Experimental investigation of film reversal evolution characteristics in gas–liquid annular flow

In natural gas wells, liquid loading is a severe problem threatening production safety. Published studies have verified that liquid loading is closely related to film reversal in gas–liquid annular flow, but the evolution of the film reversal is still unclear. This article reports on experiments conducted to reveal the film reversal evolution characteristics. Experiments were conducted in a 50-mm-diameter vertical pipe with superficial gas velocities ranging from 5.66 to 22.64 m/s and superficial liquid velocities ranging from 0.014 to 0.170 m/s. A camera and electrical resistance tomography were used to obtain qualitative and quantitative results, and an error analysis verified the experiments’ repeatability and reliability. The evolution process is divided objectively into three stages to clarify the film reversal: no film reversal (No-FR), the onset of film reversal (Onset-FR), and complete film reversal (Complete-FR). The three stages occur successively with decreasing gas velocity. The characteristics of the individual stages are elaborated, including the interfacial structures, morphological features, and motion trajectories. The void fractions are analyzed in both the time and frequency domains, where the statistical parameters of the probability density function, average value, and standard deviation are presented. The results show that the Onset-FR stage exhibits characteristics similar to both the No-FR and Complete-FR stages, indicating that it plays an intermediate role in the gradual evolutionary process. The current experimental results also achieve excellent agreement with published datasets and correlations.