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The natural convection heat transfer from extended surfaces is, so far, the primary method of electronics cooling. With finned surfaces in air and natural convection, it is possible to manage a thermal power of 0.1 W/cm2 with a temperature difference of ΔT = 80°C [1, 2]. Cooling techniques based only on natural convection are of particular interest in all those situations where demands for miniaturization and low noise are dominant, and it is also a simple, reliable and low cost. The study has been started analyzing the buoyancy induced heat transfer for an heat sink, commercially available, made by two aluminum horizontal rectangular thick fins per side positioned on a wide horizontal base plate. The time evolution of natural convection flow in the heating phase of the structure starting from t = 0 (cold) to the development of the steady state has been investigated. The visualization of the buoyancy flow for the structure was implemented numerically by developing a computational fluid dynamics (CFD) model. The numerical solution of the problem was carried out by the discretization of the space in analysis. The mesh used were optimized according to an hexa unstructured structure, with a maximum size along the three axes fixed in 0.0075 m, 0.015 m, 0.01 m and a maximum spacing of 149.8 10-6 m. The post-processing tolerance has been set in 10-4. The time discretization for the transient analysis was 0.001 s (time step) and 20 iterations for each time step, in order to perform a transient analysis. The convergence criteria imposed (residues) for transient analysis were 0.001 for the flow and 1×10-7 for energy. Each solution reached convergence with an average time of about 13 days. With a thermal load of 1 W localized in the middle of the base plate the temperature observed for the hot spot at the steady state in the commercial configuration was 60.8°C. To enhance the readability of the extens- on of the stagnation areas of overheated flow in convective motion some appropriate numerical probes were positioned. The CFD model was also developed to monitor the evolution of the boundary layer, highlighting, in the volume surrounding the heat sink, the time evolution of the low-speed flow recirculation areas. It was observed that the commercial heat sink presents a lower speed level of flow recirculation in the region between the vertical profiles in the side channels, as well as an extensive central area of stagnation. It was also studied the effect of the heat sink geometry rotation on the natural convection heat transfer. Keeping constant the structure of the heatsink in this work were investigated the effects connected with the simple rotation of the geometry and the expected increase of the heat exchange due to this choice We suppose a vertical rotation of 90 deg to match the natural buoyancy flow direction. The proposed configuration leads to a significant improved performances. The geometry rotation allows a better flow recirculation speed. Stagnation areas are quite disappeared. In this way it is possible to reach a steady state conditions in a shorter time and lower (51.2 °C) operating temperatures for the hot spot of the system.
Date of Conference: 25-27 Sept. 2012