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

Abstract

The objective of this report is to study the effects of 3D printed substrates with porous microstructures on the enhancement of saturated pool boiling heat transfer and identify the characteristics which provide optimal heat transfer performance. The porous microstructures were fabricated using the Selective Laser Melting (SLM) method, utilising aluminium alloy powder AlSi10Mg as the material of the structures. SLM is a method of Addictive Manufacturing more commonly known as 3D printing. The effect of channel height and structure geometry were evaluated in this study.

Three types of porous structures of varying heights were fabricated. Each of them varied in internal sphere diameter and external cylinder diameter. Each type was printed starting from one layer, up till four layers. The unit cell was kept constant at 2.5 mm by 2.5 mm by 2.5 mm. The base area of the structures was maintained constant at 10 mm by 10 mm. A plain copper surface was used as a benchmark. A total of 12 structures were tested, including the plain surface. All 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 increased the heat transfer performance compared to the plain copper surface used as the baseline. This could be due to the increase in surface area with increasing height. There were, however, limitations to this effect and it is incorrect to conclude that for whatever the size of the holes.

The effect of porosity structures showed that with larger sphere diameters and cylindrical hole diameters, heat transfer performance was improved. As porosity increased, the delaying of CHF increased as well. This could be due to the larger nucleation sites that promoted better fluid replenishment.

For the effect of material, at the same height and porosity level, the aluminium alloy substrate performed better than the K220 copper alloy substrate due to its higher thermal conductivity. The results of this study show that the for a possibly indefinite delay in CHF, a minimum of height of 5.0 mm and a porosity range of 34.4% to 49.3% are required. The hollow sphere with cylindrical holes design proved to provide clear fluid-vapour pathways and utilised the capillary effect to enhance the flow of fluid. These factors are significant contributors to the enhancement of heat transfer performance.