Saturated pool boiling from carbon nanotube coated surfaces at different orientations

Experiments were conducted to investigate the effects of carbon nanotube (CNT) coated surfaces and the influence of surface orientations on saturated pool boiling heat transfer and critical heat flux (CHF). A 1 cm × 1 cm fully coated CNT surface and an interlaced patterned CNT surface of coating thickness 215 μm were prepared. In FC-72 under atmospheric condition, bare silicon, fully coated CNT surface and interlaced patterned CNT surface were initially tested at constant surface orientation of 90° to determine the effect of CNT coating. Further experiments were then carried out with bare silicon and fully coated CNT surface at 0°, 30°, 60°, 120°, 150° and 180° to examine and compare the effects of surface orientation. Results showed that at constant surface orientation of 90°, both the fully coated CNT surface and the interlaced patterned CNT surface enhanced the average heat transfer coefficient by 42% as compared to the bare silicon despite the interlaced patterned CNT surface having only 70% CNT coating area. On the influence of surface orientation, results with the fully coated CNT surface showed that heat transfer performance decreases with increasing surface orientation whereas bare silicon exhibited improvement in heat transfer between 0° and 90° and deterioration in heat transfer between 90° and 180°. In addition, CNT coatings were also found to enhance CHF as the CHF of fully coated CNT surface consistently surpassed those of bare silicon for the same orientation. On the other hand, a similar trend on the dependence of CHF on orientation was observed, where CHF of bare silicon and fully coated CNT surface were seen to decrease with increasing surface orientation. The highest average heat transfer coefficient of 0.45 W/cm2 K and highest CHF of 20.1 W/cm2 were recorded for fully coated CNT surface at 0° surface orientation. Lastly, for both bare silicon and fully coated CNT surface, Rohsenow's correlation was used to characterize the pool boiling curves and the models of Kutateladze and Zuber were used to develop the CHF correlations at different surface orientations.