Comparison of mixing mechanism under different rotating directions in a horizontal self-cleaning differential rotating twin-shaft kneader

•Comparison of flow pattern in opposite-rotating, co-rotating and back-rotating differential twin-shaft kneader was performed.•Mixing performance was evaluated by particle tracking technique.•The effect of rotating directions on mixing processes between halves of kneader and a cluster was investigated.•The material exchange between left and right part in co-rotating kneader is the most efficient.•The co-rotating kneader possesses much better mixing performance than the opposite-rotating and back-rotating kneaders. Finite element method (FEM) simulation combined with the mesh superposition technique (MST) was adopted to investigate the flow field in a horizontal self-cleaning differential rotating twin-shaft kneader with a highly viscous Newtonian fluid. The distributive mixing process and mixing efficiency were investigated by using the particle tracking technique. The comparison of flow pattern, distributive mixing process and mixing efficiency for the opposite-rotating, co-rotating and back-rotating twin-shaft kneader was performed. For the opposite-rotating kneader, the material flow in the left and right region merges in the upper region of overlapping zone, moves towards the lower region and is split into two parts in the lower region of overlapping zone. For the co-rotating kneader, the main flow pattern that the material moves towards the right region from the left region. Besides, there exists the secondary flow and periodic cyclic vortex in the overlapping zone. For the back-rotating kneader, the material flow in the left and right region merges in the lower region of overlapping zone, moves towards the upper region and is split into two parts in the upper region of overlapping zone. The material exchange between the left and right part of flow domain in the co-rotating kneader is much more efficient than that in the opposite-rotating and back-rotating kneader. The mean length of stretch, instantaneous mixing efficiency and time averaged mixing efficiency of opposite-rotating kneader is very close to that of back-rotating kneader, which is much smaller than that of co-rotating kneader. Hence, the co-rotating kneader possesses much better mixing performance than the opposite-rotating and back-rotating kneaders.