Experimental investigation of terrain slugging: formation mechanism and potential mitigation methods

An experimental study on terrain-induced slugging on shallow inclines (typically less than 3°) has been conducted, using three setups with a pipe diameter of around 1”, with incline lengths of 1 m, 3 m and 24 m. In addition to an extensive characterization of the formation and evolution of slugs as a function of flow conditions and inclination angle, criteria were identified for the occurrence of countercurrent gas/liquid flow (liquid backflow) on the incline. In the characterization study, it was observed from both measurements and models and results in literature that the influence of inclination on slug frequency is small for this range of angles. Moreover, two flow regimes were distinguished. For higher superficial gas velocity, the slug frequency varies little as gas velocity is increased. This constant slug frequency does not depend on incline length, and is therefore similar for all setups. . In contrast, at low superficial gas speed, the frequency is shown to increase strongly with decreasing superficial gas speed. This increase is predicted by various slug models from literature. Unlike most earlier experiments, these experiments used a range of incline lengths. In the low-velocity regime, incline length was found to have a strong influence on the observed frequency.Although current models give an indication of expected slug frequency, they do not accurately predict this behavior. . Furthermore, whereas at higher velocities hydrodynamic slugging is observed, under low gas flow conditions, the slug appears to be initiated as a result of liquid backflow. This backflow in turn is a result of the dip filling with liquid, rather than purely a hydrodynamic interface instability. This behavior can lead to much longer slugs, and is strongly dependent on the angle of inclination. A non-dimensional analysis of the critical flow conditions revealed a universal backflow criterion that strongly reminisces of the flooding criterion known from literature, where – although the inclination angle used is very different – the flow pattern is dominated by the same force balance.