To better understand the relationship between flow, mixing and reactions in the process of preparing PP/TiO2, a 3D numerical simulation in a co-rotating twin screw extruder (TSE) was firstly employed using commercial CFD code, ANSYS Polyflow. The effects of rotating speed of screws, stagger angle of knead blocks, inlet flow rate and initial temperature of barrel on the mixing and reaction process in the TSE were investigated. The results reveal that the studied operational and geometric parameters, which determine mixing efficiency, residence time distribution, and temperature of the flows in the TSE, affect the local species concentration, reaction time and reaction rate, and hence have great influences on the conversion rate. The results show that increasing the rotating speed and inlet flow rate can decrease the time for sufficient mixing, which is not conducive to intensive reaction, and increasing the stagger angle has the opposite effect. Moreover, the conversion rate greatly affected by the initial temperature of barrel.

A triangularly-arranged tri-screw extruder (TATSE) is a new piece of equipment for polymer processing, which has a dynamic center region. To learn about the special contributions of the dynamic center region and the circumfluence phenomenon just in cross section on the mixing mechanism of the TATSE, the 2D computational fluid dynamics (CFD) model was established to reduce the axial effect. The effects of screw clearances of the center region on mixing mechanism of the TATSE were studied. The mixing evaluations for the TATSE and twin-screw extruder (TSE) with different screw clearances were performed and compared. Moreover, the effects of different blend mass ratios of LLDPE and SBS on the mixing efficiency of the TATSE were studied. The results show that as screw clearances increase, the values of mean time-averaged and instantaneous efficiencies of the tri-screw extruder increase, but the normalized segregation scale decreases. The screw clearances even change the particle conditions from stretch to compression in the center region.

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The flow of polypropylene melt in new-style four-screw extruder was analyzed using the finite element method (FEM). The pressure and velocity fields in a flight element of co-rotating intermeshing four-screw extruders were calculated. A special study of flow in the central region was employed by analysis of the velocity, pressure and resident time distribution (RTD). The extrusion characteristics of the four-screw extruder were calculated and compared with a twin-screw extruder, `-' arrayed tri-screw extruder and triangle arrayed tri-screw extruder. The results show that the pressure and velocity gradient of the material in the four intermeshing regions is larger than that in the barrel channel. There is one circumfluence in central region which prolongs RTD in the central region and reduces the ability of pressure generation for the four-screw extruder. The material in the central region cannot stagnate. The mixing efficiency of the four-screw extruder is better than that of the twin-screw extruder and tri-screw extruder.

The four-screw extruder (FSE) is a novel equipment for polymer processing. In this paper, from a new viewpoint of Lagrangian coherent structures (LCS), two-dimensional fluid transport and chaotic mixing characteristics within three kinds of novel industrial FSEs are explored based on LCS to better understand the flow and mixing natures in the FSEs. Firstly, based on the finite-time invariant manifold theory, the finite-time Lyapunov exponent (FTLE) and LCS of FSEs are calculated by considering the different initial time. Hyperbolic LCSs from the FTLE maps are adopted to identify chaotic mixing manifolds in FSEs. Moreover, particle tracking and Poincaré sections are used to illustrate the different fluid motions in the above three isolated regions. Finally, the effects of relative rotating directions and layout of four screws on the chaotic manifolds in FESs are discussed in order to enhance local mixing performance. Furthermore, quantitative mixing measures, such as the segregation scale, logarithmic of stretching, and mean-time mixing efficiency are employed to compare the mixing efficiencies in three kinds of FSEs. The results show that the relative rotating directions and positions of four screws can change the chaotic manifolds and increase mixing performance in local poor mixing regions. FTLE and LCS analysis are helpful to better understand the chaotic mixing nature in the novel screw extruders.