Computational Fluid Dynamics investigation of a single screw extruder: A flow and heat transfer analysis

This work was realized in collaboration with the Continental Group with the aim to study the fluid dynamic behaviour of a Single Screw Extruder (SSE) used in their R&D laboratories. The work was carried out using a computational fluid dynamics (CFD) code named ANSYS Polyflow which adopts the Finite Element Method (FEM) for discretising the transport equations. Two kind of geometries were realized: the one of interest to the Continental Group, referred to as the Nominal Geometry and characterized by empty spaces interrupting the thread of the screw, and the Full Screw Geometry, similar to the previous one but with a continuous thread. This was made to evalutate the main differences between the two kind of SSEs. Both geometries were created using the CAD software ANSYS SpaceClaim and once generated, they were meshed using ANSYS ICEMCFD. In the CFD model, the material considered is a polymer treated as a non-Newtonian fluid. In particular, a power law was used to model the shear rate dependence of the viscosity. On the contrary the depedence of the viscosity on the temperature was neglected. Both isothermal and non isothermal simulations were conducted to resolve both the momentum transport equation and the energy transport equation. To perform steady state simulations and thus reduce the computational cost, the rotating reference frame technique was adopted. In fact, by rotating the barrel and keeping fixed the screw, the shape of the flow domain does not change in time and stationary simulations are possible, provided that an opportune transformation of the system of reference is performed. During the simulation the inertia of the fluid and the viscous heating were took into account and the hypotesis of incompressible flow was made. By using postprocessing tools on the results of the simulations it was possible to obtain the contour plots of the main physical fields. In particular from the isothermal simulations the contour plots of velocity, pressure and shear rate were extrapolated. Results showed a substantial backflow effect into the The Nominal Geometry: a small portion of the fluid inside the extruder travels in the opposite direction of the main motion, thus enhancing mixing. From the non isothermal simulations the contour plots of the temperature and the viscous heating were extrapolated. By calculating the average temperature on different cutting planes it was possible to evaluate the variation of the temperature along the extruder. In addition, by varying the flow rate entering the system, it was possible to derive the screw characteristic lines (pressure as a function of the flow rate) which can be coupled with the die characteristic to find the operating point of the extruder. This point is the cross – point between the two characteristic line and correspond to the flow rate and pressure value at which the system will operate. The comparison between the data obtained from the CFD simulations and the experimental data shows a good agreement both in the temperature profile and pressure increase.