Effects of cell anisotropy on conductive and radiative thermal transport in polymeric foam insulation

The effects of cell anisotropy on the conductive and radiative heat transfer in closed-cell microcellular and nanocellular thermal insulation foams were investigated. The effects of the material's intrinsic gas-to-solid conductivity ratio on conduction in isotropic and anisotropic foams with a given cell geometry were investigated using finite element analysis (FEA). The Knudsen effect accounted for the gas thermal conductivity, and the increased thermal conductivity due to the presence of fillers in polymer composites was considered through the effective medium approach. In addition, scattering, absorption and re-radiation by cell walls and struts accounted for the radiative heat transfer. Because the thickness, size and orientation of cell walls varied with the heat transfer direction, they affected the cell's transparency. The theoretical prediction was verified by experimental data from the literature. It was found that both conduction and radiation increase in the longitudinal direction due to the parallel-like structure but decrease in the transverse direction due to the series-like structure. Anisotropic polypropylene foam with 40-time expansion ratio and cell aspect ratio of 3 could achieve longitudinal 0.096 W m−1 K−1 and transverse 0.034 W m−1 K−1 at 10 μm mean cell size and be minimized to 0.031 W m−1 K−1 at 200 μm.