High heat flux phase change on porous carbon nanotube structures

摘要

Carbon nanotube (CNT) forests are investigated as porous wick structures for chip-scale heat pipe cooling systems. An analytical model is developed to demonstrate the merits of phase change heat transfer on nanoscale porous structures, compared to that on microscale porous wick. Results indicate that nanoscale porous structures increase the thin-film evaporation surface area by one order of magnitude, which can significantly increase phase change heat transfer efficiency. The pertinent wick structure properties of the CNT forest are experimentally measured. Results show that the CNT forest is highly porous (～95% poros ity), and possesses large variations in effective thermal conductivity ranging from 0.8 to 180W/mK. Effective pore size of the CNT wick structure varies between 50 and 180 nm, which can generate capillary pressure up to two orders of magnitude higher than the microscale wick structure. However, its low per meability, about three to four orders of magnitude lower than the traditional wicks, underscores the necessity of bi-porous CNT wick structures. The bi-porous CNT wick structures are composed of nano scale porous CNT clusters, separated by microscale (～50 μm wide) passages. Experimental results show a maximum heat flux of 770 W/cm~2 over a 2 mm × 2 mm heating area. With enhanced thin-film evapo ration, heat transfer coefficients are improved by up to 100%, compared to the microscale wick. In con trast, the low CHF ～140 W/cm~2 over a 10 × 10 mm~2 heating area is caused by vapor occupation of the microscale pores and the reduction of wick permeability.