Determination of the anisotropic radiative properties of a porous material by radiative distribution function identification (RDFI)

International Journal of Heat and Mass Transfer

2017

a b s t r a c tRadiative-conductive heat transfer in porous media is usually investigated by decoupling the heat transfer modes and solving the volume-averaged continuum equations using effective transport properties. However, both modes are naturally coupled and coupling effects might significantly affect the results. We aim at providing quantitative understanding of the coupling effects occurring in a model geometry. This is an important first step towards improving the accuracy of heat transfer predictions in engineering applications.We developed a numerical method using a structured mesh, cell centered finite volumes and Monte Carlo ray tracing techniques in order to simulate the 3-dimensional and unsteady coupled radiativeconductive heat transfer in semitransparent macroporous media. We have optimized the numerical method with regards to memory and computational requirements leading to optimal performance and allowing to perform a parameter variation study for various steady state cases.We conducted a parameter study considering different optical and thermal material properties and boundary conditions in order to quantify the coupling effect between conduction and radiation, and to demonstrate its dependencies. In terms of thermal properties, it was found that the ratio of bulk thermal conductivities is governing the coupling effect. A distinct peak at a given conductivity ratio was found. The influence of optical properties is discussed in details. It was found that a significant coupling effect exists, reaching up to 15% of the total thermal heat flux.The verified modeling framework in conjunction with our non-dimensionalization offers a tool to investigate the importance of radiation-conduction coupling in a quantitative manner. It is an important step towards understanding the detailed mechanisms of radiation and conduction coupling and provides engineering guidelines on the importance of these effects.

Section: Introductionmentioning

confidence: 99%

Numerical quantification of coupling effects for radiation-conduction heat transfer in participating macroporous media: Investigation of a model geometry

Perraudin

International Journal of Heat and Mass Transfer

2017

“…The two-phase radiative properties ( σ refl, i , σ ij , refl, i , and ij ) depend on morphology of the multi-phase media and its interface properties (specifically roughness and refractive indexes). In the present study, the two-phase radiative properties are derived from the corresponding probability distribution functions [34,35] while they can be also derived by utilizing the mean-free-path theory applied to each phase [18] . The multi-scale radiation problem is solved in two steps.…”

Section: Multi-rte Approachmentioning

confidence: 99%

“…[1,2] ), to multiple intensities approach, previously applied to packed beds (e.g. [6,[34][35][36] ), foams (e.g. [14,18] ) and fibrous media, [33] in order to estimate the applicability of simplifying theoretical approaches for predicting radiative transfer in isotropic fibrous materials consisting of optically large fibers.…”

Section: Introductionmentioning

confidence: 99%

Radiative characterization of random fibrous media with long cylindrical fibers: Comparison of single- and multi-RTE approaches

Randrianalisoa

Journal of Quantitative Spectroscopy and Radiative Transfer

Baillis

et al. 2017

a b s t r a c tRadiative heat transfer is analyzed in participating media consisting of long cylindrical fibers with a diameter in the limit of geometrical optics. The absorption and scattering coefficients and the scattering phase function of the medium are determined based on the discrete-level medium geometry and optical properties of individual fibers. The fibers are assumed to be randomly oriented and positioned inside the medium. Two approaches are employed: a volume-averaged two-intensity approach referred to as multi-RTE approach and a homogenized single-intensity approach referred to as the single-RTE approach . Both approaches require effective properties, determined using direct Monte Carlo ray tracing techniques. The macroscopic radiative transfer equations (for single intensity or two volume-averaged intensities) with the corresponding effective properties are solved using Monte Carlo techniques and allow for the determination of the radiative flux distribution as well as overall transmittance and reflectance of the medium. The results are compared against predictions by the direct Monte Carlo simulation on the exact morphology. The effects of fiber volume fraction and optical properties on the effective radiative properties and the overall slab radiative characteristics are investigated. The single-RTE approach gives accurate predictions for high porosity fibrous media (porosity about 95%). The multi-RTE approach is recommended for isotropic fibrous media with porosity in the range of 79 −95%.

“…Previous pertinent studies on continuum-scale radiative properties of multi-component media consisting of individual components in the limit of geometrical optics include determination of the properties for media consisting of an optically thin (non-participating) component and an opaque component [4][5][6][7][8], and for media consisting of an optically-thin component and a semi-transparent component [9][10][11][12][13]. These studies show that the discrete-scale interface and component internal properties, and the exact internal medium geometry (morphology) must be known in order to predict the continuum-scale radiative characteristics.…”

Section: Introductionmentioning

confidence: 99%

Continuum radiative heat transfer modeling in media consisting of optically distinct components in the limit of geometrical optics

Lipiński

Keene

Journal of Quantitative Spectroscopy and Radiative Transfer

et al. 2010

a b s t r a c tContinuum-scale equations of radiative transfer and corresponding boundary conditions are derived for a general case of a multi-component medium consisting of arbitrary-type, non-isothermal and non-uniform components in the limit of geometrical optics. The link between the discrete and continuum scales is established by volume averaging of the discrete-scale equations of radiative transfer by applying the spatial averaging theorem. Precise definitions of the continuum-scale radiative properties are formulated while accounting for the radiative interactions between the components at their interfaces. Possible applications and simplifications of the presented general equations are discussed.Published by Elsevier Ltd.