Modeling the effect of fiber orientation on local yield stress in flow of pulp suspensions

A model that takes into account the local yield stress in pulp suspensions (2-3%) was derived. In this model the yield stress was assumed to decrease due to a reduction in the number of contact points between fibers in a network when the fibers are oriented in the shear field. The local yield stress described by the number of contact points multiplied by the friction force in the fiber connections was implemented in a CFD solver to describe the yield stress in the non-Newtonian single-phase Bingham model. Model results were compared with experimental UVP data from a study performed earlier on the flow of pulp suspensions at two concentrations after a sudden expansion. First model parameters were estimated with experimental data from a case with a concentration of 1.8% and an inflow velocity of 1.2 m/s. Then the model was used to predict five other cases. The proposed model was able to capture both the decrease in centerline velocity and the increase in the width of the jets at higher inflow velocities. At low inflow velocities the predicted jet lengths were too short compared to the experimental measured jet length due to under prediction of fiber alignment.