Liquid accumulation in a horizontal–vertical pipe system based on the interface tracking method

During valve opening in gas–liquid two-phase pipeline systems, fluid accumulation at local low points may trigger upstream cavitation, leading to complex air–water interface dynamics and pressure oscillations. To explore the underlying mechanisms, Computational Fluid Dynamics simulations based on an interface-tracking method were conducted under varying inlet flow rates, gas volume fractions, and liquid accumulation conditions, and their effects on pipeline pressure characteristics were systematically analyzed. This study investigates the gas–liquid two-phase flow behavior in a downstream pipeline–riser system under assumed operating conditions representative of a helical axial-flow gas–liquid transport unit, aiming to provide theoretical support and boundary condition references for future experimental research on such spiral transport devices. To validate the numerical model, a rapid water-filling process involving a retained air pocket in a horizontal pipeline was simulated, confirming the suitability of the two-dimensional Volume of Fluid model. The investigation addressed cavitation formation, pressure distribution, and interface evolution, with emphasis on pressure and differential pressure variations at key locations under different operating conditions. Results show that liquid blockage induces substantial flow instability. At high flow rates and low gas fractions, large pressure fluctuations and peak pressures occur near the inlet. In contrast, at low flow rates, repeated rupture and reformation of the interface boundary of the liquid plug induce sustained pressure pulsations. Transient pressure fluctuations associated with liquid film breakage and interface evolution are also captured, and the overall trend of mean pipeline pressure is analyzed. These findings enhance the understanding of two-phase flow transients and offer theoretical guidance for the design and stable operation of gas–liquid transport systems.