Visual investigation on the coalescence process and the thermal-hydraulic characteristics of the two-phase interface morphology in narrow vertical channel

A visual investigation was carried out to study the characteristics of the coalescence process of the vapor bubbles and the thermal-hydraulic characteristics with the two-phase interface morphology. The experiment was conducted in a narrow vertical channel with the size of 2 mm × 10 mm × 250 mm, and the effective heating length was 200 mm. The transparent test-section and the heating section were made of Polycarbonate and a glass plate coated with ITO (Indium Tin Oxides) film, respectively. A high speed camera was used to record the characteristics of the vapor bubble coalescence and the two-phase interface morphology from the bottom of the transparent heating wall. Also, the corresponding experimental parameters were measured simultaneously. In this study, the horizontal and vertical coalescence of two vapor bubbles, as well as the coalescence of multiple vapor bubbles were analyzed in detail. It was found that the generation of the macrolayer was closely related to the trapping process and the breaking process, where the surface tension dominated. Moreover, the evolution process from the bubbly flow to the flow with confined vapor slug based on the vapor bubble coalescence and the thermal-hydraulic characteristics with the two-phase interface morphology were studied.