CFD investigation of gas-core instability in axial swirl separators using a coupled VOF-two-fluid model

Unstable flow in gas–liquid separators significantly degrades separation efficiency and system reliability. While previous work has documented unstable flow phenomena, the statistical regularities of transient interfacial oscillations and their quantitative coupling with local flow parameters (swirl intensity, pressure gradient, and velocity slip) have not been fully quantified. To address these gaps, the present study develops and validates a coupled VOF-two-fluid model for axial swirl separators, incorporating the Reynolds Stress Model (RSM) to investigate the dynamic characteristics of transient gas-core behavior. The results show that under unstable conditions, the gas core exhibits quasi-periodic “necking-expansion” oscillations with bimodal probability distributions (peaks at 29.56 mm and 36.49 mm), contrasting sharply with the unimodal distribution observed under stable flow. The standard deviation of core diameter increases by 116% (from 1.8 mm to 3.9 mm), quantitatively confirming fluctuation amplification. Strong negative correlations are observed between core diameter and swirl intensity, with swirl intensity decaying by approximately 4% during an expansion event at Z/D = 8. Transient pressure-drop peaks reach 4.62 times the temporal mean and are synchronized with core oscillations, indicating strong pressure-interface coupling. Swirl intensity decays monotonically along the axis, by 47% from its maximum after an axial length of 10D, while velocity slip remains elevated in the region of Z/D = 3–13. This competition between weakening centrifugal constraints and sustained shear effects drives instability onset in the middle-to-downstream region, which is characterized by the proposed dimensionless stability indicator (We/Nc). These findings provide additional insight into gas-core instability behavior.