Effects of carbon nanotube coating on flow boiling in a micro-channel

Experiments were performed to assess the heat transfer enhancement benefits of coating the bottom wall of a shallow rectangular micro-channel with carbon nanotubes (CNTs). Using water as working fluid, tests were performed with a bare copper surface and three separate, yet identical CNT-coated surfaces. Each of the CNT-coated surfaces was tested repeatedly at the same mass velocity to explore any time dependence of heat transfer performance parameters, especially critical heat flux (CHF). Appreciable differences in the influence of CNT coating were observed at high mass velocities as compared to low. CHF was repeatable at low mass velocities but degraded following repeated tests at high mass velocities, proving high flow velocities cause appreciable changes to the morphology of the CNT-coated surface. SEM images show the initially near-vertical CNTs were bent upon the heated surface at high mass velocities to form a repeated ‘fish-scale’ pattern. Voids between the ‘fish scales’ provided near-zero-angle cavities that enhanced heat transfer in the nucleate boiling region compared to the bare copper surface. While CHF was enhanced by the increased heat transfer area associated with the CNT coating, the enhancement decreased following repeated tests as the CNT fin effect was compromised by the bending.