CFD Based Modeling and Performance Analysis of Single/Twin Extruder

Abstract

This thesis work is done at the SCME, NUST under MS (Process System Engineering) program. The idea behind this effort is to design and model the Single and Twin-Screw Extruder and determining the local residence time distribution (RTD) by utilizing CFD simulation software FLUENT developed by Workbench Inc. 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 have very high viscosity are mixed to get mixture having desired properties. The basic goal in mixing viscous immiscible fluids is to produce a mixture with a desired structure. The structure might be the morphology of a polymer blend or the drop size distribution of an emulsion. The objective of this work is to model a viscous fluid mixing system using CFD codes. Workbench 21.2 (CFD software) available at SCME is utilized to carry out this work. The major stages involved in this thesis, are creation of geometry of single and twin-screw extruders developed using Design Modeler (component system in workbench) and meshing of the geometry to discretize flow domain. The geometry is exported to FLUENT to simulate the system by solving the CFD equations. Two immiscible fluids i.e., Ethylene Vinyl Acetate (EVA) and Ethylene Glycol (EG) are selected for mixing. Ethylene vinyl acetate (also known as EVA) is the copolymer of ethylene and vinyl acetate. The weight percent 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, lowtemperature toughness, stress-crack resistance, hot-melt adhesive water proof properties, and resistance to UV radiation. Ethylene glycol is mixed in ethylene vinyl acetate as compatibilizer in cable insulation manufacturing. In this study EVA 28420 is selected because of comparatively low viscosity making the problem simple. The problem is set up in CFD software, FLUENT. Multiphase mixture model is used for transient simulations. The unsteady state of simulation is used because at each time step the meshes move to a new position, coinciding the flow mesh. The fitting boundary conditions of velocity inlet are set for the inlet and by default out-flow is set at the outlet. The screws are rotated 200 rpm for both conveying screws. iv As a results total pressure contours, velocity magnitude contours, shear stress contours, volume fractions contours and vorticity magnitude developed in flow domain and mixing index is evaluated. At the end RTD analysis was carried out for both the screws using a tracer injected through inlet into the flow domain and calculating the molar concentration at the outlet.