Theoretical model on the slug/churn transition based on the generation and evolution of the huge waves

Previous experimental studies have proved that the flooding of the falling film surrounding the Taylor bubble is attributed to the transition from the slug flow to the churn flow, which is related to the generation and evolution of huge waves. In the present study, theoretical models on the basis of the Kelvin–Helmholtz instability of the falling film around the Taylor bubble and kinematic analysis of the interfacial wave traveling on the falling film are, respectively, established to reveal the mechanism of the slug/churn transition. The formation of the liquid bridge or pseudo-liquid bridge is taken as the judgment basis to determine the transition. A term named “most dangerous wave” is introduced in the present study, and its criteria is related to the ratio of wave amplitude to pipe diameter. Verified by the data and models in the literature, the proposed model is demonstrated to have satisfactory predicting accuracy. In addition, parameters, including pipe diameter and system pressure, are analyzed in detail to discuss their effect on the slug/churn transition. The results indicate that the slug/churn boundaries move downward as the system pressure increases but upward with the increasing pipe diameter. We believe that the findings in this paper benefit a better understanding of the relation between the huge waves and slug/churn transition.