Transient mechanisms of slug flow: Dynamical modeling and experimental validation of slug-structure collapse

When gas supply is weakened or interrupted, stable slug flow may undergo pronounced pressure transients, whose understanding is important for predicting rapid pressure variation, flow instability, and possible disturbance to downstream equipment in gas–liquid transport systems. However, a compact dynamical description of this structural transition remains limited. In this work, the transition from stable slug flow to a liquid-dominated state is identified as a slug-structure collapse process, and a minimal lumped-parameter model is developed to describe the associated pressure response. By representing an equivalent control volume with effective hydraulic inertia, resistance, and compliance, the pressure transient is reduced to a second-order dynamical system with physically interpretable parameters. Experiments were conducted in a horizontal gas–liquid two-phase flow loop, where gas shut-off was used to trigger slug-structure collapse under multiple operating conditions. The measured responses exhibit a common pattern of rapid pressure decay, a single pressure minimum, and gradual recovery toward a final liquid-dominated steady state. The estimated parameters give ωn=0.119–0.132 rad/s and ζ=1.23–1.39, indicating overdamped behavior for all tested cases. The model reproduces the measured transients with r=0.96–0.99, R2=0.91–0.94, and mean absolute error =1.9–6.6 kPa. The peak pressure-drop error is 2%–14%, and the pressure-minimum timing deviation is 0.3–4.1 s. The results show that slug-structure collapse is governed by liquid-column inertia, hydraulic dissipation, and residual-gas compliance, providing a concise framework for interpreting this transient process.