Pool boiling heat transfer of macro/microstructured metal additively manufactured materials

• Main Author: Phoi, Wei Wen
• Other Authors: Ho Jin Yao
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/167763
• Institution: Nanyang Technological University

This project studies and characterises the pool boiling performance of macro/microstructured surfaces in dielectric fluid, HFE7100. The structures are fabricated using selective laser melting (SLM), a metal base powder additive manufacturing technique, using aluminium alloy, AlSi10Mg. There is a total of four types of macrostructured fin surfaces, i.e., pyramid, rectangular, fractal and pin fins. The pyramid fin incorporates a number of edges with a distinctive step-like shape, where each fin converges from a fixed fin width towards the fin tip to produce a fin in the shape of a pyramid. The idea of this design is to increase the number of nucleation sides using corners. The rectangular fin array consists of 10 rectangular fins, each with a fin width of 0.55 mm, a fin spacing of 0.45 mm and with two different heights of 1 mm and 2 mm. The fractal fin is an array of square fins arranged in a specific way, determined by a ratio that produces self-similarity at different scales. Square pin fin array consists of fins with different pin widths and spacings of 1.2 mm and 0.5 mm which lead to 16-fin and 100-fin configuration. To characterise the pool boiling performance of the various fin designs, the experiments were done at atmospheric pressure and near saturated temperature of HFE7100 fluid. The study investigated the effects of various surface geometries on boiling performance, including fin angle, fin height, total surface area and wall area. Heat treatment of different temperature and etching of different durations to generate microscale structures on the material surface with the aim of enhancing boiling were also studied. These processes were carried out on pyramid, rectangular, and pin fin arrays such that the microstructures were generated on the macrofins to produce macro/micro-hierarchical surfaces. Additionally, remelting of the last layer during SLM printing was performed on pyramid fins to study the effect of remelting on pool boiling performance. Videos were taken during steady-state conditions throughout all the experiments to capture the boiling characteristics of the test surface. The experimental results showed that increasing fin height enhances critical heat flux (CHF) while having little effect on the pool heat transfer coefficient (HTC) at high heat flux. Long vertical walls that are tightly packed were found to have better boiling performance as compared to structures with fin angles. Total surface area was found to be an unreliable indicator of CHF performance, whereas wall area had a better correlation with CHF and HTC. The surface with rectangular fins of 0.55 mm fin width, 2 mm fin height, and 0.5 mm fin spacing (also known as Rec 2mm) achieved the highest CHF performance of 107.9% enhancement compared to the plain surface. However, the etched Rec 2mm was unable to achieve this CHF enhancement because all etched surfaces suffered a 10% to 30% decrease in CHF performance, despite a significant enhancement in HTC of more than 100%. In the case of macro/micro-hierarchical Rec 2mm surface, HTC was enhanced up to 135%.