Experimental and Numerical Investigation of Pool Boiling Heat Transfer on Engineered Nano-Finned Surfaces

Abstract

Pool boiling experiments for nanocoatings (or nanostructured surfaces) show that despite the lower thermal conductivity values than carbon, silicon yielded higher values of CHF (critical heat flux). Subsequently numerical studies showed that the interfacial thermal resistance (Kapitza resistance or ?R_(k)?) between a nanofin and fluid molecules is the dominant component of the thermal impedance network. The values of R_(k) for silicon were predicted to be ~1000 times smaller than that of carbon in these numerical simulations. Since the total thermal impedance of silicon nanofins is lower than that of carbon they cause higher levels of enhancement of CHF.

Surface adsorption of the liquid molecules on a nanofin results in the formation of dense ?compressed phase? which in turn induces thermal capacitance and diodic behavior. This is termed as the ?nanofin effect?: which implies that CHF is more sensitive to R_(k) than the thermal conductivity of the nanofin. Hence, the objective of this study was to verify the nanofin effect.

Experimental and numerical investigation of transport phenomena during pool boiling were performed in this study for liquid subcooling of 0 ?, 5 ? and 10 ? on horizontal planar heater configuration. Surface temperature was measured using nanosensor (Thin Film thermocouple or ?TFT?) arrays. Heater surfaces (with or without nanofins of different heights) were composed of ceramic, oxide and metal surfaces. The nanofins were fabricated using Step and Flash Imprint Lithography (SFIL). Contact angle was measured both before and after the experiments.

Nucleate pool boiling heat transfer was enhanced with increase in pillar height. Numerical predictions for R_(k) obtained from Molecular Dynamics (MD) simulations were found to be consistent with the level of heat flux enhancement observed in the experiments for the different nanofin configurations. Hence this study demonstrates that R_(k) is the more dominant parameter for heat transfer enhancement during pool boiling ? compared to the thermal conduction resistance (or material properties) of the nanofin itself. As an outcome of these investigations future topics of research are also proposed (such as, using temperature nano-sensors for the investigation of controlled fouling on pool boiling phenomena for heaters with micro/nano-structured surfaces).