Onset of pseudo-slug flow in upward-inclined gas–liquid pipe flow governed by liquid-film backflow

Pseudo-slug flow is a transitional intermittent pattern between stratified and fully developed slug flows, yet its onset mechanism in upward-inclined low-liquid-loading pipes remains unclear. Air–water experiments were performed in a 50 mm inner-diameter transparent pipe at inclination angles of 2°–30°, superficial gas velocities of 2–35 m/s, and superficial liquid velocities of 0.001–0.05 m/s. High-speed visualization, pressure-drop and liquid-holdup measurements, pressure-signal cross correlation, and dye tracing were combined to identify flow patterns and quantify the hydraulic and kinematic responses. The results show that pseudo-slug flow is initiated by liquid-film thickening and local backflow, forming through a mechanism chain of liquid-film reversal, local liquid accumulation, partial blockage, gas penetration, and liquid redistribution. Unlike fully developed slug flow, the pseudo-slug body cannot stably block the pipe cross section; gas continuously penetrates the liquid-rich region, producing a finite-lifetime intermittent structure with weak pressure accumulation, non-periodic motion, and strong spatial instability. Increasing pipe inclination or superficial liquid velocity promotes axial gravitational backflow and shifts the stratified-to-pseudo-slug boundary toward higher gas velocities. Based on this physical picture, a transition model was developed by coupling a stratified-flow two-fluid momentum balance with two liquid-wall shear stress reversal criteria: cross-section-averaged shear reversal and bottom-film shear reversal. The proposed model links pseudo-slug onset to liquid-film backflow and wall shear stress reversal, providing a mechanistic criterion for predicting the stratified-to-pseudo-slug transition boundary.