On the miniaturisation of convenction cooling solutions applicable to portable electronic devices

Dimensional restrictions in electronic equipment have resulted in miniaturisation of many existing cooling technologies. In addition, cooling solutions are required to dissipate increased thermal loads to maintain component reliability and user comfort. Fans are widely used in electronics cooling to meet such thermal demands, either in standalone for direct component cooling, or in combination with a heat sink. The thermal performance of such designs when scaled to dimensions suitable for use in portable electronics has received limited attention, mainly due to the reliance on passive cooling methodologies currently employed. However, as heat flux increases, passive cooling is reaching its limit and other solutions will be required. This thesis aims to address this issue by experimentally examining the fluid dynamics and thermal performance of forced convection cooling solutions with dimensional constraints. Conventional finned and novel finless heat sink designs have been integrated with commercially available radial blowers to investigate cooling solutions with overall foot print areas as low as 487mm2, and profile heights less than 5mm. The novel finless geometry, with reduced manufacturing cost, energy consumption and weight promoted heat transfer above that of the same size classical finned designs for a range of operating points. Both geometries showed increases of up to 20% in thermal performance by aligning the fan exit flow with the heat sink channels, hence demonstrating the need for integrated fan and heat sink design of low profile applications. Optimisation and geometry selection criteria were determined by scaling profile height for both heat sink designs from 4mm to 1mm. Theoretical predictions under estimated the finless design thermal performance, which was found to scale towards that of a turbulent flow regime despite the low Reynolds number. The mechanisms of this improvement in heat flux was investigated and unique, heat transfer enhancing, features in the finless design were identified. A combined infrared thermography and heated-thin-foil technique was developed for miniature fan applications, to accurately determine local heat transfer coe cients due to radial and axial fan flows. This highlighted the non-uniform heat transfer rates produced by the three-dimensional air patterns from rotating fans, and has been shown to be an important consideration in the design stages for component cooling. For the same chip temperature, strategic positioning of electronic components resulted in up to three-fold gains in power dissipation for direct component cooling applications. Local peaks in heat transfer coe cient when using axial fan impingement were directly related to the air flow and fan motor support interaction. It was found that for optimum thermal performance, motor support dimensions should be kept to a minimum and positioned on the inlet flow plane, the opposite to the current industrial practice. Flow structures and surface heat transfer trends due to radial fan flows were found to be common over a wide range of fan aspect ratios (blade height to fan diameter). The limiting aspect ratio for heat transfer enhancement was 0.3, as larger aspect ratios were shown to result in a reduction in overall thermal performance. Results also indicate that low profile radial fan designs are not just limited to portable devices, but may also be a practical solution to thermal management issues in larger scale electronics. A practical operating condition was represented by the introduction of a uniform crossing air flow above a radial fan inlet and indirectly reduced surface heat transfer. A distorted inflow shifted the surface heat transfer distribution from an axisymmetric to asymmetric profile for a radially discharging fan design. Overall, the findings presented in this thesis are fundamentally and practically useful for the design of forced convection cooling solutions using rotating fans in space restricted applications.