Pool boiling heat transfer characteristics of nanocoating in various working fluids

An experimental pool boiling study was conducted using plain and nanocoated heater surfaces immersed in various working fluids. Working fluids include water, ethanol and HFE-7100 and pool boiling tests were performed on a flat 1 cm × 1 cm heaters. Unlike in water, CHF enhancement of the nanocoating seems to be less or marginal in ethanol and HFE-7100 at 1 atm. The reduced effect of the nanocoating in ethanol and HFE-7100 is believed to be due to the highly wetting nature of these fluids since no obvious difference in wettability through apparent contact angle measurement is observed between nanocoated and uncoated surfaces at 1 atm. Moreover, pressure effects were also investigated for the fluids mentioned above. The uncoated and nanocoated surfaces were tested in the working fluids at four different pressures. For the uncoated surface, measured CHF values closely matched those of Zuber's. In the case of the nanocoated surface, CHF enhancement of the nanocoating appeared to be dependent on the test pressure, showing the greatest CHF enhancement value at the lowest pressure and the enhancement decreased as the pressure increased. Although CHF enhancement of pure water was superior to that of other fluids, it was observed that there was also noticeable CHF enhancement as pressure decreased for the highly wetting fluids. It is believed that this enhancement could be closely related to the bubble departure diameter. As the test pressure decreases, the departure bubble size increases and this allows the nanocoating to become more influential, even for the highly wetting fluids, in delaying local dry-out, which in turn results in increasing CHF enhancement.