Terrain slugging is induced by natural transient effect due to liquid accumulation at the dip of a hilly-terrain pipeline. The slugs' characteristics of newly initiated developing slugs, such as velocity, exhibit unsteady behavior downstream of the initiation point. The objective of this study is to experimentally and theoretically investigate the evolution of slug velocity of newly initiated slug at the lower elbow of a hilly-terrain pipeline. Experimental observations show that pseudo slugs initiated at the bottom elbow initially accelerate then decelerate at a region near the uphill pipe section exit. Detail analysis on the slug velocity distribution indicates that the acceleration and deceleration phenomenon are related to the slug growth and dissipation along the pipe. Furthermore, the maximum slug velocity is observed approximately at dimensionless distant (x/D) of 110 from the bottom elbow. The investigation of operational and geometrical parameters effect showed that gas and liquid superficial velocities have an increasing effect on slug velocity. However, the superficial liquid velocity effect decreases as the flow develops along the uphill pipe section. Experimental results show that an increase of 1° inclination angle promotes the slug acceleration and deceleration trends significantly.
A theoretical model developed on the basis of momentum balance and gas law fairly predicted the acceleration trend close to the initiation point, but failed to predict the deceleration trend observed in experiments toward the pipe exit. The model error in predicting slug velocity increases as the pipe dimensionless distant increases due to model assumption of a single slug evolution. The developed model can be used as a predictive tool to estimate the distance to accelerate a slug to the maximum slug velocity and ultimately the location of flow related erosion/corrosion.
