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 (δ/ a) and fiber−matrix emissivity (ε s) e.g., for a perpendicular protrusion with δ/ a = 3 and ε s = 0.5, a maximum effective emissivity enhancement of 27% is found at Φ = 0.6, and for fibers protruding parallel to the edge with δ/ a = 3 and ε s = 0.5, a maximum enhancement of 21% is observed at Φ = 0.6. 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. 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 ε s = 0.8 and a fiber fraction of 30 vol%, the maximum error bound is 4%−5%. 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 δ from the composite matrix, with central axes either perpendicular to or into the direction of the composite external edge.
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