Stratified to annular flow transition due to drop entrainment in a flat minichannel

This study deals with the experimental and numerical investigation of the transition from stratified to annular flow in a flat minichannel over a wide range of gas and liquid velocities, including turbulent conditions. It was established that the transition to annular flow at high phase flow rates occurred not only due to sidewall wetting, as reported in the literature, but also as a result of drop entrainment. Moreover, when the contact between the liquid and the side walls of the channel was eliminated, the transition to annular flow at high liquid and gas flow rates occurred solely due to drop entrainment. To study the droplet entrainment mechanisms in a minichannel, numerical simulations were performed in both two- and three-dimensional problems using volume of fluid and coupled volume of fluid with level-set methods. It was found that the mechanism of the disturbance wave formation in the minichannel does not differ from that in large-scale channels and is associated with the merging of initial waves near the liquid inlet. However, compared to large-scale channels, in a minichannel, the wave frequencies increase significantly, while the longitudinal wave size decreases (by about an order of magnitude). The primary cause of drop entrainment was pulsations, in particular vortices, in the gas phase which induced oscillations in the vertical pressure gradient above the disturbance waves. These oscillations could lead to vertical wave growth, eventually resulting in liquid ligament formation and its subsequent breakup. An additional factor increasing the likelihood of instability with drop entrainment was wave coalescence.