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COMBINED TWO-FLUX APPROXIMATION AND MONTE CARLO MODEL FOR IDENTIFICATION OF RADIATIVE PROPERTIES OF HIGHLY SCATTERING DISPERSED MATERIALS

Leonid A. Dombrovsky
Joint Institute for High Temperatures, 17A Krasnokazarmennaya Str., Moscow, 111116, Russia; Tyumen State University, 6 Volodarsky Str., Tyumen, 625003, Russia

Krithiga Ganesan
Department of Mechanical Engineering, University of Minnesota, Minneapolis, USA

Wojciech Lipinski
Research School of Engineering, The Australian National University, Canberra ACT 2601, Australia

Abstract

An identification procedure is developed for obtaining spectral radiative properties of highly scattering dispersed materials such as porous ceramics. Traditional techniques based on measurements of the directional-hemispherical reflectance and transmittance are of limited use because of difficulties in fabricating sufficiently thin and mechanically stable samples to obtain reliable values of directional-hemispherical transmittance. However, one can use the directional-hemispherical reflectance measurements for optically thick samples to obtain the transport scattering albedo. A one-dimensional analytical solution employs the modified two-flux approximation for the identification of transport scattering albedo. An additional transmittance measurement is required to identify the transport extinction coefficient. Bi-normal narrow cone transmittance is measured for this purpose. Because the one-dimensional analytical solution is not applicable to model the bi-normal narrow cone transmittance, Monte Carlo ray-tracing technique is used to identify the transport extinction coefficient. The identification procedure is applied to obtain near-infrared radiative properties of porous ceria ceramics used in solar thermochemical reactors. The identified transport scattering coefficient is shown to be in good agreement with theoretical estimates based on the Mie theory for polydisperse pores and grains. This verifies the applicability of a model based on independent scattering and Mie theory for theoretical predictions of radiative properties of two types of ceria ceramics with porosity of 0.08 and 0.72, and for extrapolating the properties of both ceramics in a limited near-infrared range to the range of significant absorption.

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