Detailed observations of wavy interface behaviors of annular two-phase flow on rod bundle geometry

•High speed camera with backlight was used to study annular flow on BWR rod bundle.•The clear side-view images of the phenomena were achieved.•The formation of the “singlet disturbance-crest” near the inlet was observed.•Different entrainment processes were described.•The relation between singlet disturbance-crests and disturbance waves was proposed. The high speed camera technique with backlight arrangement has been applied to study the annular two-phase flow on a 3×3 simulating BWR fuel rod-bundle test-section. An attempt to keep the base liquid film from flowing on the duct wall has helped to obtain the clear side-view images of the phenomena. The data at high time and space resolutions provided the detailed descriptions of the gas–liquid interfaces’ behaviors at the region close to the inlet as well as further downstream. The formation of the “singlet disturbance-crest” near the inlet which is suggested to be the first form of the disturbance wave was observed. An explanation on the mechanism of this formation process was proposed. Obtained images of three types of the entrainment process (bag break-up, ligament break-up, and droplet impingement) not only agreed with the previously proposed mechanisms but also included the information about wavy behaviors right before and after these events and the created droplets. Moreover, the side-view images of the disturbance waves at different stages of development were firstly presented. These data can be used to evaluate other measuring techniques applied to the study of this type of waves.