Finite Element Simulation of Flow in Twin Screw Extruder Mixing Elements

Abstract

<p>In the plastics industry, twin screw extruders are widely used for melting, dispersing and homogenizing polymers. There are a diversity of designs employed throughout the polymer industry, each one having different operating principles and applications. Among the different arrangements of twin screw systems, the intermeshing co-rotating configuration has been found to be one of the most efficient mixers and it is one of the most commonly used pieces of equipment among the continuous mixers due to its self wiping properties. The problem of mixing of polymers involves aspects of fluid dynamics and rheology. Mixing is usually obtained through a combination of mechanical motion of the mixing device and the resulting deformation induced in the flowing material. The quantitative description of the flow patterns is now feasible even in the most complicated geometries through the development of computational fluid dynamics (CFD) tools and the continuous increase in computer resources at lower costs. Intermeshing co-rotating twin screw extruders (ICRTSE) are usually built in a modular fashion to meet the diversity of tasks performed by this type of machine. There are two main types of elements; full flight conveying elements and kneading block mixing elements. The kneading blocks have been the focus of attention for the theoretical analysis of flow due to their significant contribution to the mixing performance of the extruder and the fact that kneading blocks normally work under a fully filled channel condition, which is one of the fundamental assumptions in CFD simulations. The objective of this thesis is to understand the flow mechanisms in the kneading disc section of co-rotating twin screw extruders. This is done by means of the 3D numerical simulation of the flow process within the complex geometry involving intricate passages and continuously moving surfaces. A quasi-steady state finite element model was developed assuming isothermal, non-Newtonian flow. The intricate geometry of the kneading disc section required the development of a tailored finite element mesh generator. An analysis based on particle trajectories, calculated from the obtained velocity field, was carried out to study the effect of geometry on the mixing performance. The approach used for the initial location of the particle tracers was to cover the entire cross section of the kneading blocks. The problem of particles leaving the flow field due to time discretization was addressed by determining the locations where particles crossed the solid surfaces of the discs and reincorporating them into the flow field. The calculation of particle trajectories and deformation history give valuable information about the rapid fluctuations in shear stress experienced by different particles within the flow field. A rigorous examination of the model results was carried out. Comparisons of the 3D model against experimental pressure data in the radial and axial directions are presented. The simulation results were also compared against experimental results of velocity obtained via particle image velocimetry. Results confirmed the ability of the model to predict the flow behaviour. It was determined that inlet and outlet boundary conditions play a significant role in the development of flow patterns in the kneading disc section. The assumption of isothermal flow introduced limitations in the predictions made by the model. Future work should include the addition of the energy equation to the model.</p>