Numerical Simulation On Performances Of Plane And Curved Winglet Type Vortex Generator Pairs With Punched Holes And Field Synergy Analysis

Vortex generators (VGs), as a kind of passive heat transfer enhancement device,have been extensively applied to improve the air - side convective heat transfer coefficient of plate - fin and finned tube type heat exchangers. The strategically placed VGs create secondary flows, particularly longitudinal vortices which bring about great enhancement in heat transfer at low pressure loss. In the present paper, numerical simulations are carried out to investigate thermal and flow characteristics of plane and curved VGs with punched holes on the basis of known experimental results. The corresponding mechanism of heat transfer is analyzed based on field synergy principle(FSP) and intensity of secondary flow theory.3-D numerical simulations are carried out in an air channel flow (Re= 700 -26500) embedded with a pair of plane and curved delta, trapezoidal and rectangular winglet - type VGs, respectively. The effects of the these VGs are examined and compared using the dimensionless parameters Nu/Num0,f/f0 and R= (Num/Num0)(f/f0).Parametric study on curved trapezoidal winglet pair (CTWP) reveals that the attack angle of ?=450 and inclination angle of a=20° perform better than other conditions.The results show that CTWP provides the best thermo-hydraulic performance with the value of R ranging from 0.68 to 1.14 under the present conditions.The mechanism of heat transfer enhancement is explored with the help of field synergy principle (FSP) and secondary flow theory. Take the delta winglet pair (DWP)for example, the area - average synergy angle ?s between velocity vector and temperature gradient of the cross section along flow direction first increases sharply to a peak of 85.7° before the VGs. After that, it declines dramatically to 81.8° and then climbs back and stabilizes around 89°. Correspondingly, the Num/Num0 shows the opposite trend to ?s. Meanwhile, it is found that the site with smaller synergy angle are the site with higher Nu. The results indicate that the FSP can well explain the mechanism of heat transfer enhancement and can be used to guide the shape design and arrangements of VGs.When air flows over the VGs, the pressure loss is high due to the form drag of the VGs. Punching holes on the surface of VGs is an efficient way to reduce the pressure for the jet from the holes can take away the stagnant fluid behind the VGs. The size,shape and position of the punched holes are studied in the present paper. The results show that VGs with punching holes present higher heat transfer enhancement and lower flow resistance than those without holes and the value of R is increased by 9.8%under the present conditions. Further parametric study suggests that the ratio of hole area to the VG area should be optimized to achieve better heat transfer enhancement.And punching holes at a lower position and close to the leading edge gives better thermal - hydraulic performance.