Experimental investigation of heat transfer enhancement with radially arrayed delta winglets in a heat exchanger tube

Delta winglets are one of the promising and effective vortex generators (VGs) for enhancing heat transfer, with a low pressure drop penalty. This research is concerned with innovative arrangements (inline, staggered, mixed configurations) of delta winglet vortex generators (VGs) inside a circular tube to investigate their effects on thermal and flow characteristics. The VGs were arranged inside a tube as a series of four rings. Each ring had four winglets on the inner surface of the tube. Experimental measurements with VG inserts were conducted for fully developed turbulent air flows with Reynolds numbers between 6000 and 27,000 under a constant heat flux condition on the surface of the tube. The experimental results show that delta winglets with different arrangements lead to significant thermal performance enhancement. Using the staggered configuration instead of inline configuration of the VGs resulted in a significant drop in friction factor (6 %) and a significant increase in Nusselt number (16 %). Compared to the inline configuration, staggered configurations of VGs provided better heat exchanger performance. The highest Thermal Performance Enhancement (TPE) obtained, 1.33, was achieved using mixed VG directions with a staggered VG arrangement (i.e., configuration referred to as RZ1). The effects of VG length, rotational direction, and inward and backward positions of VGs on thermal performance were also characterized. Rotational directions (positive and negative) were found to have a positive impact on the Nusselt number (Nu) and a small effect on the friction factor (f). Increasing the length of the VGs resulted in an increase of f and Nu. This study suggests that innovative layouts/arrangements of VGs can contribute to the development of high-performance heat exchangers.