An experimental investigation is reported using particle image velocimetry of the flow structure in the flow through a rectangular duct containing a circular cylinder with axis normal to the flow. A duct with a narrow gap was designed to simulate a fin-tube heat exchanger containing a single circular cylinder. The time-averaged velocity vectors map, < V >, patterns of streamlines, < ψ > and corresponding vor-ticity contours, < ω > were obtained using 490 instantaneous images. Development of the entrainment process between the main flow and wake-flow regions is magnified by a helical horseshoe vortex system emerging upstream of the cylinder. The occurrence and development of boundary layer separation and the formation of horseshoe vortices upstream of the cylinder close to both upper and lower plate surfaces and the merging of these developing horseshoe vortices into a primary horseshoe vortex system were also observed in the side-view plane for the Reynolds number range of 1500 ≤ Re_d ≤ 6150.
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