The flow structure in a plate fin and tube heat exchanger model composed of single cylinder located between two parallel plates for duct height-to-cylinder diameter ratio of 0.365 for Reynolds numbers of 4000 and 7500 is investigated experimentally. Particle image velocimetry (PIV) technique is employed to obtain instantaneous, time-averaged and phase-averaged turbulent flow characteristics in the heat exchanger flow passage. Interactions between the main flow and the secondary flow are examined in detail in the horizontal and vertical planes of the flow passage. Horseshoe vortex system formed in close region of cylinder plate junction and convected downstream in the main flow direction and its evolution in the circumference of the cylinder is also investigated in detail. (C) 2005 Elsevier Inc. All rights reserved.