Kin Keong Wong

Fabrication of heat sinks by Selective Laser Melting for convective heat transfer applications

ABSTRACT In this paper, the forced convective heat transfer performance of heat sinks produced by Selective Laser Melting (SLM) was experimentally investigated. Three heat sinks comprising pin fins of circular, rectangular-rounded and aerofoil geometries were fabricated by SLM from aluminium alloy AlSi10Mg powder. The heat sinks were tested in a rectangular air flow channel for convective heat transfer performance. Experiments performed for Reynolds numbers ranging from 3400 to 24,000 show that the heat transfer performances of the aerofoil and rectangular-rounded heat sinks exceeded those of the circular heat sink. Using the cylindrical heat sink as a benchmark, the average enhancements in the normalised Nusselt numbers were computed to be 15.0% and 21.4% for the rectangular-rounded and aerofoil heat sinks, respectively. It was demonstrated that SLM can be employed to design and fabricate heat sinks of customised geometries for heat sink applications.

Enhanced Nucleate Pool Boiling From Microstructured Surfaces Fabricated by Selective Laser Melting

Selective laser melting (SLM) is a promising manufacturing method which enables the production of complex structured components from base metal powders. With the development of SLM, the possibility of fabricating functional heat transfer devices such as heat pipes and heat sinks using this technique has also gained significant interest in the recent years. In this paper, the possibilities of producing microstructured surfaces using SLM to promote nucleate pool boiling heat transfer were explored. The SLM facility (SLM 250 HL by SLM Solutions GmbH) at the Future of Manufacturing Laboratory 1 of Singapore Centre for 3D Printing (SC3DP) in Nanyang Technological University (NTU), Singapore was employed for the fabrication of the surfaces. The machine is comprised of a Gaussian distributed Yb:YAG laser with maximum power of 400 W and laser beam spot size of 80 μm which melts and fuses the AlSi10Mg base powder of distribution size 20 μm to 63 μm layer-by-layer to develop three-dimensional structures.In total, four 1 cm × 1 cm microstructured surfaces were produced; namely micro-cavity surface, micro-fin surface, micro-sized rectangular channel (MRC) surface and micro-sized square channel (MSC) surface. Saturated pool boiling experiments were conducted on these surfaces in a water-cooled thermosyphon with FC-72 as the coolant fluid under atmospheric condition. In comparison with a plain surface, the MRC and MSC surfaces exhibited marginal improvements in the average heat transfer coefficient whilst more significant enhancements of up to 51.2% were demonstrated with the micro-cavity and micro-fin surfaces. At low heat fluxes (< 7 W/cm2), minimal differences in heat transfer performances between the microstructured surfaces and plain surface were observed. For increased heat fluxes, incremental enhancements in the heat transfer coefficients were observed for the micro-cavity and micro-fin surfaces as compared to the plain surface. The highest enhancement in the heat transfer coefficient over the plain surface was determined to be 63.5% for the micro-fin surface at the heat flux of 17.9 W/cm2 and it was also observed that the heat transfer coefficient of micro-fin surface is consistently higher that of other microstructured surfaces for the range of heat fluxes tested. In addition, higher critical heat fluxes were also achieved with all microstructured surfaces as compared to the plain surface with the highest CHF of 46.2 W/cm2 for the micro-fin and MRC surface. Visual observations suggest that the enhancement in heat transfer from the microstructured surfaces is likely to be due to the increased bubble nucleation sites created from the extended surfaces and the artificial cavities. In summary, these results indicate the promising use of SLM to produce surface features that will enhance pool boiling heat transfer.Copyright

Exergy Analysis of the Revolving Vane Compressed Air Engine

Exergy analysis was applied to a revolving vane compressed air engine. The engine had a swept volume of 30 cm3. At the benchmark conditions, the suction pressure was 8 bar, the discharge pressure was 1 bar, and the operating speed was 3,000 rev·min−1. It was found that the engine had a second-law efficiency of 29.6% at the benchmark conditions. The contributors of exergy loss were friction (49%), throttling (38%), heat transfer (12%), and fluid mixing (1%). A parametric study was also conducted. The parameters to be examined were suction reservoir pressure (4 to 12 bar), operating speed (2,400 to 3,600 rev·min−1), and rotational cylinder inertia (0.94 to 2.81 g·mm2). The study found that a higher suction reservoir pressure initially increased the second-law efficiency but then plateaued at about 30%. With a higher operating speed and a higher cylinder inertia, second-law efficiency decreased. As compared to suction pressure and operating speed, cylinder inertia is the most practical and significant to be modified.