Simulation of High-Viscosity Generalized Newtonian Fluid Flows in the Mixing Section of a Screw Extruder Using the Lattice Boltzmann Model

The mixing quality of polymer melts in the mixing section of a single-screw extruder and an injection molding machine has considerable effects on the properties of the molded products. Therefore, the study of the flow field of polymer melts in the mixing section is of great importance. The lattice Boltzmann method (LBM) exhibits unique advantages in simulating non-Newtonian fluids. Many researchers have used LBM to study the flow of medium- and low-viscosity fluids. In their studies, the Reynolds number of fluid flows is generally moderate. However, polymer melts are typical high-viscosity fluids, and their flow Reynolds number is generally very small. The single-relaxation-time lattice Boltzmann method (SRT-LBM) has been used previously to study the flow field of power law fluids in the mixing section. Herein, the flow field of high-viscosity generalized Newtonian fluids in the mixing section of a single-screw extruder is studied using SRT-LBM, the two-relaxation-time lattice Boltzmann method (TRT-LBM), and the multiple-relaxation-time lattice Boltzmann method (MRT-LBM). Through comparison, TRT-LBM has been found to exhibit obvious advantages regarding stability, calculation accuracy, calculation efficiency, and selection of simulation parameters. The TRT-LBM is more suitable for studying high-viscosity generalized Newtonian fluids than SRT-LBM and MRT-LBM. SRT-LBM has low computational efficiency when simulating high-viscosity generalized Newtonian fluids, and instability is easily caused when the fluid has a yield stress. For MRT-LBM, only by studying the relaxation parameters can its advantages be fully utilized. However, optimizing the accuracy and stability of the MRT-LBM via parameter research and linear stability analysis is difficult. For non-Newtonian fluids, it is difficult to optimize the relaxation parameters to make the MRT-LBM more stable and accurate than the TRT-LBM. It is difficult for the MRT-LBM to realize its potential when simulating high-viscosity generalized Newtonian fluids. In addition, we studied the flow pattern in the cross section of the screw channel and compared it to the results reported in previous studies.

Investigations of the Elongational Deformation Induced by Pins in Pin-Barrel Cold-Feed Extruders

This paper is a study on the elongational deformation induced by pins in pin-barrel cold-feed extruders. The mathematical model is established by using simple flow field, plate model, and Newton constitutive relation. It is proved that there is elongational deformation when the fluid bypasses the pin, and the elongational deformation is quantitatively calculated. The finite element method is used to simulate the flow field of the screw mixing section of the pin-barrel cold-feed extruder with different specifications. The velocity vector nephogram in the simulation results is analyzed by numerical analysis of elongational deformation and compared with the theoretical elongational deformation value. The results show that the simulated and theoretical values of elongational deformation are approximately consistent. The numerical analysis of the key factors affecting the elongational deformation shows that the elongational deformation will gradually increase with the increase of screw diameter. For the screw of the same specification, the increasing helical angle will reduce the elongational deformation, and the increasing rotational speed will linearly increase the elongational deformation in unit time. This study can be used to roughly estimate the elongational deformation induced by pins and establish the mixing theoretical model of the pin-barrel cold-feed extruder.