Numerical study of mixing dynamics inside the novel elements of a corotating nontwin screw extruder

A novel, self-cleaning, corotating nontwin screw configuration is proposed, and its geometry is briefly introduced in this study. In this new configuration, one single-tip screw element intermeshes with another two-tip screw element at a speed ratio of 2. A numerical simulation of the three-dimensional flow field in this new kind of conveying system, as well as in the conventional twin screw configuration, was carried out using the finite element method (FEM) and the mesh superposition technique (MST) provided by the commercial CFD code POLYFLOW. The purely viscous fluid is assumed to obey the Bird–Carreau constitutive model. Based on the predicted flow field in the domain FEM mesh, post-treatment codes have been developed to predict the mixing details where standard fourth-order Runge–Kutta schemes were used. Flow patterns and mixing dynamics were presented in terms of the mixing evolution of the particle group released from different initial positions and the variation of the variance index, mixing index, strain tensor magnitude, and their time histories. Trajectories of the particle groups were captured successfully, and an obvious difference with regard to flow pattern and mixing was found for different types of screw geometries. The numerical results showed that mixing was highly dependent on the initial position of the particles and that the nontwin screw configuration had better distributive and dispersive mixing abilities than the conventional twin screw configuration.