Extensive tests were conducted to obtain thermal characteristics of pin fin heat sinks with pins of square cross-section under different top and side bypass conditions. A consistent set of aluminum heat sinks with constant planform area and variable pitch were utilized. The heat sinks were tested in an adjustable cross-section wind tunnel, which provides 0 to 1 top and 0 to 10 side clearance ratios. The base temperature of the heat sinks was measured under constant heat flux. The overall thermal resistance and average heat transfer coefficient of the heat sinks were calculated and the influence of top and side bypass was explored. The thermal experimental data were consistent with the previous hydraulic experimental data. With the increase of flow velocity, flow transition from laminar to turbulent flow was revealed by the thermal resistance data. It was found that the average heat transfer coefficient depends strongly on the flow velocity through the heat sinks. Heat sinks with side clearance only result in smaller overall thermal resistance compared to top clearance only at the same clearance ratio. In some cases, the heat sinks with a small side clearance had better thermal performance than the same heat sinks without clearance. The empirical equation for infinitely long tube bundles of circular cross-section was used to correlate the thermal experimental data.

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