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Remi Coquard
Société "Etude Conseils Calcul Modélisation" (EC2-MODELISATION), 66 Boulevard Niels Bohr, F69603 Villeurbanne, France

Jaona Harifidy Randrianalisoa
Centre de Thermique de Lyon, Institut National des Sciences Appliqués, Lyon; GRESPI, Université Reims, EA 4301, Moulin de la Housse, F51687 Reims, France

Dominique Baillis
LaMCoS, INSA-Lyon, CNRS UMR 5259,18-20 Rue des Sciences, F69621 Villeurbanne, France


Polymer foams with closed-cells are widely used as insulating material in many applications from building to space launchers. In these applications, the heat transfer within the foam is governed mainly by conduction and radiation. To enhance the insulating system capability, the knowledge of the thermal properties of these materials is indispensable.
From the thermal radiation point of view, polymer foams behave as semitransparent materials. Current knowledge shows that the radiative properties of the foam depend on the optical and surface properties of the solid phase, on the cell size and shape, and on the wall thickness (or relative density). Among these parameters, the influences of the cell morphology are not well understood. This contribution aims to investigate numerically the influences of the cell architecture on their radiative properties.
At first, three-dimensional samples are modeled through the Voronoï tessellation method. Different foam morphologies are considered ranging from the periodic assembly of the famous Kelvin'cells to assemblies of totally random cells.
Secondly, the equivalent radiative properties are determined from a Ray-Tracing (RT) method performed inside the REV [Randrianalisoa 2010, Coquard 2011]. This method consists in tracking the path of a large number of energetic rays (or photon bundles) from their possible emission location to their extinction location. The absorption and scattering coefficients are determined from the history of extinction paths while the scattering phase function is determined from the history of scattering direction distribution.
The radiative properties of the samples are compared with analytical models of the literature. The evolution of these properties with the cell randomness is analysed and discussed. An analysis of the anisotropy of the radiative behaviour is also conducted. For validation purpose, the calculated hemispherical transmittances and reflectances of plan parallel foam slices are compared with the corresponding measurements conducted on XPS foam slices. The agreement between experimental and computed transmittances and reflectance is quite satisfactory, demonstrating the suitability of the numerical approach.

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