Peter D. Jones

Correlation of Measured and Computed Radiation Intensity Exiting a Packed Bed

The spectral and directional distribution of radiation intensity is measured, using a direct radiometric technique, at the exposed boundary of a packed bed of stainless steel spheres. The purpose of these measurements is to provide an experimental data base of radiation intensity with which to correlate intensity field solutions of the radiative transfer equation in participating media. The bed is considered to be one-dimensional, is optically thick, and has measured constant-temperature boundary conditions. Intensity exiting the bed is numerically simulated using a discrete ordinates solution to the radiative transfer equation, with combined mode radiation-conduction solution of the coupled energy conservation equation. Radiative properties for the bed are computed using the large size parameter correlated scattering theory derived by Kamiuto from the general theory of dependent scattering by Tien and others. The measured intensity results show good agreement with computed results in near-normal directions, though agreement in near-grazing directions is poor. This suggests that either radiative transfer near the boundaries of this medium might not be adequately represented by a continuous form of the radiative transfer equation, or that the properties derived from correlated scattering theory are insufficient. In either case, development of a more detailed radiation model for spherical packed beds appears warranted.

Spectral-directional emittance of thermally oxidized 316 stainless steel

The spectral-directional emittance of thermally oxidized stainless steel is measured for angles from normal to grazing, wavelengths between 2 and 10μm, and temperatures between 773 and 973 K. The oxide is grown by holding the steel substrate at a high temperature over a long period while exposed to normal atmospheric conditions, until the measured emissive power of the surface achieves an asymptotic level. It is found that the emittance decreases with angle away from the surface normal at the lower end of the measured spectral range and increases with angle at the higher end. The emittance decreases with wavelength, although there is evidence of a peak near 2 pill. The variation with temperature within the measured range is insignificant. Overall higher values for the oxidized steel are measured than those reported in previous work.

Concentration distributions in cylindrical receiver/paraboloidal dish concentrator systems

Abstract Concentration distributions on a cylindrical receiver in a paraboloidal dish concentrator are computed for space applications (no atmosphere). A geometric optics method is applied which integrates over the solar disk and the concentrator projected surface, and maps analytically, in implicit closed form, through the concentrator and onto the receiver. Finite sunshape, concentrator surface errors, and pointing system zero-mean and constant offset errors are considered. Results define the section of the receiver surface which receives the majority of the concentrated flux, where the receivers aperture might be located. Results are given in terms of concentrator geometry, concentrator total system error tolerance, receiver geometry, and pointing offset error. In cases with pointing offset error (nonzero mean pointing error), circumferentially varying concentration distributions are shown.

Theoretical Investigation of Particles Having Directionally Dependent Absorption Cross Section

Examined here are a class of particles that possess highly anisotropic absorption cross sections. The particles are in the form of an eccentrically stratified sphere in which a relatively small, highly absorbing sphere is located against the interior surface of a larger, nonabsorbing sphere. For this configuration the nonabsorbing sphere can act essentially as a lens and focus radiation onto the absorbing sphere. Using an exact solution to Maxwells wave equations, the directionally-dependent absorption cross sections of the particle are predicted for refractive indices of common absorbing and nonabsorbing materials and radiative size parameters ranging from 5 to 50. Under certain conditions, the absorption cross section of the particle can be extremely sensitive to the direction of the incident radiation. Placing the absorber opposite the point of incidence can result in an absorption cross section that is two orders of magnitude greater than its isolated, single-particle value, yet for incident angles as little as 20 deg removed from the focused condition, the absorption becomes comparable to or smaller than the isolated value. Factors that optimize the absorption anisotropy and the implications with regard to radiative transfer are discussed.

Spectral-directional emittance of oxidized copper

The surface emittance of fully oxidized copper is experimentally determined as functions of wavelength between 2-10 /tin, the direction between the normal and grazing angles, and the temperature between 400-700°C. Clean copper surfaces, heated and exposed to air, begin to oxidize immediately. After a sufficient heating time, the oxide layer becomes thick enough that the radiative surface properties are those of copper oxide, independent of the properties of the underlying copper. The experimental apparatus measures emitted flux over discrete bands of wavelength and solid angles centered about a direction, and is calibrated with a radiating cavity (hohlraum). Emittance results are presented in both spectral and polar form, and also integrated to obtain the spectral-hemispherical emittance of fully oxidized copper. By assuming copper oxide to be a dielectric, the real part of the index of refraction is reduced from emittance data. This index is found to decrease with wavelength and temperature. This technique of spectral-directional emittance determination by direct emission measurement, together with index of diffraction identification (if found to fit Fresnel or other relations), should prove useful for other engineering materials as well as copper oxide.

