| Summary: | This project investigates the droplet dynamics on heated micro/nanostructured aluminium alloys and their performance for Leidenfrost suppression. A conventional aluminium alloy, Al6061, and an additively manufactured (AM) aluminium alloy, AlSi10Mg, were utilised in this investigation. In addition to the plain Al6061 and AM surfaces, through the synergistic use of heat treatment and chemical etching on aluminium alloys, three different microstructure morphologies were fabricated. Furthermore, 300 nm boehmite nanostructures were generated on the plain Al6061 and AM surfaces to produce single tier nanostructures and were incorporated onto the microstructured surfaces to fabricate hierarchical micro/nanostructured surfaces. Hence, a total of 10 test pieces with different surface structure morphologies such as plain, nanostructures, microstructures, and hierarchical micro/nanostructures were investigated in this project. A new experimental setup was also designed and fabricated to investigate the Leidenfrost phenomenon on the micro/nanostructured surfaces.
To characterise the Leidenfrost temperature and droplet dynamics, a combination of altering the temperature of the test piece and Weber number of the water droplet was performed. The test piece temperatures were ranged between 150ºC and 550ºC with intervals of 50ºC. Weber numbers were varied by changing the water droplet drop heights from 10 mm, 30 mm, 50 mm to 70 mm, giving Weber number ranging between 0 and 100. With these experimental parameters, high-speed videos were then captured and analysed to plot a regime map of the temperature against Weber number with data points depicting the four types of boiling regime, i.e., nucleate boiling, atomization, explosive boiling and Leidenfrost.
The experimental results revealed that there is no distinct Leidenfrost temperature point for certain test pieces. Furthermore, it was also found that the surface roughness played a significant role in affecting the type of boiling regime observed. Rougher surfaces tend to affect the repeatability of the boiling regime at the data points captured. This is due to the variation of boiling regime even at the same temperature and Weber number range. With increasing Weber numbers, the boiling regime tend to shift towards atomization and explosive instead of exhibiting Leidenfrost phenomenon. Lastly, boehmite formation on all surfaces showed a strong corelation to delay the transition to Leidenfrost.
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