Physically-based models for predicting the bubbly-to-slug flow transition in vertical downward gas–liquid two-phase flow

Accurately predicting flow regime transitions remains one of the key challenges in multiphase flow systems, with significant implications for design, safety, and operational reliability. In this study, novel models are introduced to predict the bubbly-to-slug flow transition in vertical downward gas–liquid flows. The transition to bubbly flow is defined in such a way that it reflects the disappearance of slug-like flow structures, offering a more intuitive physical interpretation of the underlying mechanisms. Two independent and physically meaningful criteria are proposed: (i) the vanishing of the Taylor bubble and (ii) the onset of homogeneous flow behavior. Based on these criteria, analytical expressions are derived using recent correlations for global and slug liquid holdups. The resulting transition lines are nearly identical, underscoring the internal consistency and robustness of the proposed methodology. The models’ performances were validated against an extensive experimental database from the literature and covering a broad range of pipe diameters (9.53 mm ≤ D ≤ 80 mm). It showed excellent agreement with observed transitions in most cases, confirming its predictive accuracy.