Showing papers on "Thermal radiation published in 1981"

Thermal radiation heat transfer

Abstract: A comprehensive discussion of heat transfer by thermal radiation is presented, including the radiative behavior of materials, radiation between surfaces, and gas radiation. Among the topics considered are property prediction by electromagnetic theory, the observed properties of solid materials, radiation in the presence of other modes of energy transfer, the equations of transfer for an absorbing-emitting gas, and radiative transfer in scattering and absorbing media. Also considered are radiation exchange between black isothermal surfaces, radiation exchange in enclosures composed of diffuse gray surfaces and in enclosures having some specularly reflecting surfaces, and radiation exchange between nondiffuse nongray surfaces. The use of the Monte Carlo technique in solving radiant-exchange problems and problems of radiative transfer through absorbing-emitting media is explained.

A new radiation solution method for incorporation in general combustion prediction procedures

Abstract: A new numerical solution method for the calculation of the thermal radiation transfer in combustors is described. It has been especially developed for incorporation in general combustor prediction procedures for the flow, and chemical reaction. It combines features and advantages of the zone, Monte Carlo, and flux model solution methods while avoiding their shortcomings. It is based on the solving of representatively directed beams of radiation within the enclosure between the known wall boundary conditions and on the subsequent computing of the radiation sources which arise within the finite difference control volumes of the flow procedure due to the passage of the beams. It is fast, exact applicable to complex geometries, and it retains in evidence the physics of the problem by avoiding complex mathematics.

A set of equations for full spectrum and 8- to 14-μm and 10.5- to 12.5-μm thermal radiation from cloudless skies

Abstract: A comprehensive experiment was conducted at Phoenix, Arizona, involving the monitoring of full spectrum thermal radiation and those fractions of that flux that are contained within the 8- to 14-μm and 10.5- to 12.5-μm subrogions. Also monitored were surface air temperature (T0) and vapor pressure (e0). Analyses of the data established the source of water vapor associated thermal emittance (ϵ) variations of the cloudless sky as being due to the variable atmospheric concentration of water dimers—pairs of water molecules linked together by weak hydrogen bonds. New equations ((15)–(17)) based on this physical model were thus developed for the effective emittance of the atmosphere in both the 10.5- to 12.5-µm and 8- to 14-μm wavebands, as well as for the full thermal spectrum. Equation (17) was then shown to be a significant improvement over previous equations that have attempted to model this phenomenon.

Radiative heating and cooling with spectrally selective surfaces.

Abstract: Matter continuously exchanges energy with its surroundings. This exchange can be dominated by radiation, conduction, or convection. In this brief review we discuss how proper design of radiative surface properties can be used for heating and cooling purposes. The desired properties can be understood once it is realized that solar and terrestrial radiation take place in different wavelength ranges and that only part of the solar spectrum is useful for vision and for photosynthesis in plants. These facts allow the possibility of tailoring the spectral absorptance, emittance, reflectance, and transmittance of a surface to meet different demands in different wavelength intervals, i.e., to take advantage of spectral selectivity. One example is the selective surface for efficient photothermal conversion of solar energy, which has high absorptance over the solar spectrum but low emittance for the longer wavelengths relevant to thermal reradiation. Below we discuss the pertinent spectral radiative properties of our ambience. These data are then used as background to the subsequent sections treating four examples of spectrally selective surfaces. The first example is the previously mentioned selective surface for converting solar radiation to useful heat. The second example considers surfaces capable of reaching low temperatures by benefiting from the spectral emittance of the clear night sky. The third example concerns two related types of transparent heat mirror. The fourth example, finally, treats radiative cooling of green leaves; this part is included since it gives a nice example of how nature solves a difficult problem in an elegant and efficient way. This example hence provides an interesting background to the other cruder types of artificial selective surfaces. Throughout our discussion we treat the ideal spectral properties, give an illustrative experimental example of how well this goal can be realized, and—where this is possible—show a corresponding theoretical curve indicating to what extent the measured results can be theoretically understood.

