Quantitative investigation of slug flow in an inclined pipeline

This study quantitatively and qualitatively examined the initiation and development of slug flow under varying superficial velocity conditions in a 10° inclined pipeline. Experiments were conducted in a pipeline with a total length of 6.0 m and an inner diameter of 0.051 m. The liquid (water) superficial velocity (USL) ranged from 0.10 to 0.80 m/s, and the gas (air) superficial velocity (USG) varied from 0.10 to 0.50 m/s. Using high-speed visualization, we investigated the frequency of slug flow and the evolution of liquid slug length distribution along the pipeline. Velocity fields of slugs with lengths within ±1D of the maximum probability density (MPD) were calculated using particle image velocimetry (PIV) and averaged as representative under specified superficial velocity conditions. The findings indicate that in addition to slugs initiated at the bent section (B-initiated slugs), slugs were also initiated in the upstream horizontal section (H-initiated slugs), with their frequency increasing as USL increased. Compared to horizontal pipelines, slug frequency in the inclined pipeline was significantly higher. Along the pipeline, the liquid slug length with MPD increased from 56D to 96D. In the downstream region, slug frequency and average liquid slug length were minimally affected by USG at lower USL values (0.10 and 0.20 m/s) but were impacted at a USL of 0.80 m/s. Analysis of the slug flow velocity field obtained by PIV revealed that backflow formed in the Taylor bubble region at the lower USL values but disappeared at a USL of 0.80 m/s. The presence of backflow resulted in marked differences in the velocity and turbulence intensity distributions between slug flows in the inclined and horizontal pipelines. Conversely, the absence of backflow eliminated these differences. Additionally, an improved one-dimensional mixture model was introduced, providing more accurate predictions of the translational velocity of slug flow than existing models. © 2024 Elsevier Ltd