Journal of Porous Media

COMPUTATIONAL PREDICTION OF RADIATIVE PROPERTIES OF POLYMER CLOSED-CELL FOAMS WITH RANDOM STRUCTURE

ABSTRACT

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 that absorb, emit, and scatter radiation. Current knowledge shows that the radiative properties, namely the absorption and scattering coefficients, and the phase function 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 the radiative properties of closed-cell foams. At first, three-dimensional samples are modeled through the Voronoiiessellation method. This modeling approach is chosen among other thanks to its capability to reproduce a large variety of foam microstructures and to create large sample volumes, which are representative elementary volume (REV) of the foam material. Different foam morphologies are considered ranging from the periodic assembly of Kelvin cells to assemblies of totally random cells. Second, the equivalent radiative properties are determined from a ray-tracing (RT) method performed inside the REV (Randrianalisoa and Baillis, 2010; Coquard et al., 2011). This method consists of 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 analyzed and discussed. An analysis of the anisotropy of the radiative behavior of the foams generated is also conducted. For validation purposes, the calculated hemispherical transmittances and reflectances of plane parallel polymer samples are compared with the corresponding measurements conducted on slices of extruded polystyrene (XPS) foam samples using a Fourier transform infrared (FTIR) spectrometer. The agreement between experimental and computed transmittances and reflectance is quite satisfactory, demonstrating the suitability of the numerical approach.