Emissivity of High-Temperature Fiber Composites

Effective emissivities of high-temperature fiber composites are calculated for several external edge surface structures-randomly overlapping, parallel cylinders of radius a protruding out a distance 6 from the composite matrix, with central axes either perpendicular to or into the direction of the composite external edge. First-order multiple scattering reciprocal bounds provide effective emissivity values, usually within an error of 10% or less; e.g., for carbon-carbon composites with a fiber-matrix emissivity of epsilon(s) = 0.8 and a fiber fraction of 30 vol %, the maximum error bound is 4%-5%. The fundamental behavior of the effective emissivity, as a function of protrusion depth, is examined, and significant deep-bed, blackbody radiative effects are observed, even at modest model protrusions. A maximum value for the fiber composite effective emissivity at an optimum porosity is found for any given ratio of protrusion length to fiber radius (delta/a) and fiber-matrix emissivity (epsilon(s)); e.g., for a perpendicular protrusion with delta/a = 3 and epsilon(s) = 0.5, a maximum effective emissivity enhancement of 27% is found at Phi = 0.6, and for fibers protruding parallel to the edge with delta/a = 3 and epsilon(s) = 0.5, a maximum enhancement of 21% is observed at Phi = 0.6. Model equation results agree with the measured effective emissivity values of carbon-carbon brake composites.