Two-phase slug flow across small diameter tubes with the presence of vertical return bend

This study examines the two-phase slug flow across small diameter tubes with the presence of vertical U-type return bends. The translational velocity of the air slug across return bend usually peaks at an angle of π/2–3π/4. The increase of translational velocity is especially pronounced when flow enters at the lower tube with a smaller curvature ratio. An approximately twofold increase of the slug velocity is observed at a curvature ratio of 3. Dimensionless correlations for flowing upwards and downwards with a mean deviation of 18.7% and 24.5% are proposed that can describe the variation of translational velocity within the return bend.