Virtual enclosure model for thermal radiation extinction inside porous materials with closed cell structure

We present a virtual enclosure model for thermal radiation extinction in porous materials. The geometrical structure of the model is based on series of cuboid enclosures. The key conjecture of the model is that the mean free distance of photons emitted from the real cell can be applied for finding a virtual enclosure with opaque, isothermal and diffusive surfaces. This leads into the applicability of the view factor analysis of the virtual structure providing an efficient method for evaluation of the otherwise highly complex radiation interaction between large numbers of consecutive unit cells. The model is applied to analyzing how the cell configuration affects radiative extinction properties of materials with a wide range of complex refractive indices. It is shown, e.g., that an optimum cell configuration (size, wall thickness, aspect ratio) which minimizes radiative power can be detected for materials with sufficiently high ratio between the real and imaginary part of the refractive index. In addition, it is shown that for materials with high radiation extinction properties closed cell structures have superior radiation extinction properties as compared to open cell structures.