Numerical simulation on performances of plane and curved winglet - Pair vortex generators in a rectangular channel and field synergy analysis

Thermal and flow characteristics of plane and curved longitudinal vortex generators (LVGs) are investigated and the corresponding mechanism is analyzed based on field synergy principle (FSP). 3 - D numerical simulations are carried out in a channel flow (Re = 700-26500) embedded with a pair of plane and curved delta, trapezoidal and rectangular winglet - type LVGs, respectively. The effects of the these LVGs are examined and compared using the dimensionless parameters Num/Num0, f/f0 and R = (Num/Num0)/(f/f0). The results show that the curved trapezoidal winglet pair (CTWP) provides the best thermo - hydraulic performance with the value of R ranging from 0.68 to 1.14 under the present conditions. Parametric study on CTWP reveals that the attack angle of β = 45° and inclination angle of α = 20° perform better than other conditions. The heat transfer characteristics and flow structure are explored with the help of secondary velocity vectors, streamlines and isotherms. The volume - average synergy angle θm between velocity vector and temperature gradient first decreases with β, reaches the minimum value for β = 45°, and then rises back to the maximum value for β = 90°. Correspondingly, the Num/Num0 shows the opposite trend to θm while varying with β. It is confirmed that the maximum value of heat transfer augmentation is achieved at the minimum synergy angle, which indicates that the mechanism of heat transfer enhancement can be well explained by FSP.