Abstract: Tubing collars’ influence on hydrodynamic behavior of annular duct flow has been investigated using Particle Image Velocity (PIV) technology. PIV has become an efficient method for complex transient flows visualization. A water flow loop with two replaceable variable cross-sections (VCS), 75–90 mm and 90–110 mm, in a 129 mm inner diameter (ID) pipe was used. The whole field of the variable cross-section annulus (VCSA) was visualized, including forward-facing step (FFS), narrow annulus (NA), and backward-facing step (BFS) flow. The VCSA ratio and Reynolds (Re) number influence on streamline distribution, velocity distribution, and turbulence intensity were discussed. Results showed that the recirculation is easier to form in BFS than FFS flow under the same condition. The VCSA ratio affects the formation of recirculation zones and the location of the reattachment point. Reynolds number mainly affects BFS flow by influencing the main velocity component - axial velocity. The turbulence intensity is relatively high in BFS than FFS flow and is larger at y/h>1.0 than y/h<1.0. Furthermore, the streamwise coherent structures reveal that the first two modes are predominant and represent the main characteristics of the flow by proper orthogonal decomposition (POD) method.