the escaping tail of high-energy kernel electrons in the BMS model is calculated in various limits. If this tail dissipates purely collisionally, for example, its thick-target bremsstrahlung can significantly modify the kernel spectrum at the high-energy end. The energetics of this dynamic dissipation model for thermal hard X-ray bursts also are briefly discussed.

The exchange of infrared radiative energy in the troposphere

Abstract: On the basis of an atmospheric condition of a six-layer general circulation model of the National Center for Atmospheric Research, the spatial and the spectral partition of the long-wave radiation energy exchange in the model atmosphere is investigated and its implications are discussed. It is found that most of the energy is exchanged between adjacent layers and through the wings of strong lines. The strong line approximation is found to be a good approximation for water vapor transmittance. Cooling to space, however, is found to be a poor approximation for the total cooling rate. Both the strong line and cooling to space approximation are discussed in the content of the energy exchange findings.

A finite element approach to combined conductive and radiative heat transfer in a planar medium

Abstract: Combined conductive and radiative heat transfer has been the subject of numerous investigations in recent years The exact solution to the nonlinear integrodifferential equations involved is seldom possible, and typically one must resort to numerical approximations The problem of heat transfer by simultaneous conduction and radiation in an absorbing, emitting, and scattering medium of plane parallel geometry has been solved by various numerical methods The present study was undertaken to solve this problem using the Galerkin finite-element method Results are presented for temperature profiles and heat flux at one boundary for both steady-state and transient cases

Method for the measurement of temperature in shock compression of solids

Abstract: A new optical measurement method for obtaining the temperature of solids during shock compression has been developed. Spectral measurements of thermal radiation produced during shock compression of fused silica have been made, and the shock temperature estimated from the spectrum. This method is less sensitive to variations in emissivity, and has an accuracy of about 3%.

Thermal Stress Analysis of Partially Absorbing Brittle Ceramics Subjected to Symmetric Radiation Heating

Abstract: An analysis is presented of the thermal stresses in partially absorbing ceramics, in the form of flat plates symmetrically heated by thermal radiation and cooled by convection. The absorption coefficient is introduced as a material property, which affects thermal stress resistance. For zero heat transfer coefficient and optical thickness ..mu..a ..-->.. 0 the tensile thermal stresses are proportional to (..mu..a)/sup 3/. For h = varies as and ..mu..a ..-->.. 0, the tensile thermal stresses are a linear function of ..mu..a. Appropriate thermal stress resistance parameters were derived.

An emissometer with high accuracy for determination of the total hemispherical emittance of surfaces

Abstract: To determine the total hemispherical emittance epsilon h of surfaces, a calorimetric emissometer has been designed and built. In this emissometer an electrically heated specimen with unknown emittance is placed opposite a large, liquid nitrogen-cooled black receiver surface. When thermal equilibrium is reached, the radiated heat flux from the specimen to the receiver almost equals the supplied electrical power.

Analysis of heat transfer in building thermal insulation

Abstract: The measurement of the apparent thermal properties (ie, conductivity, resistivity, and resistance) of insulation by the guarded hot-plate technique is mathematically simulated on a computer by assuming that coupled conductive and radiative heat transfer occurs in an absorbing and emitting single-phase gray medium Calculations are performed for insulation extinction coefficients between 0001 and 1000 ft-/sup 1/, thicknesses between 00208 and 10 ft, continuous-phase thermal conductivities between 01800 and 01980 Btu in/(h ft/sup 2/ /sup 0/F), hot-plate temperatures between 485 and 635/sup 0/R, and cold-plate temperatures between 435 and 585/sup 0/R A three-region approximate solution to coupled conductive and radiative heat transfer in an infinite slab of absorbing and emitting gray material bounded by black surfaces is also developed and shown to agree to within +-05% of the numerical results for most cases The approximate solution to the coupled problem and the exact solution to the uncoupled problem are used to establish the effect of test conditions (such as specimen thickness, plate emissivity, plate temperatures, and continuous-phase thermal conductivity) on the measured apparent thermal properties of an insulation specimen Examples of the temperature profiles within the insulation and a table of representative thicknesses for guarded hot-plate test specimens (ie, the minimummore » specimen thickness required for measurement of an apparent thermal resistivity that is within 2% of the value at infinite thickness) are also presented A means to extrapolate thermal resistance data from thin to thick specimens is suggested by ths analysis Predictions from the extrapolation are shown to be consistent with existing thermal resistance data on low-density mineral fiber building insulation batts« less

Resistance-network representation of radiative heat transfer with particulate scattering

Abstract: The resistance-network representation for radiative heat transfer is developed for a planar absorbing-scattering medium on the basis of the two-flux model and the linear anisotropic scattering model. Particular attention is given to the scattering effect due to particulates such as flame soot or smoke particles. Limiting relations for various radiative regimes are derived and the physical significance of the resistances are discussed. An illustrative example is presented for thermal radiation from a smoke layer. Extension to two-phase dispersed systems is also demonstrated.

