Effects of diameter on interfacial structure in horizontal two-phase flow: A review

This review critically examines the effect of pipe diameter on interfacial structure and flow regime transitions in horizontal gas–liquid two-phase flows by analyzing experimental, theoretical and computational studies. It is demonstrated that diameter dramatically alters the balance of gravitational, inertial and surface-tension forces, producing diameter-dependent interfacial structure. In pipes exceeding 100 mm, annular and intermittent flow regimes are delayed and dispersed flow prevails, whereas in smaller diameters, enhanced interfacial effects foster bubbly and slug flows. Stratified flow persists to higher gas velocities in larger pipes, and slug frequency decreases roughly in inverse proportional to diameter, reflecting the stabilizing effect of increased cross-section. Void fraction distributions and wave dynamics likewise shift from Kelvin–Helmholtz instabilities in small tubes to roll wave formation in larger diameter pipes. Current models often underpredict transitions in large diameter pipes, where reduced slug frequency and modified holdup profiles emerge. This work elucidates the mechanisms of slug stability, bubble coalescence, and wave growth, identifies gaps in transient dynamics, two-phase interactions, and fluid property influences, and recommend large-scale experiments, high-resolution diagnostics, refined dimensionless group frameworks, Reynolds number, Froude number, and Eötvös number and advanced Computational Fluid Dynamics scaling approaches to guide diameter-specific design in energy and process engineering.