Flow boiling in a micro-channel coated with carbon nanotubes

This study examines the heat transfer enhancement attributes of carbon nanotubes (CNTs) applied to the bottom wall of a shallow rectangular micro-channel. Using deionized water as working fluid, experiments were performed with both a bare copper bottom wall and a CNT-coated copper wall. Boiling curves were generated for both walls, aided by high-speed video analysis of interfacial features. CNT arrays promoted earlier, abundant and intense bubble nucleation at low mass velocities, consistent with findings from previous pool boiling studies. However, high mass velocities compromised or eliminated altogether any enhancement in the nucleate boiling region. The enhancement achieved at low mass velocities appears to be the result of deep, near-zero-angle cavities formed by the mesh of CNT arrays. On the other hand, high mass velocities tend to fold the CNTs upon the wall, greatly reducing the depth of the CNT-mesh-induced cavities, and compromising the effectiveness of CNTs at capturing embryos and sustaining the bubble nucleation process. CHF enhancement was also achieved mostly at low mass velocities. It is postulated CNT arrays enhance CHF by increasing the heat transfer area as well as by serving as very high conductivity fins that penetrate into the cooler, bulk liquid flow and take advantage of the liquid subcooling away from the wall. While these mechanisms are prevalent at low velocities, they are both weakened, especially the fin effect, at high mass velocities because of the folding of CNT arrays upon the wall.