Revealing the heat transfer enhancement mechanism of dimpled tube hybridized with twisted tape via dimple utility indicator

The dimpled tube has long been recognized as an effective method for enhancing heat transfer. However, further improvements in its performance have been constrained by challenges in optimizing the structure and arrangement of dimples. In this study, we propose a novel approach based on hybridization enhancement, where the dimpled tube is combined with a twisted tape, introducing a new mechanism for heat transfer improvement. Numerical simulations demonstrate that this hybrid design can boost the average heat transfer coefficient by up to approximately 16 % compared to the conventional dimpled tube. Systematic analyses reveal that the circumferential flow induced by the twisted tape significantly enhances the utilization of dimples, thereby improving heat transfer performance, assuming the number and shape of dimples remain constant. The degree of enhancement is found to depend on the height and pitch of the twisted tape. To quantify these effects, we introduce the dimple utility indicator (DUI), defined as the number of dimples exposed to fluid flow per unit time, effectively measuring the "dimple washing rate." Simulation results confirm that a higher DUI is the primary factor contributing to heat transfer enhancement. Furthermore, the DUI increases with the height of the tape and decreases as the tape pitch decreases.