Investigation of the gas and film flow of Taylor bubble in a tube with a short constriction employing 3D particle tracking

Taylor gas–liquid flows are present in many technical processes. However, the gas flow inside bubbles, which influences various process parameters, has been poorly investigated experimentally. These data provide new insights for more resource- and energy-efficient processes. This study explores the internal gas flow and film flow dynamics within Taylor bubbles in a cylindrical tube with a constriction. By employing particle image velocimetry for internal gas flow and highly spatially resolving double helical particle tracking velocimetry (DH-PTV) for film flow, experiments are conducted with downward water flow. The Reynolds number Re for the liquid and air flows ranges from 127 to 1460 and 317.5 to 1016, respectively. With an Eötvös number Eo<5 and a capillary number Ca<10^–3, the internal flow in a plain tube exhibits a toroidal vortex in the upper part of the bubble, whereas the constriction introduces significant alterations, generating two toroidal vortices with elevated shear velocities. Complementary film flow measurements below constrictions reveal a correlation with internal flow velocities at the bubble surface. The DH-PTV technique, which uses a spatial light modulator, proves effective in measuring micrometer-scale film thickness and reducing aberrations due to curved optical interfaces. This work contributes insights into multiphase flow dynamics, offering potential applications for particle separation studies and more efficient flow geometries in various processes.