Mechanism of slug aeration in horizontal gas-liquid intermittent flow: Insights from S-PLIF and PIV analysis

Horizontal gas-liquid two-phase flows are commonly observed in chemical engineering processes, particularly in multiphase flow equipment such as reactors, fluidized beds, and cyclones. One of the most prevalent flow patterns in horizontal pipes is intermittent flow. A comprehensive investigation of the structural evolution characteristics and slug aeration mechanisms of gas-liquid intermittent flow is essential for revealing its hydrodynamic behavior and mass/heat transfer performance, as well as providing guidance for establishing the physical model of flow parameters. In this research, we design an optical measurement system suitable for smalldiameter pipes, i.e., structured planar laser-induced fluorescence and particle image velocimetry (S-PLIF&PIV). The system is equipped with a Ronchi ruling plate for traditional PLIF&PIV, which can effectively eliminate the impact of total internal reflection (TIR) and refraction at the gas-liquid interface, allowing for the simultaneous measurement of the true gas-liquid interface and velocity distribution in the liquid phase. The interfacial structure of gas-liquid intermittent flow (namely plug flow and slug flow) is accurately detected by S-PLIF&PIV during the experiment on horizontal gas-liquid two-phase flow. Based on the evolution of interfacial morphology, three slug aeration mechanisms are identified, i.e., the shear stress mechanism, the front vortex mechanism, and the gas carry-under mechanism. Additionally, high-aerated slug flow is further subdivided into three sub-regimes. The velocity distribution and the average axial velocity profile in the liquid phase are extracted, enabling us to examine the influence of liquid phase motion on the gas-liquid interface morphology and the bubble detachment process. Additionally, the turbulence intensity and Reynolds stress in the liquid phase are calculated, further elucidating the slug aeration mechanisms from the perspective of microscopic flow motion.