Numerical investigation of the effect of branching angle onthe heat transfer performance of a fractal heat sink

Heat sinks are passive heat exchangers widely used in power electronics, LED lighting systems, computer technologies. Through their finned structure, they can quickly transfer waste heat energy to a cold fluid medium so that the temperature of the component they connected to can be kept under control. With the development of systems consuming more power, the need for efficient and compact heat sinks is also increasing. In this context, fractal heat sinks, which are allowing to obtain a higher heat transfer area even at lower volumes and heights owing to their tree-like branched structure, were investigated. It was aimed to numerically examine the effect of branching angle on the heat transfer performance. In the study, a radial plate-fin heat sink has an outer diameter of 120mm and a height of 40mm was taken as a reference model and 3 of new fractal heat sinks having branches at 30 °, 60 °, and 120 ° angles were designed by keeping the total surface area of each constant and equal to that of the reference model. To make sure that the solution is independent of the numerical model, mesh independence analyzes have been performed by including a single segment of the most complex fractal heat sink which was designed with branches at the angle of 30° and optimal mesh parameters have been determined. Based on these model parameters, computational fluid dynamics models of reference and fractal heat sinks were prepared. Since the fractal heat sinks have complex flow paths, a turbulence model giving more accurate solutions in regions where the flow separated or swirled was required to solve the thermal boundary layers precisely. Therefore k-w SST turbulence model was used. In order to investigate the effects of mass flow rates on heat transfer performances, CFD analyzes were repeated at 3 different airflow speeds of 2.5m/s, 5m/s, and 10m/s. In the reference heat sink model it is observed that the flow activity is weak and turbulence density is low around the regions close to the heat source, hence most of the fin area cannot be used effectively. On the other hand, in all fractal models, it is understood that the flow accelerates and is directed towards the bottom of the heat sink so that the heat is mostly dissipated from the fins close to the center. Compared to the reference model, a decrease was observed in the mean base temperatures of all the fractal heat sinks. In the IK-120 model branched at the angle of 120 °, a decrease of 5C ° in the base temperature and a 15% improvement in thermal resistance were calculated compared to the reference heat sink. Even though fractal heat sinks show better thermal performance than the reference model, due to their complex structures flow tends to accelerate and separate resulting in higher pressure drops. In this context, the lowest pressure losses among fractal models were obtained in the IK-120 model. With this study, it is understood that lightweight fractal heat sinks which show similar or better thermal performance compared to the conventional heat sink can be designed, and the effects of branching angles, fin spacing, fin alignment and orientation on performance have been experienced. The number of studies on fractal heat sinks in the literature is quite limited. Therefore, an important contribution has been made to the literature on understanding the effects of geometry and inlet velocity on heat transfer performance of fractal heat sinks cooled by forced convection.