Enhancing nucleate pool boiling heat transfer of water using micro/nanostructured metal additively manufactured interfaces

• Main Author: Yeo, Shin Huay
• Other Authors: Ho Jin Yao
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering
• Online Access: https://hdl.handle.net/10356/177617
• Institution: Nanyang Technological University

This project investigates the pool boiling heat transfer characteristics of micro/nanostructured aluminium alloy surfaces in deionised water. The aluminium alloys investigated in this study were Al6061 and AlSi10Mg, which were fabricated by conventional machining and metal additive manufacturing, selective laser melting (SLM), respectively. The micro/nanostructuring processes of the aluminium alloys consist of heat treatment, chemical etching, and oxidation (boehmitisation). Three heat treatment temperatures were applied to AlSi10Mg, viz, 0°C, 300°C, and 400°C, while no heat treatment was performed on Al6061. Chemical etching was performed on the surfaces to reveal the underlying microstructures while oxidation was subsequently performed to form 300 nm boehmite nanostructures to generate hierarchical micro/nanostructures. A post heat treatment process of 500°C was performed on the AM samples to ensure a stable thermal conductivity. Additionally, plain Al6061 and AM samples were investigated, and the former was used as a baseline for comparison against other samples. The pool boiling performance metrics such as the critical heat flux (CHF) and heat transfer coefficient (HTC) of the surfaces were characterised and analysed. The study investigated the effects of boehmite, as well as the structure morphology and length scale, on boiling performance. Different characteristics that affect CHF and HTC were identified. The boiling performance characteristics, such as nucleation site density and bubble dynamics, and the surface characteristics, such as micro/nanostructure morphology, surface roughness and wickability were discussed in this study. To characterise the pool boiling performance of the various surfaces, the experiments were conducted at atmospheric pressure using saturated water as a coolant under quasi-steady state conditions. Throughout the boiling experiments, videos were taken to capture the bubble dynamics of the test specimens. The experimental results showed that AM surfaces performed better than Al6061 surfaces and some of the micro/nanostructured AM surfaces resulted in further enhancements in CHF and HTC. The micro/nanostructured AM sample that underwent 300°C heat treatment temperature, i.e., AM-H(300)E(10)H(500)B achieved the highest HTC enhancement of 259.9%. The micro/nanostructured AM sample that underwent 400°C heat treatment temperature, i.e., AM-H(400)E(5)H(500)B enhanced CHF by 26.6% which was the highest compared to the other surfaces. Surface roughness was found to have some significant impact on CHF performance, while wickability had a small impact on CHF performance. Nucleation site density played an important role in enhancing HTC. Additionally, performing boehmitisation before the experiment was found to stabilise the performance of the surfaces because similar boiling performance of the repeat experiments was obtained. The post heat treatment of 500°C also seemed to stabilise the data, as the thermal conductivity of AM increases when temperature increases.