The flow-induced void fraction transition phenomenon was observed in an upward air-water two-phase flow in a vertical pipe with inner diameter D = 200 mm and height z = 25 m. As the two-phase flow develops in a vertical pipe, the void fraction increases firstly in the flow direction in bubbly flow, then decreases in the flow direction, finally increase again. The flow-induced void fraction transition shows an N-shaped changing manner. The present experimental investigation revealed that this phenomenon was attributed to the formation and the growth of local dominant large bubbles in the flow. According to the bubble sizes and behaviors observed in the experiment, the flow regimes were classified into bubbly, churn and slug flows in a vertical large-diameter pipe. The drift velocities in the three flow regimes were measured in this paper. New constitutive equation for drift velocities in bubbly, churn and slug flows was proposed and confirmed in this study. The flow-induced void fraction transition in N-shaped manner can be predicted by using the drift flux model with the newly developed constitutive equations.

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