Experimental and numerical studies of the flow of pulp suspensions

Doctoral thesis, 2014

Large amounts of pulp suspensions are handled in the pulp and paper industry daily. In order to produce the final product, paper, a lot of energy is required, e.g. for pumping, mixing and screening the suspensions. A lack of knowledge about the flow and fluid properties of pulp suspensions, however, aggravates the optimization of the design and the operation of process equipment. The aim of this research is to advance the understanding of the nature of fiber flow and fiber interaction by experimentally studying pulp suspensions at a consistency of 0.5 to 3%. These pulp suspensions were studied in flows through a sudden expansion, over a backward-facing step and along a flat plate. These simple flow geometries were chosen because of their potential to generate complex flow structures, including several flow regimes. Flow structures were studied with Ultrasonic Velocity Profiling technique (UVP) and Laser Doppler Anemometry (LDA). The experimental data was later used to develop a model that describes the local yield stress of the pulp suspension. The results show that jet dimensions are influenced by the consistency and velocity of the pulp suspension. In simulations, it was found that the size of the yield stress, i.e. fiber network strength, is the main parameter that affects the jet length. From these findings, a model that describes the local yield stress of pulp suspensions at a consistency higher than 1% was derived and connected to a single-phase rheology model. Validation of the model shows that it captures the trends in the experimental data. In studies of pulp suspensions after a backward-facing step, it was found that the reattachment length decreased with an increase in suspension consistency or a decrease in flow rate, except at the lowest consistency. It was also found that the reattachment length may be affected by the fluid regime at the separation point at the end of the step. In the study of the development of the near-wall boundary layer, width was found to increase with a decrease in consistency or an increase in velocity. An additional finding was that a constant boundary layer thickness was reached after 20 mm, regardless of the consistency or the velocity of the pulp suspension in the flow pipe.