Numerical Evaluation of Clearance Requirements Around Obstructions in Finned Heat Sinks

Abstract

This study uses CFD to consider the effects of obstructions (bosses) on the fluid flow and heat transfer in finned heat sinks used for cooling electronic components. In particular, the effect of bosses, used for mounting components, on the fluid flow distribution and temperature distribution in the heat sink are evaluated. A typical heat sink has fins sandwiched between top and bottom plates, with electronic components mounted on the plates. The top and bottom plates spread the heat generated in the components to reduce the local heat flux. The fins substantially increase the heat transfer area, reducing the temperature rise from the coolant to the top and bottom plates. In this case a uniform distribution of flow across the heat sink can be achieved and there will be no localized hot spots. Ideally there are no protrusions into the finned portion of the heat sink which would cause disruptions in the uniform flow through the heat sink. However, a boss may be needed to bolt a component to the heat sink. The presence of the boss has three effects on the heat sink performance. The boss disrupts the flow in its immediate vicinity, increasing the thermal resistance. This will cause an increase in operating temperature at that location. In addition, the boss will change the flow distribution in the heat sink. Locations upstream and downstream of the boss may see reduced flow due to the obstruction, which in turn will cause an increase in operating temperature for these areas of the heat sink. Finally, the change in flow distribution may increase the pressure drop through the entire heat sink, increasing the power required to operate the system. The purpose of this study is to numerically evaluate the clearance requirements around circular bosses. Comparisons between an unobstructed heat sink and a heat sink with an obstruction are made for the maximum component temperature rise, the pressure drop and the flow distribution. Clearance ratios, diameter of the fin cut out to boss diameter, were varied from 1.1 to 3.3. The Reynolds number for the flow was varied from roughly 3000 to 70,000 based on the hydraulic diameter of flow passage.