EXPERIMENTAL POOL BOILING HEAT TRANSFER STUDY OF THE NANOPOROUS COATING IN VARIOUS FLUIDS

Abstract

An experimental pool boiling study was conducted using plain and nanoporous coated heater surfaces immersed in various working fluids: water, ethanol and HFE-7100. Pool boiling tests were performed on flat 1 cm × 1 cm heaters. Unlike in water, the critical heat flux (CHF) enhancement of the nanoporous coating seems to be less or marginal in ethanol and HFE-7100 at 1 atm. The reduced effect of the nanoporous coating in ethanol and HFE-7100 is believed to be due to the highly wetting nature of these fluids since no obvious difference in wettability is observed between nanoporous coated and uncoated surfaces through apparent contact angle measurement. Moreover, pressure effects were also investigated for the fluids mentioned above. For the nanoporous coated surface, CHF enhancement of the nanoporous coating appeared to be dependent on the test pressure, showing greater CHF enhancement at lower pressure. It is believed that this pressure dependent CHF enhancement behavior could be closely related to the bubble departure diameter. As pressure lowers, the departure bubble size increases and this allows the nanoporous coating to become more influential, even for the highly wetting fluids, in delaying local dry-out, which in turn results in increasing CHF enhancement.