Contribution of the radiative transfer mechanism to the total thermal conductivity of anisotropic porous materials

Abstract

In order to understand the performance of polymeric porous materials as heat insulators, the contribution of the radiative transfer mechanism in porous materials with high ratios of anisotropy is studied. Porous materials based on extruded polystyrene (XPS) with relative density in the range of 0.03 − 0.05 and with a range of anisotropy ratios of 0.6 − 1.4 have been selected for this research. The study begins with a characterization of the selected materials in terms of porous structure and thermal conductivity as a function of temperature. Then, the radiative contribution for each of the three main directions of the materials is obtained by three independent methodologies: Fourier Transform Infrared (FTIR) spectroscopy, derivation from the total conductivity using theoretical models, and theoretical calculation from the model proposed by Glicksman. The results show similar trends for all methods and confirm clear differences between each direction, showing a significant reduction of the radiative contribution and, thus, the total conductivity, if the material is oriented geometrically towards the direction in which the pore size is the smallest. Indeed, reductions of 17 − 20% in the total conductivity can be achieved at temperatures ranging from 10 − 40 ◦C if the material is reoriented as stated.