Pitch and aspect ratio effects on single-phase heat transfer through microscale pin fin heat sinks

Abstract Heat transfer and pressure drop of single-phase liquid flow is characterized in eight micro pin fin heat sinks with varied pitch and aspect ratios. The pins are diamond shaped with respect to the flow and have transverse pitch-to-diameter ( S T / D h ) and aspect ( H pin / D h ) ratio variations in the range of 1.7–3.0 and 0.7–3.2, respectively. The fluid used is PF-5060 over a Reynolds numbers (based on pin fin hydraulic diameter) range of 8–1189. Flow visualization is performed on all the heat sinks and flow transition into unsteady vortex shedding is observed only in those with specific pitch and aspect ratios. Flow visualization reveals upstream propagation of the onset of vortex shedding along the length of heat sink with an increase in Reynolds number. The existence of vortex shedding in micro pin fin heat sinks affects the prediction error of heat transfer correlations in literature. To address this gap, together with data from a prior study using liquid nitrogen [1], separate correlations are developed to predict Nu in the steady and unsteady regimes. The resulting correlation for the unsteady regime shows significantly decreased dependency of Nusselt on the Prandtl number compared to the non-vortex-shedding condition.