Stratified flow distribution during gas–liquid downflow in the mesodomain

The paper discusses the formation and characteristics of stratified air–water downflow in the mesodomain. The stratified pattern, observed at low phase velocities, is characterized by air and water flowing side by side and both wetting the conduit wall, similar to stratified distributions in horizontal conduits. Such flow segregation without the influence of gravity is counterintuitive and no detailed investigation on the formation and flow physics of this distribution is reported till date. We have performed extensive experiments in glass conduits of 2.5–12.5 mm diameter where the two phases are introduced through T and Y junctions with different included angles between the entry arms. Our experiments reveal that stratified flows are formed up to a critical angle $ϕc$ subtended by the edges of the circumferential liquid film and beyond the critical wetting angle, the flow pattern is annular with liquid completely wetting the conduit wall. We further note $ϕc$ to depend on liquid properties, included angle of Y-entry, conduit diameter, and phase flow rates. Based on experimental observations, we propose a simplistic analysis to relate the liquid properties, conduit dimension, and Y-entry included angle and the liquid and gas flow rates to describe the formation of stratified and annular flow at the junction. The analysis also estimates the in situ void fraction during stratified gas–liquid downflow.