Effects of various thin film coating techniques on pool boiling heat transfer

Abstract

To investigate the effects of thin film, chromium (Cr) and carbon nanotube (CNT) films were fabricated on a stainless steel grade 316 (SS316) substrate. The Cr coated heaters was produced via a sputtering process. The sputtering powers and times were carefully controlled to produce well-defined Cr layers on the substrate. On the other hand, CNT coated heaters were produced through a vacuum filtration processes. After preparing the test heaters, the coating surface was characterized using various measurements. The surface wettability was quantified by the measurement of contact angles. The surface morphology was examined by scanning electron microscopy (SEM). The contact angle measurement shows a great increase in wettability for a Cr coated heater while the wettability of the CNT coated heater decreases slightly. Moreover, the SEM images indicate that the Cr structures were compactly distributed showing nanoscale cavities. The experimental results of pool boiling tests show that, although a great increase in wettability was evaluated after the sputtering process, no enhancement of CHF was observed for the Cr coated heater. However, though the surface wettability and roughness decreased slightly, the porous structure of CNT arrays significantly enhanced the CHF.