Flow Characteristics and Evolution Mechanisms of a Liquid–Liquid Compound Jet

The disintegration and evolution of liquid–liquid compound jets significantly impact the mixing and mass transfer of two-phase fluids, yet the underlying mechanisms remain unclear. In this study, the flow characteristics and evolution mechanisms of a compound jet were investigated by using high-speed imaging. Three types of inner fluid structures were identified: liquid droplets, liquid dumbbells, and liquid ligaments, among which liquid dumbbells and ligaments were transitional inner fluid structures. Liquid dumbbells evolve into droplets through pinch-off or recoil, whereas liquid ligaments first contract into liquid dumbbells and then undergo a similar evolution to form droplets. A model was developed to investigate the competing mechanisms between pinch-off and recoil in the evolution of liquid dumbbells. The effects of flow rate ratio, flow velocity, interfacial tension, and liquid viscosity on the mean droplet diameter (d32) and jet breakup length (L) were systematically examined. Based on the experimental results, correlations for predicting d32 and L were proposed, with deviations within ± 10% and ± 20%, respectively.