Moving-orientation and position effects of the piezoelectric fan on thermal characteristics of the heat sink partially filled in a channel with axial flow

This study experimentally investigated the heat transfer and fluid flow behaviors of the heat sink partially filled in a rectangular channel with the axial mainstream interacted by the oscillating movement of the upstream piezoelectric fan. Total eight heat sinks, including square pin-fin heat sinks (Models A-C), plate-fin heat sinks (Models D-F) and aluminum-foam heat sinks (Models G and H), were employed. The relevant parameters were the Reynolds number of the mainstream (Re), the oscillating-movement orientation of the piezoelectric fan and the location of the piezoelectric fan (the relative horizontal distance S/L = 0.25 and the relative vertical distance Hp/L = 0.22-0.62). The smoke flow visualizations demonstrate that the oscillating movement of the piezoelectric fan did strengthen the turbulent intensity of the mainstream at low Reynolds number, giving the additional disturbance momentum to the mainstream and making the fluid flow through the heat sink with turbulent flow characteristic. The heat transfer results indicate that the heat sink gained more heat transfer enhancement at smaller Re when the piezoelectric fan was operating. For example, when the piezoelectric fan was operating with transverse oscillation at Re = 850, S/L = 0.25 and Hp/L = 0.62, the Nu/Nu0 (Nu and Nu0 separately mean the Nusselt number of the case with and without the piezoelectric fan) of square pin-fin heat sink Model C would exceed 2. The values of Nu/Nu0 for various heat sinks would drop and approach to unity as increasing Re. In general, under the oscillating condition, the square pin-fin heat sink gained the best heat transfer enhancement; the longitudinal plate-fin heat sink gained slightly less heat transfer enhancement than the square pin-fin heat sink did; the aluminum-foam heat sink gained poor heat transfer enhancement. In addition, the transverse oscillation promoted slightly more heat transfer enhancement than the axial oscillation did. Finally, this study proposed reasonable and accurate empirical correlations of Nusselt number in view of the pure axial-flow, the pure piezoelectric-fan and the both-combined conditions.