In the direct-water-cooled power module, there is a small gap between the heatsink and the channel wall. This gap results in bypass flow that reduces the pressure drop while maintaining high heat transfer. In this paper, we discuss the effect of this gap on both pressure drop and heat transfer over pinfin heatsinks using our semi-analytical model based on mass, momentum, and energy conservation within two control volumes. The first control volume in the model is located within the finned area, and the second one is located in the gap between the tip of the pins and the flow channel. Dimensionless pressure drops could be predicted within an error of 30%, and the predicted Nusselt numbers agreed within an error of 50%.
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