Multi-Wavelength Pyrometry

Abstract: The main problem in the application of radiation pyrometry to the measurement of true surface temperatures is the large error arising from the unknown or varying emissivity of the surface. The nature of the additional information required for the class of techniques which utilize only the thermal radiation emitted from the surface is discussed. The suggestions that multi-wavelength pyrometry, requiring measurements at many different wavelengths, may be employed to solve the problem without additional information are analyzed, and shown to possess distinct disadvantages.

Stationary and transient thermal properties of semiconductor laser diodes

Abstract: Numerical analyses and experimental results on thermal behavior of AlGaAs laser diodes for CW and modulated operations are presented. Theoretical stationary and transient temperature rises are calculated using Fourier and Laplace transformations. Effects of inhomogeneous heat flow to a submount and heat radiation from the diode surface into the ambient atmosphere are taken into account in the calculation. Thermal resistances of laser arrays are also discussed. Experimental temperature behavior at the active layer is obtained from the wavelength shift measurement of an oscillation mode using a birefringent filter, which provides fast and precise measurement. Three components of temperature rise are observed in step and periodic responses. Numerical and experimental results are in good agreement.

Radiative heat transfer in rotary kilns

Abstract: Radiative heat transfer between a nongray freeboard gas and the interior surfaces of a rotary kiln has been studied by evaluating the fundamental radiative exchange integrals using numerical methods. Direct gas-to-surface exchange, reflection of the gas radiation by the kiln wall, and kiln wall-to-solids exchange have been considered. Graphical representations of the results have been developed which facilitate the determination of the gas mean beamlength and the total heat flux to the wall and to the solids. These charts can be used to account for both kiln size and solids fill ratio as well as composition and temperature of the gas. Calculations using these charts and an equimolar CO2−H2O mixture at 1110 K indicate that gas-to-surface exchange is a very localized phenomenon. Radiation to a surface element from gas more than half a kiln diameter away is quite small and, as a result, even large axial gas temperature gradients have a negligible effect on total heat flux. Results are also presented which show that the radiant energy either reflected or emitted by a surface element is limited to regions less than 0.75 kiln diameters away. The radiative exchange integrals have been used, together with a modified reflection method, to develop a model for the net heat flux to the solids and to the kiln wall from a nongray gas. This model is compared to a simple resistive network/gray-gas model and it is shown that substantial errors may be incurred by the use of the simple models.

Absolute measurements of the thermal conductivity of liquids at pressures up to 500 MPa

Abstract: An apparatus for the accurate measurement of the thermal conductivity of liquids is described. The instrument, which is of the transient hot-wire type, operates over the pressure range 0.1 to 700 MPa and the temperature range 20–200°C. The precision of the thermal conductivity measurements is estimated to be one of ±0.2% over the entire range. — Experimental data are reported for the thermal conductivity of n-heptane in the temperature range 35–75°C and the pressure range 50–500 MPa. The accuracy of the present results is estimated to be one of ±0.5%, which is slightly worse than their precision because the lack of suitable data for the optical properties of the liquid at high pressures only allows an approximate correction to be applied for the systematic effects of radiative heat transfer. Along an isochore the thermal conductivity of n-heptane is found to increase with temperature although along an isobar the temperature derivative is negative. The density dependence of the thermal conductivity is almost independent of temperature.

Chromosphere-corona transition region models with magnetic field and fluid flow

Abstract: Energy balance models of the chromosphere-corona transition region are computed for a segment of average Sun network using assumed magnetic field geometry. The energy fluxes considered include radiation, conduction, enthalpy, and gravitational potential energy, but do not include mechanical heating. Two classes of models are considered: conduction driven and flow driven. In the former class, the thermal conductive flux vanishes at the base of the transition region, and in the latter class the thermal conductive flux increases with depth or remains constant. The conduction driven models require primarily outflow, whereas the flow driven models require downflow. It appears that the flow-driven models give best overall agreement with observations for 3 x 10/sup 5/ K< or =T< or =1 x 10/sup 6/ K, but that neither class of models is satisfactory for T< or =1 x 10/sup 5/ K. It is concluded, however, that the flow-driven models have ample energy supply even in the low transition region, and there is no apparent need for additional energy sources.

