The heat transfer performance of a flat plate behind a 10 mm high (h) and 20 mm long delta winglet was studied at a distance up to 30h in a wind tunnel at a Reynolds number based on h of 6000. The focus was on the role of the attack angle, which was varied from 30 to 60 degrees in 15-degree increments. The bottom side of the flat plate was uniformly heated by condensing steam at 100 degrees C. Surface thermal imaging results indicated that the peak Nusselt number (Nu) increases with the attack angle, and this augmentation was attributed to the larger share of the transverse vortex at the larger attack angles. Peak Nu dropped sharply in the near wake (X/h < 10), presumably due to the rapid fading of the transverse vortex. It subsequently decreased more gradually and became less sensitive to attack angle farther downstream. This extended heat transfer enhancement was postulated to be caused by the slowly-decaying longitudinal vortices which persisted beyond the studied span. The prevailing longitudinal vortex induced heat transfer enhancement was explained in terms of the detailed flow characteristics scrutinized via a triple hot wire at 20h. The Inflow region, where cooler freestream air was brought into the hot plate, corresponded to the maximum Nu boost; while the Outflow region, where heated air began to leave the hot surface, correlated with the Nu valley. Further analysis revealed Nu relations with the local near-surface stream wise velocity, out-of-plate velocity, and turbulence intensity. The specific heat-flow correlations subtly differed between the Inflow and Outflow regions, and thus also the effect of the winglet attack angle. (C) 2017 Elsevier Ltd. All rights reserved.