The analogy between heat and mass transfer has been used to obtain local and average heat transfer characteristics over a complete flat tube-fin element with four vortex generators (VGs) per tube. Several types of surfaces involved in heat transfer process such as fin surface mounted with VGs, its back surface (mounted without VGs) and flat tube surface are considered. The mass transfer experiments are performed using naphthalene sublimation method. The effects of the fin spacing and VG parameters such as height and attack angle on heat transfer and pressure drop are investigated. The comparisons of heat transfer enhancement with flat tube-fin element without VG enhancement under three constraints are carried out. The local Nusselt number distribution reveals that VGs can efficiently enhance the heat transfer in the region near flat tube on fin surface mounted with VGs. On its back surface the enhancement is almost the same as on the fin surface mounted with VGs but enhanced region is away from flat tube wall with some distance. Average results reveal that increasing of VG height and attack angle increases the enhancement of heat transfer and pressure drop, whereas small fin spacing causes greater increase of pressure drop. The heat transfer performance, correlations of Nusselt number and friction factor are also given.

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Winglet vortex generators can be used to enhance the heat transfer performance of finned flat tube bank fin. The effects of the height of vortex generators (VG) on local heat transfer were studied using the naphthalene sublimation method and the optimum height of winglet VG are screened by using JF, a dimensionless factor of the larger the better characteristics. In order to get JF, the local heat transfer coefficient obtained in experiments and a numerical method were used to get the heat transferred from the fin. For the configurations studied in this paper: for local characteristic, as increasing height of VG, heat transfer is enhanced, but the mostly enhanced region moves away from the tube wall; with increasing height of VG to certain degree, the width of enhanced region does not increase significantly; the effects of VG’s height on span-average Nusselt number (Nu) are more mixed on fin surface mounted with VGs and its back surface, with increasing height of VG, in some region heat transfer is worsened, and in other region heat transfer is enhanced; in real working condition, the heat transferred from fin surface mounted with VGs is larger than the heat transferred from the other surface of the fin; increasing the height of VG (H) increases average Nu and friction factor (f ), but with considering the fin efficiency, there is an optimum H to get best heat transfer performance; the optimum height of VG is dependent on the thickness of fin and its heat conductivity, for mostly used fin thickness and material, the optimum height of VG is 0.8 times of net fin spacing.

To investigate the effects of the angle of attack of delta winglet vortex generators (VGs) on heat transfer performance and pressure drop penalty of finned flat tube bank, local and average heat transfer characteristics were studied by using analogy between heat and mass transfer with naphthalene sublimation experiments. In order to consider nonuniform fin temperature, the heat transfer rate from fin surfaces was investigated by solving the conduction equation in the fin with a Finite Volume Method on curvilinear coordinate systems based on experimental results of local heat transfer. Three angles of attack (θ = 25 • , 35 • , 45 • ) are investigated. Without considering the nonuniform fin temperature, increasing the angle of attack, average Nusselt number and friction factor are increased, the heat transfer enhancement on fin surface without mounted VGs is reduced, but the heat transfer enhancement on fin surface mounted with VGs is less affected. Considering the nonuniform fin temperature, the heat transfer enhancement becomes weak as the angle of attack greater than certain value. The numerical calculation shows that the total heat transfer rate from the fin surface mounted with VGs is larger than the total heat transfer rate from the fin surface constructing the same flow channel. Based on a dimensionless factor of the larger the better, JF, the results show that the angle of attack, 40 • , has better heat transfer performance for the configurations discussed in this article.

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