Scale effects on evaporative heat transfer in carbon nanotube wick in heat pipes

The heat transfer capability of conventional materials like copper or devices like heat pipes are challenged by the growing trend of miniaturization of integrated circuit and exponentially increasing density of heat generation. Vertically aligned carbon nanotube (CNT) array has been recognized as a promising wicking structure of new generation heat pipes for higher heat transfer capability and more compact size. In contrast to conventional grooved or powder-sintered wicks, the CNT wick behaves differently due to the scale effects, which has been investigated through physical models extracted from its working conditions. The influence by the velocity slip of the liquid at the CNT wall is evaluated, which shows that the slip leads to considerable enhancement of the flow near the CNT wall. Calculation of the overall flow resistance indicates higher permeability due to the slip. The gaps between CNTs create exceptionally high capillary pressure due to the nanoscale pores. It proves to be apparently superior to conventional structures in micrometer scale. The effective thermal conductance of the CNT wick benefits from the extremely high thermal conductivity of CNTs thanks to the non-Fourier conduction. The effects emerged in the non-evaporating and thin-film regions are analyzed, which clearly shows that the evaporation at the liquid-vapor interface is dramatically affected by these nanoscale effects. The present study on scale effects identifies the advantages of the CNT wick in capillary ability, effective conductivity, and evaporative flux, etc. Meanwhile, it also reveals the disadvantage of relatively lower dry-out limit of the CNT wick.