Coaxial liquid-liquid flows were numerically studied in a nesting two-tube system. Calculations were carried out when various exit-lengths (meaning length differences between the two tubes) were used. Numerical results indicated that there exists a certain range of exit-length for the liquid-liquid flows to form stable and smooth interfaces, which requires that the exit-length should roughly be less than 5.6 times the outer tube diameter. In this range, interface instability is effectively restrained and the core fluid shows a phenomenon of die swell. When the exit-length is about 1.6 times the outer tube diameter, the core fluid has the greatest diameter size in the shell fluid. Velocity distributions at the outer tube exit favor formation of a continuous and stable core-shell structure.
