Abstract
Computational fluid dynamics (CFD) simulation is an important tool as it enables engineers to study different design options without a time-consuming experimental workload. However, the prediction accuracy of any CFD simulation depends upon the set boundary conditions and upon the applied rheological constitutive equation. In the present study the viscoelastic nature of an unfilled gum acrylonitrile butadiene rubber (NBR) is considered by applying the integral and time-dependent Kaye–Bernstein–Kearsley–Zapas (K-BKZ) rheological model. First, exhaustive testing is carried out in the linear viscoelastic (LVE) and non-LVE deformation range including small amplitude oscillatory shear (SAOS) as well as high pressure capillary rheometer (HPCR) tests. Next, three abrupt capillary dies and one tapered orifice die are modeled in Ansys POLYFLOW. The pressure prediction accuracy of the K-BKZ/Wagner model was found to be excellent and insensitive to the applied normal force in SAOS testing as well as to the relation of first and second normal stress differences, provided that damping parameters are fitted to steady-state rheological data. Moreover, the crucial importance of viscoelastic modeling is proven for rubber materials, as two generalized Newtonian fluid (GNF) flow models severely underestimate measured pressure data, especially in contraction flow-dominated geometries.
Highlights
In order to solve macroscale flow problems like mold filling or extrusion, use is made of continuum mechanics by omitting microscopic discontinuities of the investigated fluid
Discretizing the area of interest by an appropriate mesh, conservation equations of mass, momentum, and energy, are applied, leading to a set of partial differential equations, which are typically solved by means of computational fluid dynamics (CFD) simulation
The prediction accuracy depends upon the set boundary conditions and upon the applied rheological constitutive equation
Summary
In order to solve macroscale flow problems like mold filling or extrusion, use is made of continuum mechanics by omitting microscopic discontinuities of the investigated fluid. Instead, generalized Newtonian fluid (GNF) flow models are used to describe the relation between the stress and rate of deformation tensors. These models fail to reflect important rheological properties (e.g., normal stress differences or transient data). They intrinsically assume a ratio of three (Trouton ratio) between steady-state shear and uniaxial elongational viscosities. As a result, they fail to predict extrudate (die) swells or inlet vortices and massively underestimate pressure drops in contraction flow areas [1,2]. Developing and improving viscoelastic constitutive equations is an eminent subject in polymer rheology research [3]
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