Enhancement of saturated pool boiling using 3D printed substrates with microstructures

• Main Author: Chew, Timothy Jia Wei
• Other Authors: Leong Kai Choong
• Format: Final Year Project
• Language: English
• Published: 2016
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/67800
• Institution: Nanyang Technological University

The objective of this paper is to study the effects of 3D porous structures on the enhancement of saturated pool boiling heat transfer and identify the characteristics which provide optimal heat transfer performance. The porous structures were fabricated using the Selective Laser Melting (SLM) of Aluminium alloy powder AlSi10Mg; SLM is a method of Additive Manufacturing more commonly known as 3D printing. The effect of channel height, structure geometry and cell size were evaluated in this study. Three types of porous structures of varying heights were fabricated. The first two were Octet-truss and Rhombus unit cells of heights 2.5 mm, 5.0 mm, 7.5 mm and 10.0 mm. The third and fourth were parallel vertical channels with one and three rows of horizontal channels at the bottom respectively, and variation of heights 5.0 mm, 10.0 mm and 15.0 mm. The base area of the structures was maintained constant at 10 mm by 10 mm. A plain surface made from aluminium alloy (Al 6061) was used as a benchmark. A total of twenty-one structures were tested, including the plain surface. The pool boiling experiments were conducted in a thermosyphon with FC-72 dielectric coolant as the working medium, under atmospheric conditions and at saturated temperature. For the effect of height, it was found that increasing the porous structure heights generally increases the heat transfer coefficients as far as 125% improvement compared to the benchmark, which could be due to the increased nucleation sites as the surface area increases with increasing height. However, it was also found that there was a limit to the improvement in performance with the increment of height. As height was increased beyond a certain limit, the performance decreased. This could be due to the temperature drop along the height of the structure becoming significant enough to prevent nucleation from occurring at the top of the structure, which then also becomes additional bubble escape resistance. The porous structures all showed substantial delay of Critical Heat Flux (CHF) of more than 100%, which is likely due to improved fluid replenishment. The effect of structures showed that a structure with lower pore density exhibited better performance, likely due to decreased bubble resistance. In addition, it was found that when the design of the structures promotes orderly flow of bubbles during boiling, heat transfer performance and CHF delay were significantly improved. For the effect of cell size, the results showed that for a given porous structure, there exist a minimum cell size where below that the effect of bubble resistance becomes significant and heat transfer performance deteriorates. The results of this study shows that the balancing of the multiple counteracting factors is necessary for optimal heat transfer performance. Also, having clear definition of fluid-vapour pathways and the utilisation of capillary effect to enhance the flow of fluid have been identified as significant contributors to the enhancement of performance.