Radiation heat loss detector.

Abstract: Need for a simple to use device that gives good estimates of heat loss from a building or machines exists This device uses a radiation and convection heat loss detector to answer this need Radiation from an extended surface area as at (22) is detected with a sensor (40) having a wide field of view closely matching that of an associated camera (24) By simultaneously photographing the surface area at (22) of interest and detecting the radiation from that same area at (22), a clear record as in Fig 2 is provided for future use A wide band radiation sensor (40) is used Compensation for environmental radiation is made by first viewing the environment with the sensor (40) and holding the sensed signal and then subtracting that signal from the flux detected when the sensor (40) faces the extended surface area The date and time are also recorded on the photograph as in Fig 2 A sonar device (42) positioned on the camera provides a distance indication which, with a known field of view, allows for a determination of the total flux from the surface area at (22), a determination of convective heat losses from the surface and correction for atmospheric absorption of the radiant energy

Optical fiber temperature sensor

Abstract: The self-generating thermal radiation properties of an optical fiber element are employed to determine the temperature, location and length of a hot spot, or region, of an object.

Energy Balance in Solar and Stellar Chromospheres

Abstract: The spectrum from a specific region of the sun or from a star can be used to determine the temperature-density stratification and other physical properties of the emitting atmospheric layers. We need such a description of the atmosphere in order to understand the causes of solar activity and the nature of various stellar phenomena. Computer programs are available for solving the detailed radiative transfer and statistical equilibrium equations to calculate the spectrum emerging from an optically thick gaseous medium of prescribed properties. It is usually possible to adjust the properties of the atmospheric model to obtain a spectrum which agrees with an observed one. Current models of the solar atmosphere indicate that a meaningful temperature-density stratification can be determined from the, spectrum: using relatively few atmospheric parameters to define the model, good agreement can be obtained between the calculated spectrum and the observed one. In this paper we consider the chromospheric models corresponding to faint, average, and bright components of the quiet sun, a solar plage and flare, and the stars a Boo and λ And. In each case we calculate as a function of depth the net radiative cooling rate per unit volume due to the transitions of various atoms and ions (principally H, Mg II, Ca II, and H-). This radiative cooling must be balanced by the same amount of mechanical heating at each depth. The chromospheric cooling rate, obtained from models based on observations, provides a detailed constraint on theories which explain how the chromosphere is heated.

Radiative transfer in an isotropically scattering two-region slab with reflecting boundaries

Abstract: The problem of radiative heat transfer in an absorbing, emitting, isotropically scattering two-layer slab with diffusely and specularly reflecting boundaries is solved by the FN method and results are presented for the transmissivity and reflectivity of the slab.

Thermal radiation measuring arrangement

Abstract: In a thermal radiation measuring arrangement, a thermal radiation detector is located at the focal point of a collecting mirror, upon which incident thermal radiation from a surface, such as a building wall, is directed. The thermal radiation detector may be, for example, a thermopile, and provides an output signal having a magnitude proportional to the amount of thermal radiation which it receives. The temperature detection means detects the temperature of the thermal radiation detector and, for example, may detect the cold junction of the thermopile. In a first operating condition, a signal summing means receives the output signal from the thermal radiation detector and the temperature detection means and provides a third output signal proportional to the sum of these first and second output signals. In a second operating condition, a signal biasing means is connected into the signal summing means. The signal biasing means provides a signal to the signal summing means to cause the third output signal to become zero when radiation is received from a reference surface. When the arrangement is in the second operating condition and directed to receive thermal radiation from a second surface different from the reference surface, the signal biasing means maintains the same level of bias to the signal summing means as it did when detecting the radiation from the reference surface.

