Experimental study of two-phase flow pattern evolution in a horizontal circular tube of small diameter in laminar flow conditions

• The explored ranges of phase velocities are different from previous studies.
• Evolution of flow pattern from the mixing zone at L/D = 10–420.
• New flow patterns, like bubbly disk flow, are visualized in the entrance region.
• Flow maps in both explored regions (L/D = 10 and L/D = 420) are drawn.

The carried out experimental study aims at analyzing air–water two-phase flow patterns in a horizontal tube for laminar flow with carefully controlled conditions. An experimental test rig has been setup to produce a two-phase flow in a glass tube of 3 mm inner diameter with co-current air and water flows. Unlike previous studies, the investigated ranges of superficial velocity of the liquid and gas are from 0.78 × 10−3 ms−1 to 79 × 10−3 ms−1 and from 2.3 × 10−3 ms−1 to 3.54 ms−1 respectively. Flow visualizations are recorded with a high-speed camera in the entrance region at L/D = 10 corresponding to a mixing zone of the two phases and far downstream at L/D = 420. The results are used to highlight the flow patterns in both zones. Within the explored ranges of superficial velocities, several flow patterns were obtained namely bubbly, slug and annular flow pattern. More complex flow patterns were visualized in the entrance region that evolve to fewer and simplified flow structure with smoother interfaces far downstream. Flow maps are drawn and proposed. They reveal some pattern rearrangements. A detailed description and the physical interpretation of the change in the flow structure between the two investigated zones are given in the paper.