SIMULATIVE STUDY ON IMMISCIBLE VISCOUS FLUIDS MIXING IN SCREW EXTRUDER

Abstract

Single and twin screw extruders are widely used machines for the mixing of viscous fluids. The idea behind this effort is to model the mixing of two immiscible viscous fluids in screw extruder by utilizing CFD software FLUENT available at the SCME. The subject work has wide range of applications in process industries such as polymer, food and pharmaceutical in which two immiscible fluids with one having high viscosity are mixed to achieve desired properties. The basic goal in mixing viscous immiscible fluids is to produce a mixture with a desired structure and morphology. The structure might consist of a polymer blend or the drop size distribution in an emulsion. Mixing in viscous fluids systems is a challenging task because no turbulent eddies are present to help distribute components. Because of the high matrix viscosity, diffusion coefficients for even very small molecules are exceedingly low. Mixing can be brought about in viscous systems only by mechanical action or by the forced shear or elongational flow of the matrix. Solids with a cohesive nature (such as agglomerated particles) or other immiscible fluids (drops with interfacial tension) will require intensive mechanical stress to achieve the required diminution. The first phase in this work is the creation of geometries of single and twin screw extruders in compatible software GAMBIT and meshing of the geometries to discretize the flow domain. In the second phase, export of both geometries in FLUENT is done for simulation by solving the fluid flow equations utilizing CFD codes. Two immiscible fluids i.e Ethylene Vinyl Acetate (EVA) and Ethylene Glycol (EG) are selected for mixing. EVA is the copolymer of ethylene and vinyl acetate. The weight percent of vinyl acetate usually varies from 10 to 40%, with the remainder being ethylene. It is a polymer that approaches elastomeric materials in softness and flexibility, yet can be processed like other thermoplastics. The material has good clarity and gloss, barrier properties, low-temperature toughness, stress-crack resistance, hot-melt adhesive water proof properties, and resistance to radiation. Ethylene glycol is mixed in ethylene vinyl acetate as compatabilizer. However viscosity ratio of continuous phase (EVA) to dispersed phase (EG) is 5.095e 03 making it difficult for mixing to be done by turbulence. To model the mixing in such system, problem is set up in FLUENT 6.2. The appropriate boundary conditions at the inlet and outlet are set. The screws are rotated at 200 rpm for both single and twin screw extruder. Multiphase mixture model is used for transient simulations. The unsteady state simulations are done because at each time step the screw meshes are moved to a new position, overlapping the flow mesh. For each node of this new domain that lies within a given screw, a special formulation is used that imposes a velocity that matches the rotation speed of the screw. After solution convergence, results of pressure, velocity, shear stress, volume fraction, shear rate, vorticity magnitude distributions developed in the flow domain are studied and mixing index is evaluated to observe the dispersive and distributive mixing. Simulation results indicate shear stress gradient from the tip of the screw flight near the wall to the root of the screw. The axial velocity gradient is also observed from the inlet to the outlet indicting elongational stretching. The combination of shear stress gradient and axial velocity gradient show high shear and elongational stretching necessary for the dispersive mixing. Twin screw extruder is a better choice because it generates more elongational stretching in the fluids with less shear stress.