Experimental investigation on heat transfer and pressure drop of ZnO/ethylene glycol-water nanofluids in transition

•Passive LED heat sink with composite aluminum-graphite pin fins.•Graphite to aluminum pin fins improves the thermal conductivity and reduces LED substrate temperature.•Graphite concentration is 60 vol.% of the optimal composite material design. Light emitting diodes (LEDs) have many advantages, including a low power consumption, a long lifetime and a small size. However, at temperatures greater than 120 °C, a significant deterioration of the lamp body and packaging materials occurs. As a result, effective thermal management schemes are essential in maintaining the performance and reliability of high-power LED modules. One of the most common forms of LED cooling system is a heat sink consisting of an array of vertical fins designed in such a way as to optimize the heat dissipation efficiency through conduction, convection and radiation effects. In practice, the efficiency of such systems is largely dependent on the thermal conductivity of the fin material. Accordingly, the present study performs a numerical investigation into the cooling efficiency of a heat sink consisting of aluminum-graphite pin fins under ambient temperature conditions of 35 °C, 70 °C and 80 °C and natural convection air flow velocities of 0.5 ms−1, 2 ms−1 and 4 ms−1. The simulations assume the pin fins to be fabricated of aluminum-graphite composite material with graphite contents ranging from 0 to 90 vol.%. In general, the results confirm the effectiveness of graphite addition in improving the heat transfer performance of passive aluminum pin fin heat sinks. It is found that for all values of the ambient temperature and convection air flow velocity, the heat transfer performance improves as the graphite addition is increased from 0 to 60 vol.%. However, at higher values of the graphite concentration, only a minor further improvement in the heat transfer rate is obtained. Thus, it is inferred that a graphite content of 60 vol.% represents the optimal design for the present aluminum-graphite pin fins.