Research on bubbly to slug flow regime transition of air-water co-current upward flow in vertical rectangular channels with different gap sizes

To investigate the transitional criterion of bubbly to slug flow influenced by gap size in rectangular channels, an experimental investigation of air-water two-phase co-current upward flow has been conducted in adiabatic vertical channels under atmospheric conditions. Experiments were carried out in three channels with different cross-sections of 4 × 66 mm, 6 × 66 mm, 8 × 66 mm, respectively. The superficial velocities are 0≤jg≤20m/s and 0.25≤jf≤3m/s for gas and liquid phase. The range of liquid superficial Reynolds numbers is 1900≤Ref≤42000 which have covered purely laminar flow to turbulent flow. Flow regimes were defined according to photos captured by a high-speed camera and characteristics of void fraction measured by flat electrodes, and a random forest algorithm was used to identify the flow regimes at the transitional region. Results show that with the increase of the gap size, the transition from bubbly to slug flow is delayed, and the regime transitional void fraction changes from 0.17 to 0.25. The mechanisms of random collision, wake entrainment, and turbulence breakup were analyzed according to the experimental phenomena. A new criterion of the bubbly to slug flow regime transition applied in rectangular channels was proposed considering the impact of the gap size. •The transition from the bubbly to slug flow delays to the right with the increased gap size.•According to the PDF of void fraction, the void fraction of liquid block is a key factor for flow regime transition.•A new criterion has been proposed based on mechanisms of random collision, wake entrainment, and turbulence breakup.