Pattern transition of a gas–liquid flow with zero liquid superficial velocity in a vertical tube

The flow pattern transition is of importance in the analyses of gas–liquid two-phase flows. However, few studies have been carried out on the pattern transition of the gas–liquid flow with zero liquid superficial velocity (ZLSV). We carried out an experimental study on the ZLSV flow with air and water as working fluids in a vertical pipe having a length of 12 m and internal diameter of 100 mm. The void fraction at the bubble-to-slug flow boundary is in the range of 0.06–0.10. The gas superficial velocity at the slug-to-churn flow transition boundary is approximately 0.30 m/s. The predicted results of these two transitions by the current models are larger than the experimental results. For the bubble-to-slug flow transition, we propose a transition mechanism based on the coalescence of central bubbles. Considering the radial velocity distribution in the bubble flow, a new flow pattern transition criterion is proposed. For the slug-to-churn flow transition, the transition mechanism is the breakdown of liquid slug caused by the strong wake effect. By combining the hydraulic model of the slug flow with the wake effect, the slug-to-churn flow transition criterion is proposed. The predicted results are consistent with the experimental data. This study provides valuable insights into the flow pattern transitions and paves the way for further studies on ZLSV flows.