Predicting particle orientation: Is an accurate flow field more important than the actual orientation model?

Analytical orientation prediction models (OPMs) are widely applied to efficiently predict the orientation of particles in suspensions or pastes. One application of these prediction models are extrusion processes where the model is used to identify combination of process and paste parameters that yield anisotropic orientation within the extruded filament. The accuracy of these predictions mainly depends on two elements that are the correct choice of the orientation model itself and the rheology model. The rheology of a paste is commonly modeled with the help of the Herschel Bulkley (HB) rheology model. The optimal OPM is still topic of today’s research. Which of the two elements, however, has a stronger influence on the orientation has not been addressed, yet.
We make use of a wide range of HB parameters reported in literature with a particular focus on data that is reported for varying paste compositions. Based on this data, we use the HB model to predict the velocity and shear rate profile within a pipe and perform orientation predictions with the help of the models by Jeffery and Folgar and Tucker (FT). These predictions are used to estimate the sensitivity of the final orientation on the HB parameters and on the OPM.
For our study, we assume that the paste can be described by the standard Herschel Bulkley rheology model. We assume no-slip boundary conditions and no influence of the particle orientation on the rheology. Second, we assume that the initial particle orientation before entering the pipe is random.
Our results suggest that capturing the rheology model correctly is generally more important than capturing the OPM correctly. This applies in particular when densely filled states are approached. We also provide universal relations for predicting the degree of particle alignment and the mean flow velocity as functions of process parameters and material properties.