Statistical analysis and hydrodynamic modeling of unsteady gas–liquid slug velocity behavior

Abstract

Abstract Slug flow is the dominant two-phase flow pattern in horizontal and slightly inclined multiphase transport pipelines. Slugs generate and dissipate along pipelines due to the unsteady nature of slug flow, which gives rise to the time-dependent behavior, where the pressure, mass flow and velocity vary with time. The velocity of a newly initiated developing slug exhibits unsteady behavior downstream of the initiation point. The objective of this study is to experimentally investigate the unsteady velocity behavior of newly initiated slugs at a pipeline dip. A further objective is to develop a simple transient model to predict the development of slug velocity in time and space beyond the initiation point. Experimental observations show that pseudo-slugs initiated at the bottom elbow initially accelerate, then decelerate at a region near the uphill pipe section exit. Detailed analysis of the slug velocity distribution shows that the acceleration and deceleration phenomena are related to the slug growth and dissipation along the pipe. Furthermore, the maximum slug velocity is observed approximately at a dimensionless distance ( x / D ) of 110 from the pipeline dip where slugs are initiated. The investigation of operational and geometrical parameter effects showed that gas and liquid superficial velocities have a significant effect on slug velocity. However, the effect of superficial liquid velocity decreases as the flow develops along the pipeline. The experimental results show that an increase of 1° inclination angle promotes slug acceleration and deceleration trends significantly. A theoretical model based on a momentum balance and the equation of state for gases predicted the acceleration trend quite well close to the initiation point. The model error in predicting slug velocity increases as the pipe dimensionless distance increases due to the model's assumption of a single slug evolution. The developed model can be used as a predictive tool to estimate the distance required to accelerate a slug to its maximum velocity, and ultimately the location of flow related erosion/corrosion and produced sand accumulation and transportation.