Pore-scale radiative heat transfer and accuracy of reduced radiation models in porous and fibrous insulation materials

Reduced radiation models of high-temperature porous and fibrous insulation compress the pore geometry into a few scalar optical properties. We test whether that description transfers between morphologies with different directional pore paths. Four digital microstructures (open-cell foam, random and aligned fiber networks, tortuous layered stack) are compared at matched solid volume fraction, material properties, and boundary conditions, with pore-scale surface-to-surface radiation as the common reference in prescribed-temperature and strictly coupled radiation-conduction calculations. At a representative condition (700 K mean temperature, ɛ = 0.6 , 10 mm slab), rearranging the same amount of solid changes the coupled heat flux by a factor of 2.62, and radiation carries about 14% to 74% of the total. Turning the transfer axis changes the flux by almost a factor of two for the layered structure and by about 40% for the aligned fiber network, against less than 10% for the near-isotropic foam and random fiber. An aligned and a random fiber geometry matched in solid volume fraction, interfacial area density, and mean beam length still swap their flux ordering between transfer axes. A transport correction lowers the median wall-flux error of the homogenized discrete ordinates model from about 28% to 12%, with about 8% for the near-isotropic structures but 23% and 62% for the aligned fiber and layered ones. Reduced-model accuracy thus depends jointly on pore morphology and on how the equivalent properties are obtained; within the structures examined, equivalent-medium prediction is adequate for near-isotropic pore spaces, whereas strongly directional ones require an angular representation of extinction.