Intertube Aggregation-Dependent Convective Heat Transfer in Vertically Aligned Carbon Nanotube Channels

The heat transfer of carbon nanotube fin geometry has received considerable attention. However, the flow typically occurred over or around the pillars of nanotubes due to the greater flow resistance between the tubes. Here, we investigated the forced convective heat transfer of water through the interstitial space of vertically aligned multiwalled carbon nanotubes (VAMWNTs, intertube distance = 69 nm). The water flow provided significantly a greater Reynolds number (Re) and Nusselt number (Nu) than air flow due to the greater density, heat capacity, and thermal conductivity. However, it resulted in surface tension-induced nanotube aggregation after the flow and drying process, generating random voids in the nanotube channel. This increased permeability (1.27 × 10^-11 m²) and Re (2.83 × 10^-1) but decreased the heat transfer coefficient (h, 9900 W m^-2 K^-1) and Nu (53.77), demonstrating a trade-off relationship. The h (25,927 W m^-2 K^-1) and Nu (153.49) could be further increased, at an equivalent permeability or Re, by increasing nanotube areal density from 2.08 × 10¹⁰ to 1.04 × 10¹¹ cm^-2. The area-normalized thermal resistance of the densified and aggregated VAMWNTs was smaller than those of the Ni foam, Si microchannel, and carbon nanotube fin array, demonstrating excellent heat transfer characteristics.