Heat exchange performance of stainless steel and carbon foams modified with carbon nano fibers

Carbon nanofibers (CNF), with fishbone and parallel wall structures, were grown by catalytic chemical vapor deposition on the surface of carbon foam and stainless steel foam, in order to improve their heat exchange performance. Enhancement in heat transfer efficiency between 30% and 75% was achieved with CNF-modified stainless steel foam as measured in a filled copper channel for varied lengths of heat exchanger (between 0.05 and 0.01 m). The heat transfer coefficient of carbon foam decreased when modified with CNF by ∼40% in average. The increase in heat transfer efficiency of the CNF-modified stainless steel foam is explained by an increase in surface area provided by the carbon filaments grown on stainless steel of one order of magnitude, and by the carbon fibers’ specific parallel wall structure, providing excellent thermal conductivity in the axial direction (h = 1130 W/K−1m−2). However, nanofibers grown on carbon foam have fishbone type structure, exhibiting lesser conductivity in the axial direction resulting in lower thermal conductivity of the fibers from the wall to the air (h = 428 W/K−1m−2). The higher crystallinity of carbon nanotubes of modified stainless steel material in contrast to chaotic mal-alignment and relatively high concentration of structural defects of carbon nanofibers grown on carbon foam can also contribute to the big difference in heat exchange properties.