Parametric study of major factors affecting heat transfer enhancement in a circular tube with vortex generator pairs

Abstract This paper deals with the parametric study of major factors influencing thermal and flow behaviors for delta winglet vortex generator (DWVG) pairs in a circular tube. The investigated parameters include six pitch ratios ( PR = P/D  = 2.4, 3.6, 4.8, 6.4, 9.6, and 19.2), two flow directions (upstream and downstream), and two winglet configurations (common-flow-down (CFD) and common-flow-up (CFU)). The air-flow and heat transfer behaviors in the tube are examined for Reynolds number Re  = 5 × 10 3 –2.5 × 10 4 . The experimental results indicate that PR has a significant effect on the heat transfer enhancement. A smaller PR results in a higher Nusselt number and a higher friction factor. The PR  = 9.6, corresponding to two winglet rings, yields the largest thermal enhancement factor ( TEF ) for the Re range examined. The combinations of winglet configuration, flow direction and angle of attack lead to the best TEF performance for α 30°- s 15- h 7.5, α 20°-s15- h 7.5, and α 30°- s 20- h 10 where α , s and h represent the angle of attack, spacing between VGs and height, respectively. As to winglet configuration, CFD configuration yields better thermal-hydraulic performance than CFU. Furthermore, the downstream flow renders 30–33% higher TEF than the upstream flow.