Examining biological effects on foodstuffs of seeds - by measuring intensity of ultra-weak photon radiation in vitro

Abstract: A measurement of the spontaneous or stimulated emission of ultra-weak photon radiation is used as an in vitro parameter of a cell lot. The parameter is used to detect possible cell-damaging or regenerating effects or to act as a quality control. The measured quantity is either the photon intensity or a photon statistic e.g. the distribution of numbers of photons emitted in a measuring interval. The ultra-weak radiation is typically in the infra red band and has an energy very much less than that of thermal radiation. Typically the radiation is 10 power (-10) less than thermal radiation. The radiation is detected by a photo multiplier with a gain of over 10 power 6. The method may be used to determine whether a cell lot is in a healthy state. Alternatively it can be used to determine the effect of an agent on the cells. The method is partic. suitable for quality control in foodstuffs.

Infrared radiation transfer in molecular gases

Abstract: Physical models of the theory of infrared radiation transfer in molecular gases are reviewed and their usefulness in the solution of particular problems is demonstrated. Theoretical predictions are compared with experimental data. Particular attention is devoted to analytic methods capable of providing (1) clear descriptions of the physics of the phenomenon, (2) a better understanding of the dependence of the final result on the parameters of the problem, and (3) a reasonably reliable final result that is valid over a broad range of variation of the parameters of the gaseous medium. The radiation transfer process is examined both for equilibrium and nonequilibrium gaseous media. The final section deals with the analysis of certain processes occurring in the Earth's atmosphere.

Heat transfer including particle and gas radiation in subsonic MHD diffuser-II

Abstract: Heat transfer in subsonic MHD diffusers, by convection and by gas and slag particle radiation, is analyzed by simultaneously solving the radiation transport equation and the quasi-three-dimensional gasdynamic equations. The efficiency factors for extinction and scattering by particles are calculated from the Mie theory. For a reference diffuser geometry, the heat transfer by convection is found to be 25 MW, and the radiative heat transfer varies from 44 MW to 79 MW, depending on the rate of ash carryover into the channel. Results reveal that the heat transfer is sensitive to the ash carryover into the channel, slag particles spectrum, electrical conductivity of ash, gas composition, and wall emissivity. It is observed that, because of multiple scattering, the particles shield the short wavelength radiation emitted by potassium atoms. The impacts of heat transfer enhancement by gas radiation in the channel and by gas-plus-particles radiation in the diffuser on MHD system design are assessed. It is suggested that, from the system design point of view, the diffuser be regarded as a part of the radiant boiler. No significant effect of radiation enhancement on the ability to decompose NO/sub x/ is anticipated.

Deflagration Waves Supported by Thermal Radiation

Abstract: It is shown that deflagration waves supported by thermal radiation are classified into two types, a supercritical and a subcritical deflagration. Their characteristics and structures are investigated in the framework of the diffusion approximation for radiation flux. The subcritical deflagration is found to be easily obtained and to lead to larger compression than the supercritical. The efficiency of compression driven by the deflagration waves is estimated to be 0.1∼0.3.

Method of making a gaseous medium opaque in the optical and infrared ranges of the electromagnetic spectrum

Abstract: A method of opacifying a transparent gaseous medium to optical and thermal radiation. Method characterized in that it consists in diffusing in the medium, for example air, an aerosol (5) such as boron trichloride (BCl The invention finds application in the field of electro-against-measures.

Thermal stresses in a partially absorbing flat plate symmetrically heated by thermal radiation and cooled by convection

Abstract: The stresses due to “thermal trapping” of internally absorbed radiation in solid materials are analyzed for a flat plate symmetrically heated by radiation and cooled by convection for finite values of the heat transfer coefficient. The thermal-stress state consists of an initial transient followed by a steady-state stress at t -> ∞ and involves a reversal in the sign of the stresses. The maximum value of the transient stress increases with optical thickness, whereas the maximum value of the steady-state stress occurs at an optical thickness μa = 1·3 with zero stresses at μa. = 0 and ∞.

Arrangement in order to prevent false alarms of a passive infrared motion alarm

Abstract: In an infrared motion alarm an optical filter arranged in front of an input of an infrared detector is attached to a heat sink in the form of a solid metal crucible in order to reduce the sensitivity to glass-penetrating electromagnetic radiation. Changes in radiation absorbed in the optical filter are either captured in the thermal sink or decelerated at least as to the characteristic thermal radiation of the optical filter and no longer fall within the characteristic range of change of the motion alarm.