The onset of gas entrainment from a flowing stratified gas–liquid regime in dual discharging branches: Part I: Flow visualization and related phenomena

Experiments were performed in a 50.8 mm diameter horizontal pipe with three 6.35 mm diameter branches located at the test section mid-span. The inlet length was 1.8 m and three branch orientations were tested at 0° (side), 45° (inclined), and 90° (bottom) from the horizontal. Water and air, operating at 206 kPa, were used and both fluids flowed co-currently within the inlet in the smooth-stratified regime. The inlet superficial velocity of the liquid phase ranged between 0.04 and 0.15 m/s while in the gas phase values of 0.3, 0.4, and 1 m/s were tested. Three different dual discharge combinations were tested and included side-inclined, side-bottom, and inclined-bottom. In the first two combinations air flowed in the side branch while water flowed in the inclined and bottom branches. In the third combination water flowed in both the inclined and bottom branches. The tested branch flow Froude numbers were limited between low to moderate values which ranged between 1 and 23. In the first of this two part investigation flow visualization was used to identify the related flow phenomena in single and dual discharging branches. A thorough description of these observations was provided in Part I and included the onset of gas entrainment, liquid entrainment, and inlet flow regime transition from smooth to wavy and slug regimes. The critical conditions at the onset of gas entrainment are reported in Part I for single discharge experiments including the critical height and inlet superficial velocities. The critical height at the onset of gas entrainment results are compared to published models and data sets and poor agreement was found with studies conducted in stratified gas–liquid reservoirs.