Mixing of highly viscous fluids in a cavity with internal moving parts is a common scenario found in many engineering applications. It provides a challenge for numerical simulations. In this paper, asymmetric rotors were designed to enhance mixing, and the effect of different centerline distances on mixing was investigated numerically. The novel rotors co-rotate at a speed ratio of 2 and hence have different geometries to meet the requirement of self-cleaning. The finite element method was used to solve the time-dependent flow, in which the mesh superposition technique was used to include the internal moving parts in the fixed meshes of the flow domain. A non-Newtonian fluid obeying the Carreau–Yasuda constitutive model was used. A standard fourth-order Runge–Kutta scheme was successfully developed to perform the particle tracking calculations. Distributive mixing was examined through the flow patterns and spatial positions of the tracked particles. The centerline distance was the key factor for controlling the gap between the rotors that influence mixing and energy consumption. Different mixing subzones alternated in sequence. On the one hand, this gap introduced a bifurcation in the intermeshing zone. On the other hand, stretching, folding, and reorientations, as well as cutting and dividing actions, were encountered in the sequence. This procedure was similar to a Baker’s transformation. By contrast, for a Newtonian fluid, mixing became worse and consumed slightly more energy.

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.