The synergistic effect of micro/nanostructure length scale and fluid thermophysical properties on pool boiling heat transfer

Abstract

Facile surface micro/nanostructuring techniques for additively-manufactured (AM) aluminum alloy (AlSi10Mg) have recently been developed. The structuring techniques are not only highly scalable, but they also enable the tailoring of structure length scale and morphology to enhance the pool boiling heat transfer coefficient. Our past study revealed that the structure cavity size of 5 µm is favorable for bubble nucleation during pool boiling of dielectric fluid, HFE-7100, resulting in significant enhancements in the heat transfer coefficients (h). However, owing to the differences in thermophysical properties between different coolant fluids, including saturation temperature, latent heat of vaporization and surface tension, the required structure size range for bubble nucleation and capillary wicking force for liquid re-supply are expected to differ significantly. To explore the effect of structure length scale on the pool boiling performance of coolants with different thermophysical properties, this work develops a new surface structuring technique consisting of a dual-stage metallurgic heat treatment process and single-stage crystallographic etching process to tune the structure length scale across nearly two orders of magnitude, viz., from 0.3 μm to 15 μm. Using coolant media of vastly different thermophysical properties, i.e., dielectric fluid HFE-7100 and deionized water, we show that while microcavities with sizes ranging from 3 to 8 μm are favorable bubble nucleation sites for boiling of HFE7100, which results in the enhancement of the maximum heat transfer coefficient (hmax) by 83.4 to 103.8 % as compared to a conventional plain Al6061 surface, larger microcavity sizes of 10 to 15 μm are required to effectively promote bubble nucleation of water. This large microcavity size range of 10 to 15 μm, produced through rational nanoparticle agglomeration of the rich Si-phase in AM AlSi10Mg in elevated temperature, followed by an indirect removal process using a chemical process, is found to significantly increase hmax of water by up to 259.9 % as compared to conventional nanostructures formed on Al6061. In addition, the new AM structured surfaces also exhibit up to 26.6 % enhancement in critical heat flux (CHF) as compared to highly-wicking conventional nanostructured Al6061. In summary, by utilizing scalable fabrication techniques to tailor the structure length scale on AM AlSi10Mg, this work not only reveals the favorable microcavity sizes for bubble nucleation of different coolant fluids to enhance boiling, but also provides useful micro/nanostructure design guidelines that can be adopted to enhance boiling of other coolants and phase change applications.