Radiative characterization of random fibrous media with optically large long cylindrical fibers

Radiative heat transfer is analyzed in participating media consisting of long cylindrical fibers with a diameter in the limit of geometrical optics. The absorption and scattering coefficients and the scattering phase function of the medium are determined based on the discrete-level medium geometry and optical properties of individual fibers. The fibers are assumed to be randomly oriented and positioned inside the medium. Two approaches are employed: a volume-averaged two intensities approach referred to as multi-RTE approach as it has been done in several other papers, and a homogenized single intensity approach referred to as the single RTE approach. Both approaches require effective properties determined by utilizing Monte Carlo ray tracing techniques. The macroscopic radiative transfer equations (for single intensity or volume averaged two intensities) with the corresponding effective properties are solved by Monte Carlo techniques and allow for the determination of the radiative flux distribution as well as overall transmittance and reflectance of the medium. The results are compared against predictions by the direct Monte Carlo simulation on the exact morphology. The effects of optical properties of the fiber substance and volume fraction on the effective radiative properties and the overall slab radiative characteristics are investigated. The single RTE approach gives accurate prediction for high porosity fibrous media (porosity about 95%). Advanced radiative transfer models such as the multi-RTE approach are more suitable for isotropic fibrous media with porosity in the range 79−95%.