Heat transfer and flow characteristics in a circular tube fitted with rectangular winglet vortex generators

In this paper, the heat transfer and flow behavior of a circular tube fitted with rectangular winglet vortex generators (RWVGs) are investigated numerically and experimentally with uniform heat flux boundary conditions for Reynolds number ranging from 5000 to 17,000. Water is employed as working fluid. The RWVGs are inserted into the tube, with the slant angle β of 10°, 20°, 30°, 35°, respectively. The asymmetric winglet height is set on the one side at H1/D = 0.5, and on the other side at H2/D = 0.2, 0.3, 0.4, and 0.5, respectively. The effects of β and H2/D on the heat transfer and flow behavior are studied numerically by employing FLUENT software. The results show that the RWVGs can agitate the cold fluid from the core flow region to the tube wall, and therefore, enhancing the mixing of hot and cold fluids. The Nusselt number and friction factor increase with the increase of β, or H2/D. Specifically, the Nusselt number and friction factor are increased at 1.16-2.49 times and 2.09-12.32 times, respectively, compared with the plain tube. The Performance Evaluation Criterion (PEC) increases first, and then decreases with the increase of β and H2/D. The maximum PEC is found to be 1.18 for β = 30°, H2/D = 0.5 and Re = 5000. Based on numerical data, new correlations of Nu and f by using four nondimensional parameters are also developed. The experimental platform is set up and the numerical results agree well with those of the experiment. In addition, the mechanism of heat transfer enhancement of RWVGs can be well explained by the principle of field synergy.