CFD simulation of the turbulent flow of pulp fibre suspensions

Abstract

Pulp fibre suspensions are complex solid-liquid systems, since their components (fibre, flocs, additives…) present singular and complex interactions between them. As a consequence of this complexity, the understanding of the suspensions flow dynamics remains poor and incomplete, what usually leads to a conservative industrial equipment design and, hence, equipment oversizing. The purpose of this study is to obtain further knowledge about the dynamic mechanisms of the turbulent flow regime of industrial pulp suspensions, by producing and testing the viability of a model for the turbulent flow of fibre suspensions in conveying pipes, based on fundamental principles of conservation, and also on some experimental information (namely the rheological information) to adjust, empirically, unknown parameters as the fluid viscosity. With this purpose a numerical model was developed, based on a CFD code, using the Chemical Engineering module of COMSOL Multiphysics Software version 3.4. The K −ε Turbulence Model was chosen to simulate the pulp suspensions flow. Additionally, the final model was tested and validated using four different industrial pulp suspensions, which were previously fully studied experimentally (flow and rheological tests). The obtained results demonstrated that the pressure drop profiles obtained using COMSOL Multiphysics Software, for the turbulent flow regime, agree very well with the experimental results obtained in a pilot rig. Additionally, the use of the k −ε Turbulence Model for the simulation of pulp fibre suspensions flow, associated with the rheological data acquired experimentally, revealed to be a prompt and accurate strategy to attain good prediction of pressure drop values for fibre suspensions flow. Moreover, the adjustment of the turbulence parameters confirmed previous studies’ results, where it was concluded that the existence of particles, such as fibres, in a fluid flow, induces a turbulence damping.