Characterization of two-phase slug flow sub-regimes using flow visualization

Abstract

Abstract Flow visualisation experiments are carried out to analyse the internal structure of different sub-regimes of slug flow. Five distinct slug flow sub-regimes are identified based on the visual observations. These sub-regimes correspond to slug at formation stage (onset of slug); less aerated slug; highly aerated slug; slug and plug; and slug and wavy. While superficial liquid velocity influences the formation of slug, it is observed that superficial gas velocity has a significant influence on the transition of sub-regimes. Maximum and minimum slug frequencies are observed in highly aerated zone and slug formation zone respectively. Highly aerated slug zone is extremely unstable and exhibits complex flow structure. Depending on superficial gas and liquid velocities, maximum value of slug frequency is observed at a distance between 30D and 60D from the pipe inlet. Slug frequency reduces along the length of the pipe due to viscous damping. An empirical correlation is developed for predicting the non-dimensional slug frequency (product of Strouhal number and Froude number) in terms of non-dimensional superficial liquid and gas velocities (represented by superficial Reynolds number of liquid ( Re SL ) and gas ( Re SG )) and length to diameter ratio ( L / D ). Identification of slug flow sub-regimes and their flow behaviour reported in this paper are of significant interest to oil production and transport processes where control of flow regime transitions is important in order to avoid severe unwanted jigging due to the formation and growth of slugs.