Axial development of annular, churn and slug flows in a long vertical tube

• Development of annular, churn and slug flows in a 42-m long, 48-mm ID vertical pipe.
• Good agreement with flow transition models only for low outlet pressure (1.4 bar).
• Data suggest that churn flow exists where others saw it as underdeveloped slug flow.
• The holdup reaches a developed condition for annular flow at low liquid flow rates.
• The frequency of flow structures is weakly dependent on the axial distance.

Experimental data are presented on the axial development of gas–liquid flows in a 42-m long, 0.048-m ID tube system. Different transitional models for slug, churn and annular flow regimes were compared with visual observations and showed reasonable agreement for locations L/D = 560 and 820. The behavior of liquid holdup and frequency of flow structures (large waves, disturbance waves) were also analyzed for three different axial positions (L/D = 102, 521 and 815), over a range of pressure between 1.4 and 5.2 bar, liquid mass fluxes ranging from 17 to 319 kg/m2-s and dimensionless gas velocities from 0.05 to 1.6. The liquid holdup showed significant axial variation at all monitoring locations for higher liquid mass fluxes, while it appeared that a kind of developed flow was reached at L/D = 521 for lower liquid mass fluxes. The flow structure frequency did not exhibit significant axial variation for dimensionless gas velocities (Wallis parameter) between 0.2 and 1.6.