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Slip at the wall is often encountered during the flow of polymeric materials and during the mixing of multiphase systems, such as concentrated suspensions. This article is concerned with the development of a numerical method to simulate flows that show such slip. The flow of a generalized Newtonian fluid with Bingham-like viscosity curve is considered and a linear relation between the slip velocity and the shear stress at the wall is used. A novel method based on a penalty parameter is developed to implement the slip boundary conditions into the finite element method. The flow in the conveying elements of the extruder is used as an example of the application of this method. It is shown that the wall slip decreases the pressurization capability of the extruder and increases the leakage flow occurring over the flight tips, as well as its mixing efficiency by reducing the rate of recirculation and the stress levels. The technique developed in this article can be applied to many flows that show slip phenomena and may be used to gain better understanding of the dynamics of mixing of commercially important materials.

Numerical Study of Slip at the Walls in the Extruder