As a novel equipment for polymer fluid processing

the tri-screw extruder has very complex mixing mechanism due to periodic change in geometrical structures and stresses at its unique center region. Chaotic mixing in tri-screw extruder was analyzed by Lagrangian system

which is much different from the traditional linear mixing analysis technique. With the finite-time Lyapunov exponent (FTLE)

Lagrangian coherent structure (LCS)

Poincaré section and particle visualization technology

mechanism of fluid transfer and chaotic mixing in 2D flow field as well as influence of dynamic structure characteristics at the center region to the FTLE's and LCS's distributions were studied in tri-screw extruder and compared to those in single-and twin-screw extruders. The results show that LCS divided the flow field in tri-screw extruder into three regions of different flow characteristics

namely

the near screw region

the far screw region and the center region

where a snarl is mass transfer bridge connecting these three regions. With the increase of mixing time

the degree of bending and folding in the snarls increased gradually

that enhanced chaotic mixing in flow field of tri-screw extruder by increasing transportation of polymer melt across three regions. Three hyperbolic fixing points near the kneading block indicated good mixing capability. The existence of elliptical cycle points in the Poincaré cross-sections implied the presence of non-chaotic spots in the center of flow field. Thus

mixing in the center region of tri-screw extruder was relatively poor.