Experimental determination of the role of increased surface area in pool boiling from nanostructured surfaces

摘要

The use of nanostructured surfaces to enhance pool boiling heat transfer performance has previously been demonstrated for a variety of outwardly-projecting nanostructures such as nanowires and nanotubes. Such enhancement has been attributed to a variety of factors, including greater surface area, improved wickability, and superior nucleation site density as compared to unmodified surfaces. However, since these three phenomena are inherently interlinked with the presence of the nanostructures, isolating each one for independent study to truly understand its relative importance in enhancing pool boiling heat transfer has remained a challenge. In this work, nanoporous anodized aluminum oxide (AAO) films on metallic aluminum (Al) substrates are used to serve as an inverse representation of an aligned nanowire array with similar increase in surface area but without inter-nanowire/nanotube wicking action or significant change in observed static wetting behavior relative to Al control samples with a solid native oxide film. Further, it is shown via a combination of experiment and analytical modeling that the AAO-covered Al samples studied here do not represent a significant increase in nucleation site density relative to the control samples. In this way, the influence of enhanced surface area of a nanostructured sample on pool boiling performance was isolated and quantitatively determined. Pool boiling performance for Al samples with solid native oxides and AAO films was measured using a custom-built test setup, with the commercial, low surface tension, dielectric coolant Novec (TM) HFE-7100 as the working fluid. Results were interpreted via a nanopore wetting model along with imaging analysis of bubble size, which collectively point to wickability and nucleation site density playing a greater role than increased surface area in nanostructure-based pool boiling enhancement.