Three-dimensional finite element modeling of polymer melt flowing in a new co-rotating tri-screw extruder was established with mesh superposition technique. Based on the particle tracking technology, three typical particle trajectories in the tri-screw extruder were calculated using a 4th-order-Runge-Kutta method to study the dynamic motions of the particles. Then the flow visualizations in the local center region were carried out. Moreover, the dispersive, distributive and stretching mixing efficiencies of the tri-screw and twin-screw extruders were compared, respectively. The results show that when the particles move from one screw to another, there are great abrupt changes in the velocities and displacements, which induce the abrupt change in the stress magnitude. Most of particles, which are initially distributed in the inlet plane of the center region, fast flow out the outlet and don’t pass through any screw. This special phenomenon induces a series of new characteristics in the residence time distribution (RTD), flow number, segregation scale and time averaged efficiency. In comparison with the twin-screw extruder, the tri-screw extruder has better mixing efficiency. 1. Introduction Screw extruders are mostly used as pumping and mixing devices in polymer processing such as injection or blow molding. Their mixing efficiency and uniformity significantly affect the properties of the final product, energy consumption and costs. With the development of polymer industry, it is necessary to devise a new type of extrusion equipment with high output and better mixing efficiency. Based on the traditional single-and twin-screw extruders, a new type of triangle arranged tri-screw extruder (hereinafter referred to as tri-screw extruder) is put forward, which has three intermeshing regions and one dynamic center region [1]. Recently, the tri-screw extruder has been paid more and more attention due to its high mixing ability and output. However, as a new mixing setup for polymer processing, the complex modeling and computational simulation of the tri-screw extruder still remain very challenging, especially for the special dynamic center region with period changes of areas and geometric shapes. The mixing efficiency of the tri-screw extruder depends on its flow profiles, especially axial flow. The dynamic and period changes of areas and geometric shapes of the center region cause the local variations in the axial flow, which can affect the local and overall mixing efficiencies of the tri-screw extruder. With the recent advances in computational fluid dynamics,
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