Entrainment of Gas Into Slugs and its Subsequent Transport in Two-Phase Slug Flow

This paper reports measurements of two-phase gas-liquid slug flows in a 4-inch diameter pipe using a fast response X-ray tomography system, installed in the Well Flow Loop at IFE. The main focus of this work is phenomenological understanding of the distribution and transport of gas in the slug body. The observations made with the X-ray CT indicated different types of slugs with respect to gas distribution, (1) "two-zone" slugs that contain distinguishable mixing and migration zones, (2) "unmixed" slugs for which the mixing region is either very short or absent and (3) "frothy" slugs which are highly aerated, with nearly uniform mixing of gas within the slug. For typical two-zone slugs, a highly turbulent mixing region is observed at the slug front followed by a ‘migration region’ where small bubbles migrate towards the upper part of the pipe due to buoyancy. For frothy slugs, a significant amount of gas can be carried over from the slug into the liquid layer of the following stratified zone. Gas bubbles trapped in the liquid layer are then scooped up by the next slug together with the liquid. The occurrence of the different types of slugs is found to be dependent on the mixture velocity and the gas density. The development of the fast response X-ray CT system and its application in multiphase flow research have provided us with a valuable tool to capture the details and gain better insight into such complex flow behaviour. The quantitative information gleaned in this study is also helpful in validating the closures used in CFD simulations and 1D models.