The flow pattern of air-water two-phase flows in a vertical, circular pipe was experimentally investigated. The wettability of the pipe was changed by surface treatment. The original acrylic pipe had a contact angle θc of 77°, while two pipes after the surface treatment had θc of 36°and 104°, respectively. The flow patterns in the pipes having θc of 36° and 77° were the same and agreed favourably with previously published results for pipes of good wettability (θc<90°). On the other hand, different types of bubbly and slug flows appeared in the pipe of θc=104°, being caused by attachment of bubbles and slugs to the wall of the pipe. The bubbly flows for θc of 104° were classified into two categories and slug flows for the same contact angle were classified into three categories. The boundary between the bubbly and slug flows, however, was hardly dependent on the wettability of the pipe.