Coordinate systems for the radiative transfer equation in curvilinear media

Abstract Spatial/directional coordinate systems are presented on which general directional boundary conditions may be satisfied for the solution of the radiative transfer equation in curvilinear, participating media. The general directional boundary conditions described herein may be applied to curvilinear media without regard to spatial symmetry, although their application depends upon use of a local coordinate system, defining the intensity direction, which has a polar axis directed normal to any nearby surfaces. In the most commonly used spatial/directional coordinate systems for cylindrical and spherical media, the orientation of the directional polar axis indicates that the non-symmetric boundary conditions may be applied only to, respectively, parallel disk and spherical annulus boundary geometries. Application of the general directional boundary conditions to non-symmetric curvilinear media of these geometries, referred to only partially or obliquely in previous works, is demonstrated completely and consisely in the paper. Also, new spatial/directional coordinate systems are derived, based on reorientation of the local directional coordinate system in spatially cylindrical and spherical geometries. These new coordinate systems allow application of the general directional boundary conditions to other curvilinear geometries, such as a cylindrical annulus, cylindrical wedge, conical annulus, and spherical wedge. For each coordinate system, expressions for the pathlength derivative of radiation intensity are given in simple, conservative, and discrete ordinates form. The techniques illustrated in the derivation of these spatial/directional coordinate systems for non-symmetric curvilinear media may be applied to other, more complicated boundary geometries as well.

Radiation, Conduction, and Convection From a Sphere in an Absorbing, Emitting, Gray Medium

Combined mode heat transfer is solved for an emitting, reflecting sphere in low Peclet number motion through a gray, nonscattering, absorbing, emitting, and conducting infinite medium. The coupled formulation of the energy and radiative transfer equations is solved numerically. The radiative transfer equation is expressed in a unique spatial-directional coordinate system, whose object is to exploit the axisymmetry of the problem. The radiation intensity field is solved using the discrete ordinates method. Results are presented in terms of the Planck and Peclet numbers, and serve as a combined radiation/convection analog to the well-known Nusselt number results for a radiatively nonparticipating medium.

RADIATIVE ENHANCEMENT OF HEAT TRANSFER TO A GRAY GAS THROUGH PARTICLE SEEDING

Combined effects of radiation and convection are examined in turbulent flow of gray gases, dispersely seeded with absorbing, emitting, scattering particles. Seeding particles are used as a heat transfer enhancement technique, whereby radiatively participating particles receive radiation from hot tube walls, and transfer this heat to the gas. It is found that enhanced heating of the gas is only achieved for low gas optical thicknesses and when the seeding particles are small and either hollow or of low material density. Enhancements are identified for both radiation-dominated and combined mode (radiation/convection) cases, as functions of dimensionless parameters.

Non-Kirchhoff surface using media with directionally varying absorption efficiency

A unique radiation trapping surface is proposed in which the effective monochromatic absorptivity-to-emissivity ratio is much greater than one (a non-Kirchhoff surface). The surface is comprised of a plane-parallel layer of a disperse suspension of compound particles. Each particle has a small absorber sphere embedded eccentricly in a larger lens sphere, resulting in an absorption coefficient for the medium that is highly dependent on direction. In combination with an opaque, specularly reflecting boundary opposite the incident flux side of the medium, such a device acts as a planar radiation trap. Computational results show that an effective monochromatic absorptivity-to-emissivity ratio of up to 70 can be achieved, along with a through-flux that is superior to that of an opaque surface that obeys Kirchhoffs law. The effects of optical thickness, incidence angle, particle orientation error, and suspending substrate conductivity are investigated. dp = / = //, = Ir = J = / = k = M = m = N = Nf, = Ns = n — Q(l = Qc =

Spectral-Directional Emittance of High Temperature Oxidized Aluminum

Spectral-directional emittance measurements for high purity aluminum, oxidized in air for an extended period of time (150h) at high temperature are presented. The data presented here cover the spectral range between 3 and 14 μm, directional range from surface normal to 72° polar angle and temperatures from 400 to 600°C with a step increment of 100°C. The measurements were performed using a radiometric, direct emission method. The experimental setup used was comprised of a Fourier transform infrared spectrometer and a blackbody cavity. The Al sample has a nominal surface roughness of 0.635 μm. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used as surface techniques to characterize the aluminum oxide film that formed on the metallic surface.Copyright