Heat sinks with cross airflow are commonly used for enhancing the cooling of electronic components. When using heat sinks in avionics applications, the primary heat transfer challenges are due to low air densities, which occur when operating at high altitudes, and space and mass constraints. Because of the spatial constraints, heat sinks with a large surface area per unit volume are advantageous. In general, cylindrical pin-fin heat sinks offer such characteristics. The Nusselt number is used as an indication of the thermal performance of the heat sink for a given Reynolds number. At high altitude, we expect the Reynolds number (based on the fin diameter and maximum velocity, Red,max ) to be smaller than 1000. Empirical correlations for the Nusselt number of cylindrical pin-fin heat sinks are available in the literature; however, these correlations were obtained for larger values of Red,max . The objective of this work is to correlate the Nusselt number and the friction factor of an in-line cylindrical pin-fin heat sink with its non-dimensional geometric parameters, and the airflow Reynolds number. The emphasis is on Red,max range between 25 and 1000, which allows the evaluation of the thermal performance of the heat sink for altitudes up to 70,000 feet. The results are obtained using three-dimensional numerical simulations with the commercial CFD software Flotherm. The numerical model is validated against experimental data. The results show that for a given Red,max, the average Nusselt number and friction factor are independent of the altitude for a given heat sink configuration. However, for a given air inlet velocity, an important drop in the average Nusselt number is observed as the altitude increases due to the reduction in air density. The effect of the variation of the fin span-wise and stream-wise pitches, as well as height is also studied.

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