In the study Time-resolved micro - PIV technique was used to obtain new experimental data of flow vortex structure in wire-wrapped rod bundles. Experimental investigations were made on hydrodynamic setup of IT SB of RAS. Instantaneous fields of velocity were measured in transverse sections near central rod. Experimental data analysis showed that flow pattern is strong unsteady and contains longitudinal large-scale vortex structure. It was shown that in flow near wire-wrapper secondary flow exists.