T.W. Tong

Enhancement of thermal emission from porous radiant burners

Abstract A theoretical study on using coatings to enhance the emittance of porous radiant burners is presented. Burners made of coated silica fibers are considered along with coating materials of silicon carbide, graphite, and platinum. The radiative properties of the fibers with and without coatings are computed using a radiative scattering theory and are used in a P-3 solution of the radiative transfer equation to predict the emittance of the burners. Graphite and platinum coatings are shown to enhance the emittance by a factor of three for a burner temperature of 1000 K, and by a factor of six for a temperature of 1500 K. It is also shown that it is possible to protect the graphite and platinum coatings from oxidation by applying a silica coating over them.

Radiative heat transfer in isothermal spherical media

Abstract Radiative heat transfer in emitting, absorbing, and scattering spherical media is analyzed. The medium is assumed to be gray, isothermal, and linear-anisotropically scattering. The medium is confined in the space between two gray concentric spheres, which diffusely emit, and specularly and diffusely reflect radiation. Approximate solutions of the equation of radiative transfer are obtained using the spherical harmonics method. Results presented include the irradiance and the net radiative heat flux. The effects of the different governing parameters and the particular type of boundary reflections are examined.

Scattering of electromagnetic waves by cylinders coated with a radially-inhomogeneous layer

Abstract Scattering of electromagnetic waves by cylinders coated with a radially inhomogeneous layer is addressed in this paper. Approximate solutions are obtained using two approaches to solve the electromagnetic wave equations. The first approach is a finite-difference solution of the equations. The second approach is to discretize the inhomogeneous layer and apply a close-form solution for cylinders coated with multiple homogeneous layers. The two approximate solutions are used to obtain the scattering and extinction efficiencies of three fibers having different coatings. The efficiency results and computing time required are compared in order to judge the relative merits of the two approximate solutions.

A COMBINED NARROW- AND WIDE-BAND MODEL FOR COMPUTING THE SPECTRAL ABSORPTION COEFFICIENT OF CO2, CO, H2O, CH4, C2H2, AND NO

Abstract This paper presents a new computational procedure for computing the spectral absorption coefficient of CO 2 , CO, H 2 O, CH 4 , C 2 H 2 , and NO. The procedure is based on a combined narrow- and wide-band model which gives the absorption coefficient as a function of radiation wavelength, temperature, total pressure and partial pressure of the gases. The average line spacing is the only correlation parameter that is adjusted to fit integrated band absorptance data. Results show that the accuracy of the present procedure is comparable to those obtained from using existing correlations. A positive feature of the present procedure is, however, a single model that is applicable to all the gases considered.

Using porous inserts to enhance heat transfer in laminar fully-developed flows

Abstract The objective of this work is to investigate if it is possible to use porous inserts to enhance heat transfer in rectangular channels. A mathematical model that includes inertia and viscous effects is used to determined the velocity profile in the porous region. For the fluid region, momentum transfer is modeled using the Navier-Stokes equation. These equations and the energy equations are solved numerically via a finite-difference method. Heat transfer between the channel walls and the fluid is determined as a function of Darcy number, inertia parameter, ratio of the fluid and porous medium thermal conductivities, and the porous insert thickness. It is shown that heat transfer could be enhanced by placing a porous insert in the channel. Moreover, for some conditions heat transfer is maximized by using a porous insert thinner than the channel height while a porous insert that completely fills the channel is needed for other conditions.

Reduction of radiative heat transfer in thermal insulations by use of dielectric coated fibers

Abstract This paper reports an analysis on using coated silica fibers to reduce radiative heat transfer through thermal insulations. Considerations were given to silica fibers of diameters 2, 5, and 10 μm. They were coated with either a 0.2 μm silicon coating or a 0.2 μm silicon inner coating and a 0.1 μm silica outer coating. Calculations were performed to determine the ratio of radiative heat flux for coated fibers to that for uncoated fibers. The calculations were made for both constant fiber number density and constant bulk density. It was found that keeping the fiber number density constant resulted in larger reductions in radiative heat flow. For the test conditions examined, reduction as high as 75 percent was shown to be possible.

Angular scattering of radiation from coated cylindrical fibers

Abstract We present the results of an experimental study for testing the validity of an analytical model describing scattering of radiation by coated fibers. The experiments involved measuring radiation intensities scattered by a 7.9-μm dia silica fiber coated with a 0.25-μm thick silicon layer at wavelengths of 0.633 and 10.6 μm. The measured results have been compared with predicted values. The comparisons revealed good agreement for the locations of the maxima and minima of the scattered intensities, acceptable agreement for the backscattered fraction, and qualitative agreement for the intensity amplitudes. We conclude that there is sufficient evidence to support use of the model for analysing radiation scattering by coated fibers.

A new algorithm for computing the scattering coefficients of highly absorbing cylinders

Abstract Many researchers have reported difficulties in computing scattering coefficients for highly absorbing materials. A new algorithm, which overcomes these difficulties and permits the determination of the scattering coefficients, is presented here. A new correlation for estimating the number of terms required for the series solutions of the scattering and extinction efficiencies to converge is also described.

Heat Transfer in Rectangular Reflective Insulation Systems

The results of a numerical study on simultaneous natural convection and radiation in air filled vertical rectangular enclosures are presented. A non- partitioned enclosure, one with a non-reflecting partition, and an enclosure with a reflective partition were considered. Heat transfer characteristics were studied for a wide range of the governing parameters. The mean enclosure temperature ranged from 250 to 350 K. The enclosure temperature difference ranged from 10 to 40 K. A highly reflective partition may reduce heat flux in the enclosure more than 80% for the temperature conditions considered. The reflective partition modulates the heat flux variation with the change in temperature conditions.

Reflectance of two-layer composite porous media with linear-anisotropic scattering

Abstract We report an analysis of the hemispherical reflectance of composite slabs made up of two porous layers. To allow realistic modeling of most porous materials, anisotropic scattering is considered. The reflectance is obtained by using the method of spherical harmonics to solve the equation of transfer. Results from the P -11 approximation are presented for a wide range of governing parameters, including the single-scattering albedos and scattering phase-function coefficients for both porous layers. The effects of anisotropic scattering are illustrated.