CFD transient model of the natural convection heat transfer for an heat sink effects of increasing surface and fins spacing

In this work the study has been started modeling an heat sink, commercially available, made by two aluminum horizontal rectangular thick fins per side positioned on a wide horizontal base plate. The visualization of the natural convection flow for the structure was implemented numerically by developing an appropriate model of computational fluid dynamics (CFD). Particular attention has been devoted to 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. 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 postprocessing tolerance has been set in 10^-4. A transient analysis was performed, with a time discretization of 0.001 s (time step) and 20 iterations for each time step. The convergence criteria imposed (residues) for transient analysis are 0.001 for the flow and 1×10^-7 for energy. Each solution reached convergence with an average time of about 13 days. By keeping constant the external dimension of the heat sink horizontal base plate we proposed two different geometries. In a first case was increased the spacing between the vertical fins in order to allow a greater lateral working volume for the convective process. In a second case the surface has been extended with the introduction of an additional vertical profile fin on each side. 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. It stabilized at 59.5°C for the configuration with two fins profiles each side separated by increased distance and 58.2°C for the three-finned side proposal. 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. To enhance the readability of the extension of the stagnation areas of overheated flow in convective motion some appropriate numerical probes were positioned. 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. The effects of increasing the spacing between the side vertical fins of the structure results in a higher speed of the natural convective flows in the channels. The most intense recirculation speed was observed in the central region of the three-side profiles prototype. The increasing of the surface by the insertion of a third vertical profile further reduces the operating temperature. The natural convection heat transfer from extended surfaces is, so far, the primary method of electronics cooling. 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. 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].