Experimental and numerical study on the flow topology of finned heat sinks with tip clearance

An experimental and numerical study is presented that focuses on the flow topology in finned heat sinks with tip clearances. As it is well known, the use of tip clearances has, normally, a global beneficial effect. Typically, heat transfer may degrade (a negative effect) by a somewhat small percentage while pressure drop may decrease (a positive effect) by a substantial amount. In this context, the important question from an R&D standpoint is to understand the flow topology so that the actual design of the tip clearance optimizes the balance between heat transfer and pressure drop. In this study, a 3D numerical method is validated, first, comparing with the Particle Image Velocimetry based experimental results obtained in the actual setup in isothermal conditions. Then, the flow solver thus validated is used in a series of thermal cases in which both the tip clearance height and Reynolds number are varied so as to clarify the flow topology. In particular, it has been found that the behavior of both heat transfer and pressure drop cannot be explained in view, only, of flow development and thermal development aspects. This is so because the Nusselt versus Graetz curves that have been generated do not collapse into a single fit; instead, they collapse into several families that are governed by the tip clearance parameter. Distinct heat transfer rates have been observed for the different fin walls. The transfer rate of the side walls is nearly three times larger than that of the bottom walls, and this suggests the optimum place to locate the heat sources in a practical engineering application.