This study presents experimental data for pressure gradient, liquid holdup, and flow regimes. To the knowledge of the authors, there are no studies in the literature on vertical downward two-phase flow in annulus.

The experimental results for downward two-phase flow in the annulus is compared with downward flow in pipes. The comparison results show some differences between downward two-phase flow in annulus and pipes for the liquid holdup and flow regimes. This comparison shows a significant disagreement between flow in annulus and pipes for a wide range of conditions.

The experimental results show that the liquid holdup is consistently higher for two-phase downward flow in annulus than in pipes for the annular flow regime, and these differences are as high as 45%. When the flow regime map for downward flow in annulus is compared to the ones in the literature for flow in pipes, intermittent flow is observed for flow in pipes with liquid velocities lower than the velocities for flow in annulus.

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