AbstractsEngineering

The flow momentum in free convection heat transfer is generally lower than forced convection, and the flow direction is determined by the gravitational direction. Hence, the thermal performance of heat sinks relying on free convection could vary significantly depending on the mounting orientation. This is confirmed by the present work, a computational fluid dynamics and heat transfer study with focus on the relation between the design of heat sinks and their thermal performance under varied mounting orientations. A heat sink with rectangular straight fins, a very commonly used type of heat sink, is first studied. The results show poor thermal performance when the heat sink is placed vertically and fin length horizontally to the ground. The fluid dynamics analysis indicates interruption of buoyancy flow by the horizontally placed fins, which in turn impedes heat transfer. An experimental work on straight fin heat sink is also performed, and with the help of infrared camera and thermocouples the orientation dependencies are observed for a wide range of orientation angles. In order to minimize the negative effect of mounting orientations, two other types of heat sinks are studied. One is angled-fin heat sink, which has plate fins parallel to the diagonal rather than the side of the heat sink base. The other type is pin-fin heat sinks. The two types of heat sinks are shown to provide similar thermal performances for both horizontal and vertical orientations, and the low orientation dependency is attributed to the low orientation effect on the buoyancy flow. The fin thickness, fin spacing and number of fins are varied for both the heat sinks. The purpose is to study the effect of these changes on thermal performance for different mounting orientations. For angled-fin, the effects of fin interruptions are also studied, where horizontal orientation shows better cooling performance than the vertical. In case of pin fin heat sink, the temperature differences between the vertical and horizontal orientations show that, the fin spacing plays an important role in the selection of mounting orientation. This study is useful for designing free convection heat sinks for orientation-independent cooling performance.