Extreme ultraviolet radiation transport in laser-irradiated high-Z metal foils

Abstract: Spatially resolved extreme ultraviolet spectra were measured from the rear side of gold foils irradiated with 1.05-..mu..m wavelength, 100-ps pulses at intensities of 3 x 10/sup 14/ W/cm/sup 2/. In the energy range of 0.1 to 1.0 keV the radiation intensity decayed exponentially with foil thickness up to 1 ..mu..m but remained almost constant over the range of 1 to 6 ..mu..m. These results indicate that radiation heat conduction plays an important role in high-Z plasma energy transport. Ablation by the radiation heat flux is briefly discussed.

Heat and Mass Transfer in MHD Channels

Abstract: Accurate estimates of convective and radiative heat transfer in the magnetohydrodynamic channel are provided by simultaneousl y solving the radiation transport equation and the quasi-three-dimensional gasdynamic equations. Carbon dioxide, water vapor, and potassium atoms are considered as the principal constituents participating in gas radiation. Calculations performed for a base load-size channel indicate that heat transfer by gas radiation almost equals that by convection for smooth walls, and amounts to three-fourths as much as the convective heat transfer for rough walls. The evolution of slag particles by homogeneous nucleation and condensation is also investigated. The slag particle size spectrum is computed as a function of the ash carryover rate from the combustor to channel. The average particle size is found to increase and the total number density to decrease with ash carryover rate.

Flange and coupling cooling means and method

Abstract: Reaction vessels, furnace tubes, heating and cooling enclosures frequently have parts such as access ports, inlet tubes, inspection windows, etc. made of thermally transparent materials such as plastic, glass, quartz, oxides, nitrides and sulfides. When these parts extend outside the hot zone, they can act as "light pipes" carrying appreciable amounts of thermal radiation which can damage thermally sensitive gaskets or other materials used to secure external couplings or end closure flanges to these parts. Thermal radiation induced gasket damage is a frequent cause of stuck flanges and couplings. This problem is avoided by inserting a thermal radiation scattering region in the thermally transparent material between the hot zone and the end closure or gaskets. The thermal radiation is scattered and dispersed, so that the end zones receive less radiation and remain cooler. Milky quartz is a suitable scattering material for use with quartz furnace tubes or bell jars.

Rapidly Convergent Algorithm for Non-Linear Humidity and Thermal Radiation Terms

Abstract: In many studies involving energy exchange at a surface, a rigorous energy balance equation can be written However, the equation is difficult to solve for the desired surface temperature because the temperature appears in non-linear humidity and thermal radiation terms In this paper an algorithm is presented for solving these non-linear equations The method uses analytical equations for the temperature gradients, and therefore it is rapidly convergent An example of its use is presente

Radiation from nonhomogeneous combustion products

Abstract: A simple method is presented for predicting the thermal radiation from nonisothermal combustion systems containing variable concentrations of water vapor, carbon dioxide, and soot. Less complex than narrow and even wide-band models, the simplified technique has been shown to agree quantitatively with results of a complex narrow-band model. For combustion conditions found in typical fires and furnaces (20 to 200 cm pathlengths, temperatures between 800 and 1800 K), calculations can be performed within about 8% of those made by the complex model, and with savings of two orders-of-magnitude in computation time. The soot and gas radiances are determined separately and then added together with an allowance for spectral overlap. The soot calculation assumes an inverse wavelength dependence for the absorption coefficient, to yield an exact spectral integration in terms of the hexagamma function. The gas model, which has been independently verified, is based upon published total transmittance data for homogeneous gases. The pressure-pathlength and temperature are gas concentration weighted averages along the line-of-sight. The overlap between the gas bands and the soot continuum is corrected for by either diminishing the gas radiance with the effective soot transmittance, or diminishing the soot radiance with the effective gas transmittance. Both of these techniques are evaluated. With this model, phenomena characteristic of nonhomogeneous radiation are successfully predicted, a task impossible to perform with any homogeneous model; and yet the method is simple enough that it can be coupled to complex fluid mechanical computer codes with only a small sacrifice in computation time.