Delta Winglets for Enhancing Solar Energy: Turbulent Strain Rate-Heat Convection Relationship

Abstract

Delta winglets are an effective means for enhancing heat exchange and thus the performance of renewable energy technologies, including solar energy. A pair of 0.1 mm thick, 15 mm high (h) and 30 mm long aluminum winglets separated with transversal spacing, s, of 2h, h and 0 were scrutinized in a closed-loop wind tunnel at a Reynolds number based on h of 6300. The turbulent flow was characterized using a 3D hotwire probe, and the heat convection augmentation was quantified in terms of the normalized Nusselt number (Nu/Nu0), indicating the heat transfer improvement compared to the reference case without the winglets. The interaction of the organized counter-rotational vortices intensifies and they become indiscernible at s = 0. The peak strain rate at 10h downstream increased from 390 s–1, to 478 s–1, to 514 s–1, when the spacing decreased from 2h to h to 0, respectively. The zero-spaced winglet pair provided the largest Nu/Nu0, of around 1.21, at X/h = 10 and Y/h = 0, approximately 21% higher than that of 2h-spaced winglet pair, due to the strongest strain rate and the absence of upwash flow. On the other hand, the 2h-spaced winglet pair provided the largest span-averaged Nu/Nu0, which is of practical significance.