A direct comparison of local fin-surface heat transfer coefficients for flow around a circular tube without (a) and with (b) winglets at $ReH∼1200$ is presented in Fig. 1. The comparison reveals that, for this winglet location, the horseshoe vortex produced by the interaction of the flow with the circular cylinder is disrupted by the winglets. There is a reduction in the width of the low-heat-transfer wake region, but heat transfer coefficients directly downstream of the cylinder are actually slightly reduced for the winglet case compared to the no-winglet case. Stagnation-region heat transfer coefficients are slightly higher for the winglet case compared to the no-winglet case. The experiments were performed in a narrow rectangular duct designed to simulate a single passage of a fin-tube heat exchanger. A transient heat transfer measurement technique was employed for obtaining detailed local...