Numerical Simulation of Wire Coating Pseudoplastic and Viscoplastic Fluids

Abstract

Numerical simulation of the wire-coating process is undertaken for non-Newtonian pseudoplastic and viscoplastic fluids. The Herschel-Bulkley model of viscoplasticity is used, which reduces with appropriate modifications to the Bingham, power-law and Newtonian models. The analysis is based both on the Lubrication Approximation Theory (LAT), which regards locally a fully developed shear flow, and on a two-dimensional axisymmetric Finite Element Method (FEM). For a given die design the results give distributions of important variables, such as pressure, shear stresses along the die walls and the wire, and the wire tension due to the shearing forces of the fluids on the moving wire. These results are obtained from a full parametric study of the dimensionless power-law index (in the case of pseudoplasticity) and the dimensionless yield stress or Bingham number (in the case of viscoplasticity). Increasing the power-law index or the Bingham number leads to an increase in dimensionless pressure and stresses. In the case of viscoplastic fluids, LAT predicts interesting yielded/unyielded zones, which are however erroneous, as a consequence of using the lubrication approximation. The full 2-D analysis based on FEM shows that such zones exist only after the die exit, where the coating fluid moves on the wire as a rigid body.