Multiphase flow models require models for each flow regime, and models for the transition between the regimes. Dynamic slug flow models also aim at predicting the slug formation and decay as well as the statistical distributions of lengths, frequencies and velocities.

Previous experimental studies have demonstrated the existence of a region in the flow regime map where both stratified and slug flow can be stable flow regimes, typically for low pressure systems. This phenomenon can also lead to large flow transients at the point of stratified-slug transition, with excess liquid transported out of the pipeline in the form of a large liquid slug, with length depending on the pipe geometry and length.

This phenomenon is experimentally investigated for 3 degrees downwards air-water flow in a 16 m long pipe of 60 mm internal diameter. An inlet section of 3 m long was configured at 14.6 degree downward and 25 degrees upward, in order to generate either stratified flow or slug flow. The statistical analysis of the slug frequency and length distribution was examined, and the results show a premature stratified-slug transition when slug flow is set as inlet condition. The slug formation and evolution along the pipeline was studied by tracking the slugs at different locations in the downward pipe. Results were compared against flow patterns predicted by a commercial simulator.

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