Analysis of Convective Thermal Resistance in Ducted Fan-Heat Sinks

The thermal performance of plate fin, round pin-fin, and offset strip-fin heat sinks with a duct-flow type fan arrangement was analytically evaluated. Heat sinks of 65mm$times hbox60, hboxmm hboxplan hboxareatimes hbox50 hboxmm hboxheight$with a 4300-RPM dc fan (60mm$times$15mm) were chosen for the performance comparison. A constant temperature, 6-mm thick heat sink base plate is assumed so that thermal spreading resistance is not involved. The operating point on the fan curve is based on the flow pressure drop impedance curve through a heat sink using the friction factor correlation for the chosen heat sink. The loss coefficients at both the entrance and the exit of the heat sink are included in the flow impedance curve. The operating point is defined by the balance point of the flow impedance curve and the fan performance curve. After determining the operating air velocity, the convective thermal resistance of heat sinks is evaluated from the Nusselt number correlation for the chosen heat sink. Results obtained show that optimized round pin-fin heat sinks provide 32.8%–46.4% higher convective thermal resistance compared to an optimized plate-fin heat sink. The optimized offset strip-fin heat sink shows a slightly lower convective thermal resistance than the plate-fin heat sink. As the offset strip length decreases, however, thermal performance seriously deteriorates.