Large Eddy Simulations (LES) of the flow in repeating arrangements of staggered rectangular winglet pairs are presented to get a better understanding of the detailed flow and heat transfer enhancing mechanisms. Simulations are performed for various geometrical configurations with the winglet height h and longitudinal pitch distance L-P, being varied in the ranges h/H is an element of[0.3; 0.7] and L-P/H is an element of [3; 7] at Re = u(b)H/v = 700 and Pr = 0.71. The results show that three different types of vortices are generated by the winglet pairs: main longitudinal vortices, corner vortices and induced vortices, with the main longitudinal vortices being the main contributor to heat transfer enhancement. It is found that the heat transfer and pressure loss increase with increasing winglet height and decrease with increasing longitudinal pitch distance. The winglet height proves to have the highest impact on both the heat transfer and pressure loss. Furthermore, the results show that local heat transfer can effectively be increased on the fin side by utilising smaller wing let heights. For higher winglet heights local heat transfer is observed to be more equally distributed on both sides. Overall, pressure loss increments f/f(0) between 3.3 and 33.5 and heat transfer enhancements Nu/Nu(0) between 2.2 and 4.6 are found. When introducing the performance factor eta = (Nu/Nu(0))/(f/f(0))(1/3) as a measure for heat transfer enhancement relative to pressure loss, the optimal geometries generally have a combination of lower winglet heights and higher longitudinal pitch distances. (C) 2018 Elsevier Ltd. All rights reserved.