Morphological change of plain and nano-porous surfaces during boiling and its effect on nucleate pool boiling heat transfer

In order to investigate nucleate pool boiling heat transfer characteristics in saturated water, a plain aluminum alloy surface (6061, untreated) and a uniquely fabricated aluminum oxide Nano-Porous Surface (NPS) sample were prepared. Generally, the NPS exhibited a lower wall superheat at the onset of nucleate boiling and a higher nucleate boiling heat transfer coefficient than the plain surface. It was also noted that the nucleate boiling heat transfer coefficient decreased by 30% on the plain surface and by 37% on the NPS after five repeated boiling tests. It was found that such performance-degradation in both of the test samples was due primarily to the formation of aluminum hydroxide on the boiling surfaces. It is our belief that the aluminum hydroxide, formed on both boiling surfaces, significantly altered the surface morphologies as shown in the microscopic images of the post-boiling surfaces, which resulted in the decrease of the active nucleation sites. Although NPS is an excellent tool to enhance the nucleate pool boiling heat transfer coefficient, its life span and usefulness can be impeded by undesirable surface reactions with working fluids.