The flow pattern characteristics for a mixture of naphtha and nitrogen in a 52-m high, 189-mm diameter vertical pipe at 20 and 90 bar are reported. Time varying, void fraction and pressure variations along the riser were measured. For the former, gamma densitometers were employed. It was found that the classic slug flow Pattern was not present in these experiments. The observed flow patterns are classified as bubble, intermittent, semiannular, and annular. All the methods for flow pattern transitions in vertical two-phase flow tested against the present experiments give poor predictions. The identification and delineation of flow patterns using probability density function distribution of the cross-section ally averaged void fraction is consistent with the methods of employing pressure gradient fluctuations and structure velocity. (c) 2007 American Institute of Chemical Engineers.