Two-phase fluid flow is distinguished from single-phase flow in two respects: (1) the cross section for flow of either fluid is not defined by the conduit alone and (2) not only the extent but the manner of frictional energy exchange for each fluid depends on the individual rates of flow for both fluids. It was believed therefore that an empirical approach would not adequately describe the various situations encountered in two-phase flow, and so study was undertaken to obtain some understanding of the mechanisms of the flow of liquid with a free surface and the momentum exchange between fluids at that surface. It resulted in the development of a method of predicting liquid holdup and pressure drop for flowing systems in which the liquid, lifted by the gas flowing as a central core, moves upward as an annular film along the pipe wall. In order to clarify the relationship of annular flow to the entire range of vertical two-phase flow modes, a discussion of vertical two-phase flow is presented, followed by an analysis of the special case of vertical, upward, annular flow; a description of the experimental work; and a comparison of experimental data with predictions
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