An experimental study of the flow of concentrated pulp suspension through a sudden expansion and over a backward facing step

Large amounts of pulp suspensions are handled in the pulp and paper industry annually. In order to produce the final product, paper, a lot of energy is required, e.g. for pumping and drying the suspensions. The lack of knowledge of the flow and fluid properties of pulp suspensions, however, aggravates the optimization of the design and the operation of process equipment. Pulp suspensions are two-phase solid liquid mixtures that consist of fibers and water. Already at a consistency of 1%, coherent networks are formed that affect the flow structure. Valid rheology models that capture the two-phase effects have not been developed yet due to the difficulties in describing pulp suspension in all flow regimes. The aim of this study was to make progress with the understanding of the nature of the flow and fiber interaction by experimentally studying concentrated pulp suspensions flowing through a sudden expansion as well as over a backward facing step. These two simple flow geometries were chosen because of their potential to generate complex flow structures, including several flow regimes as well as recirculation flow, separating flows and free mixing layers. The flow structures of the suspensions that flow through a sudden expansion were studied with Ultrasonic Velocity Profiling technique (UVP) and the flow over a backward facing step was studied with Laser Doppler Anemometry (LDA). Both measurement techniques were successfully used in the study of pulp suspensions at consistencies up to 3% at various flow rates. From the results of the flow through a sudden expansion it was concluded that the dimensions of the formed jet were influenced by the velocity and the consistency of the pulp suspensions. This proves that a similarity solution could not describe the suspensions at the velocities studied. Conclusions from the flow over a backward facing step show that greater consistency or a drop in velocity decrease the reattachment length in all suspensions except those with a consistency lower than 1.5%. The reason for the change in reattachment length seems to be related to the flow regime valid in the separation boundary layer. The results from both studies indicate that pulp suspensions are best described by rheology models that include a yield stress, but the available models cannot capture the flow structure over the whole flow range investigated. Therefore new models, probably including two-phase effects, need to be derived