Network Disruption and Turbulence in Fibre Suspensions

Doctoral thesis, 1998

The production of paper pulp involves the handling of large quantities of fibre suspensions. Today's call for energy efficiency and environmentally sound processes has led to the development of MC (Medium Consistency) technology. Most unit operations in the pulp industry are now routinely performed at concentrations higher than 10 %, at which fibre suspensions behave as solids at low shear rates. If the shear rate is sufficiently high, however, the behaviour of MC suspensions resembles that of pure water. This thesis deals with the network strength (or yield stress) of fibre suspensions, which is a suspension property that may be related to the flow transitions of fibre suspensions. The main cause of network strength is the frictional force in the contact points between single fibres. A novel measurement method was developed to measure the inter-fibre friction of single pulp fibres. The coefficient of friction was found to be around 0.6 for dry kraft fibres and range between 0.6-0.8 for wet kraft fibres. A mechanistic model has been developed for predicting the network strength of homogeneous suspensions. It gives fair agreement with experimental yield stress data for homogeneous suspensions; the experimental values for flocculated suspensions were lower. The turbulent flow of pulp fibre suspensions at Medium Consistency has been subject to much speculation due to the absence of direct measurements. A rotary shear-tester, resembling an industrial mixer, was designed and constructed to permit direct velocity measurements using Laser Doppler Anemometry (LDA). The fibre concentrations were up to 20 wt % using a refractive index matched glass fibre suspension. Both mean and fluctuating velocities were found to be close to those of single-phase flow at flows above a critical rotational speed. At lower rotational speeds, the turbulence was strongly suppressed, whereas the mean velocities were close to those of fully developed turbulent flow. The single-phase flow was compared to single-phase CFD (Computational Fluid Dynamics) calculations using the sliding-grid technique. The flow field was predicted well by using the standard k-epsilon turbulence model, but the absolute values were 25 % too low. Boundary-layer measurements, including the viscous sub-layer, were performed as well. Linear velocity profiles were obtained close to the wall for both the single-phase and the tested suspensions. Further from the wall, flatter velocity profiles were obtained in the fibre suspensions than for single-phase flow.