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

•Parametric study of delta winglet vortex generator (DWVG) pairs in a tube.•Effect of pitch ratio on Nu, f and TEF is the most significant.•Heat transfer can be improved significantly with DWVG pairs. 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 × 103–2.5 × 104. 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 α30°-s15-h7.5, α20°-s15-h7.5, and α30°-s20-h10 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.