A biomimetic approach to improve convective heat transfer using self-excited flaps

Abstract

Bio-inspired flexible plates have shown promising results as a passive method to enhance convective heat transfer and mixing in flow. Flap fluttering is a self-excited phenomenon caused by the fluid-structure-interaction between the main flow and the flap. Among all the parameters, the direction of the flap relative to the incoming flow appears to be the key factor in determining the transition into the flapping mode. In this study, flaps made of a 50-micron thick polyester sheet were cut into rectangular shapes and placed at the centerline of the channels of a shrouded straight fin heatsink. Both hanging and vertical positions of the flap in the axial flow were investigated using a custom-built testbed in our lab. We measured the convective heat transfer rates and pressure drop in the heatsink for both arrangements. We observed that: i) the rectangular cantilevered flap is more unstable in the hanging flap configuration compared to the vertically installed flap, i.e., the transition to the fluttering mode occurs at a lower airflow velocity; ii) the critical flow velocity – the velocity at which the flap becomes unstable– is lower for the flap aspect ratio of 0.5. We attribute this behavior to the longer viscous boundary layer around the vertical case, which stabilizes the flap; iii) from a thermal performance point of view, the hanging configuration outperforms the vertical one with the same flap dimensions; iv) at a Reynolds number of 8,940, large amplitude vibrations and coherent fluttering in a hanging configuration render a superior thermal performance of 39 percent improvement in the Nusselt number compared to when bare channels are measured. On the contrary, at higher flowrates, the difference falls in the range of measurement uncertainty; and v) as for the pressure drop, unlike the hanging configuration, the vertical flap creates a higher pressure drop as it tends to block half the channel at the end of the trailing edge stroke. Furthermore, the presence of the flap holder adds to the pressure drop increase, which is eliminated in the hanging position and therefore, flaps could be connected to the shroud directly.