International Journal of Heat and Mass Transfer, Vol.44, No.20, 3799-3810, 2001
Fully developed turbulent flow and heat transfer at fiber-flocked surfaces
High-conductivity carbon fibers can be "flocked", or perpendicularly attached onto surfaces, thus enabling heat transfer enhancement for such fiber-flocked surfaces. An analysis is performed for fully developed turbulent flow and heat transfer in ducts with high-conductivity fibers covering the walls. The fiber volumetric packing density is sparse such that single-cylinder correlations are used for the fiber drag and Nusselt number; this gives rise to body-type terms in the momentum and energy equations for the region near the wall covered with fibers. An eddy-diffusivity type turbulence model is employed, including Laufer core-flow distribution and Van Driest damping, but with a shifted origin, The resulting core-and fiber-region equations are solved by the singular perturbation theory, and matched at the fiber-tip interface, yielding a friction factor function that is fitted to available experimental data with a 32% fiber-length coordinate shift. The results show the variation of the duct Nusselt number with Reynolds number to be similar to that for smooth surfaces, but with an enhancement factor of four for gases. A very strong variation with Prandtl number is predicted for the fiber-flocked surfaces: for liquid metals and gases a moderate enhancement is predicted, whereas for water and viscous liquids an order-of-magnitude enhancement appears possible.