Tomography-based analysis of apparent directional spectral emissivity of high-porosity nickel foams

The apparent radiative behaviors of open cell nickel foams and their dependency on textural features and intrinsic optical properties are numerically investigated in the limit of geometrical optics. Five realistic nickel foams with high porosity from 0.87 to 0.97 and pore density from 10 to 40 PPI were used as samples. First, the representations of foams and their microstructures were obtained by using computed tomography and scanning electron microscope techniques. Second, a Backward Monte Carlo method that employed the realistic representations and their intrinsic optical properties facilitated the computation of the apparent directional spectral emissivity. The apparent emissivity and its dependency on textural features and intrinsic optical properties were subsequently investigated based on a specially defined Representative Elementary Volume. It was noted that at pore scale, the non-homogenized apparent emissivity fields can vary according to the local compositions near to the foam surface (solid strut and void space) and the void pore behaves somewhat like a “black cavity” for thermal radiation. For open cell foams composed of low emittance nickel struts, high-porosity cell network may increase the intrinsic emissivity by a maximum of nearly 4 times. A predictive relationship was established that links the homogenized apparent emissivity to the open porosity and the intrinsic emissivity of solid struts. These findings will provide a deeper understanding of both the non-homogenized and homogenized radiative behaviors of open cell foams.