In this work, the flow behaviors of multiple circular cylinders in a staggered arrangement, located in a rectangular duct with a narrow gap are studied experimentally using laser particle image velocimetry (PIV). Each cylinder is positioned at the corner of an equilateral triangle with a pitch ratio of 2d. It is known that multiple cylinders in staggered arrangements are more efficient than in-line tube arrangements in the case of heat transfer. With this in mind, this study examines the effect of a combination of multiple cylinders on flow characteristics. The staggered arrangement of the multiple cylinders guides the flow to be in a mode of repeated growth-and-destruction of the boundary-layer. This flow phenomenon increases the disorder of the vortical flow characteristics profoundly. Since the efficiency of heat exchangers depends mainly on the hydrodynamic, this investigation focuses on the flow characterization particularly on the development of horseshoe vortices. In this regard, distributions of velocity vectors, streamlines, vorticity and Reynolds stress, turbulent kinetic energy and single point spectral analysis are used to analyze the flow characteristics in detail. (C) 2016 Elsevier Inc. All rights reserved.