The onset of gas entrainment from a flowing stratified gas–liquid regime in dual discharging branches: Part II: Critical conditions at low to moderate branch Froude numbers

This is the second of a two part investigation. 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 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. The tested branch flow Froude numbers were limited between low to moderate values which ranged between 1 and 23. Extensive experimental data are reported for the critical conditions at the onset of gas entrainment during dual discharge. A novel map was developed for the inclined-bottom branch configuration showing the relationship between the inlet superficial liquid velocity and branch Froude numbers. This map was used to quantify the three observed modes of gas entrainment during dual discharge. These modes were classified as onset of gas entrainment in the inclined branch only, in the bottom branch only, or both the inclined and bottom branches simultaneously. 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.