Liquid falling behavior in vertical partial annular pipes under gas–liquid countercurrent flow

Gas–liquid two-phase flow in annular pipes is fundamental to heat and mass transfer processes in petroleum, chemical, and related engineering fields. This study experimentally and numerically investigates liquid falling dynamics in vertical partially annular pipes under top-injection conditions. The numerically predicted falling ratio agrees well with experimental data, and the average error is 6.93%. Based on visual observations, five typical flow morphologies are qualitatively classified, and key governing parameters, including gas velocity, liquid velocity, rheological properties, viscosity, and surface tension, are identified. The evolution of the liquid falling ratio is quantitatively examined, and its nonlinear variation from 0 to 1 is illustrated for Newtonian and non-Newtonian fluids. The results show that at low gas velocities, the critical gas velocity for complete falling and the falling ratio both increase with liquid viscosity, density, and surface tension, while they decrease with superficial liquid velocity. Furthermore, these quantities exhibit a non-monotonic trend with increasing liquid yield stress, initially rising and then declining. At high gas velocities, thinning of the liquid film weakens interphase momentum transfer and greatly reduces the sensitivity of the falling ratio to these parameters. Compared with shear-thinning fluids, shear-thickening fluids are more prone to complete falling at low liquid velocities, whereas the opposite occurs at high liquid velocities. This behavior originates primarily from the reversal of the effective viscosity gradient due to differences in shear rate. The findings of this study can provide an important theoretical foundation for blowout control and gas-lift optimization in petroleum engineering.