Shear-induced particle migration in a short-dwell coater

Abstract

This paper aims at simulating the flow of a pigment suspension in a short-dwell coater (SDTA), a common device used for the production of value-added paper. The process consists of depositing onto a paper web a thin coating layer, the uniformity and the quality of which depend on the short-dwell coater geometry and the coating color rheology. In order to understand the possible interactions between the hydrodynamics in the pond and the rheological behavior of the suspension, this study focuses on a phenomenon known as shear-induced particle migration, which may result in an non-homogeneous suspension. For this purpose, a numerical methodology based on a shear-induced particle migration model (Sim) and the finite element method is presented and validated against analytical results and experimental data. This numerical approach constitutes the first step towards a better understanding of the hydrodynamic mechanisms that govern particle migration in a short-dwell paper coater. In this work, a parametric study of the suspension flow behavior is carried out via the introduction of a new dimensionless number that characterizes the importance of the shear-induced migration with respect to particle convection. The proposed model is used to generate particle distribution templates. It is shown that the particle size and Reynolds number are preponderant factors that affect the extent at which migration occurs in the short-dwell coater. In particular, it is shown that increasing the Reynolds number amplifies the migration phenomena and the formation of high concentration zones within a ring located in a vortex just before the nip. Such a concentration overshoot may then be dragged into the nip, resulting in runnability problems such as web breaks or coating color spitting.