Role of transverse dimples in thermal-hydraulic performance of dimpled enhanced tubes

This study numerically investigates the effects of transverse dimples in thermal and hydraulic performance improvement of Teardrop, Ellipsoidal-0^o and Ellipsoidal-45^o dimpled tubes under constant surface heat flux (q′′ = 10kW/m²) for turbulent Reynolds numbers (5000 ≤ Re ≤ 40,000). The number of dimples in the transverse direction (Star) are varied from 4 to 8 while maintaining the tube length to pitch (spacing between successive dimples) of 10 for each tube configuration. It is observed from Performance Evaluation Criteria (PEC) that Ellipsoidal-45^o-Star 6 dimpled tube (Tube 7), as compared to the smooth tube, enhances the performance by 88.2% at Re = 5000 however, it reduces to 23.9% at Re = 20,000 and becomes negligible at Re = 40,000. The Ellipsoidal-45^o dimples generate a non-symmetric wake and create a flow path between downstream dimples. As Star increases the flow momentum increases and a swirl flow pattern is generated and consequently enhances fluid-dimple interaction at low turbulent Re. On the contrary, Teardrop-Star 6 (Tube 2) and Ellipsoidal-0^o-Star 6 (Tube 5) perform better at 20000 ≤ Re ≤ 40,000. Tube 5 produces distinct flow streams and enhances the fluid-surfaces interactions at higher Re. Therefore, it is recommended for high turbulent Re.