A detailed numerical study is performed to investigate radiative and convective heat transfer enhancement in pipes filled with small diameter (similar to 100 mu m) silicon carbide fibers. Radiation between fibers and the tube wall, conduction within fibers and convection from the fibers to the surrounding fluid drive the heat transfer enhancement. Macroscopic (porous media) modeling is used to determine the velocity, pressure, and temperatures fields for periodic fiber arrays of various porosites under laminar flow conditions (Re-D = 1000). Key features of the macroscopic model include two-dimensional effects, nongray radiative exchange, and the relaxation of the local thermodynamic equilibrium assumption. Results show that fiber arrays increase heat transfer largely by the radiative mode, with significant enhancement shown for porosities as high as 0.99. The increased pressure drop due to the presence of the fibers rises monotonically as the porosity is reduced.