Mechanism Study Of Chaotic Mixing In Eccentric Twin-screws Extruder

Twin-screws extruder is common extrusion equipment.The screw element of the ordinary twin-screws extruder can only cause the fluid interface to increase linearly,and it is difficult to produce the chaotic effect of the fluid.The chaotic effect is an important means to enhance the laminar mixing efficiency of high viscosity fluid.In recent years,eccentric twin-screws extruder based on chaos effect has appeared.Compared with the traditional twin-screws extruder,the flow field structure of the eccentric twin-screws extruder is asymmetric,and the fluid movement form is nonlinear.When working,the two eccentric screw elements are tightly kneaded and synchronously rotated,the radial gap between the two is small,which has the advantages of good exhaust,better self-cleaning,uniform shear rate distribution and high conveying efficiency.In this paper,based on chaos dynamics theory and Lagrangian method,the chaotic mixing mechanism of the eccentric twin-screws extruder is analyzed by the finite element method.Adopt eccentric kneading-disk as power element.With the help of FTLE(finite-time Lyapunov exponent),LCSs(Lagrangian coherent structure),Poincaré cross-section and particle tracking,etc.Three eccentric twin-screws extruders with eccentricities of 4mm,2mm and 0mm were studies,the chaotic mixing mechanism of the two-dimensional end face in the flow field was analyzed under the conditions of co-rotation and counter-rotation of the two screw elements.The positions of hyperbolic fixed points and the local of non-chaotic mixing area were determined.The effect of eccentricity on the mixing in the flow field is discussed in detail.Particle tracking and LE(Lyapunov exponent)are used to analyze the local mixing characteristics of specific positions in the flow field.Through instantaneous mixing efficiency,time average mixing efficiency and logarithmic stretching curve,the mixing ability of different eccentricity models were compared and analyzed.Combining the velocity vector field,mixing index and rate of shear cloud map of different cross-sectional position,the mixing characteristics of the flow field in three-dimensional state are analyzed,and the trajectories distribution and chaotic characteristics of individual particles are analyzes.The results show that for the traversal range of fluid particles in the eccentric twin-screws extruders are large,the Lagrangian coherent structures are relatively complete,forming the "lobes" structure,there are hyperbolic fixed points in the flow field.The lobes structures strengthen the material exchange in strong and weak mixing zone,the hyperbolic fixed points promote the material exchange in the nearby particles cluster,and the fluid particles will experience relatively good stretching and compression.The chaotic area in the flow field is large,and the fluid particles move randomly,which is conducive to distributed mixing and dispersed mixing.In the three-dimensional state,the shape of the particle trajectory is complicated,the velocity vector field is densely distributed,and the mixing index and shear rate are relatively large.And the larger the eccentricity is,the better the mixing effect is,and the greater the degree of fluid particles undergoing stretching and folding.However,in the non-eccentric(e=0mm)twin-screws extruder,the flow field is symmetrically distributed,the movement state of fluid particles is ordered,most of particles belongs to linear motion,and the flow field is not in a chaos state.