Dissertations / Theses on the topic 'Fibre suspensions'

Godkänd; 2006; 20061115 (ysko)

This thesis deals with fluid dynamic mechanisms related to papermaking, specif- ically: the initial dewatering mechanisms during roll-forming and fibre motion in sedimentation and in shear flow.

Pressure and wire position measurements have been conducted in a model resembling the forming zone and the measured pressure distributions are shown to have more complex patterns than the simple model p / T/R (where T is the wire tension and R is the roll radius). It is shown that an increase in wire tension has a similar effect as a decrease in flow-rate on the shape of the pressure distribution. In addition, it is shown that the drainage has a stabilizing effect on the dewatering pressure.

The flow around the forming roll has also been modelled with the assump- tion that the wire is impermeable. A non-linear equation for the position of the wire is derived that clearly shows that the Weber number, We, is an im- portant parameter. The equation is linearized around the trivial solution and has a standing wave solution with a specific wavelength that scales with the We-number.

Motion of non-Brownian fibre settling in a Newtonian fluid at a small but finite Reynolds number has been studied experimentally. Two different regimes of sedimentation were identified. For dilute suspensions, fibres gener- ally fall without flipping and may travel at velocities larger than that of an isolated particle. In the semi-dilute regime we found the settling process to be dominated by large-scale fluctuations. The velocity fluctuations scale with the suspension volume concentrationφ according toφ1/3, which is similar to the findings for settling spheres.

The influence of shear on fibre orientation in the near wall region was studied in cellulose acetate fibre suspensions. At low concentration and low aspect ratio fibres were observed to orient perpendicular to the streamwise direction (named rollers) in the near wall region whereas the orientation further into the suspension was unchanged. As the concentration and aspect ratio increased the fraction of rollers decreased.

Finally, an evaluation of a commercial Ultra Velocity Profiler unit in fibre suspensions are presented. The idea was to determine the velocity and characterise the turbulence from ultra sound echoes from particles in the fluid. However, the spatial and/or temporal resolution of the measurements did not permit turbulence characterisation. These limitations might be possible to overcome and some procedures are proposed and evaluated.

QC 20141003

A flow loop system for study of fibre flocculation insuspensions has been developed. The system is designed tosimulate the flow conditions in a paper machine headbox. It isequipped with a radial distributor feeding a step diffuser pipepackage, after which the flow is contracted in a 2-D nozzle.The flow system is also equipped with a secondary flowcontraction with an area reduction ratio of 2:1 after theheadbox nozzle, mimicking accelerations that may take placeduring forming. The flow system is equipped with heating andcooling devices for the study of temperature effects on fibresuspension flocculation. An online dosage device for the studyof chemical effects on fibre suspension flocculation is alsoincluded. The maximum flow velocity in the system is 16m/s.

Flowing fibre suspensions were studied using a high speedCCD video camera and transmitted infra-red laser light pulseillumination. Images were taken either separately before andafter or along the secondary contraction. Images of fibreflocculation were evaluated by power spectrum analysis, and themean floc size and the flocculation index were calculated. Aconcept of mean floc area reduction, based on power spectrum,has been introduced to characterise the fibre network insuspensions. By comparingthe fibre flocculation before andafter the secondary contraction, or by following the fibreflocs along the secondary contraction, floc rheologyinformation can be obtained. The effects of chemical additivesand fibre surface modification can also be studied by comparingthe corresponding fibre flocculation.

For a bleached softwood kraft pulp suspension at a fibreconcentration of 5 g/l, the fibre flocs along the secondarycontraction have been manually evaluated, and the resultsconfirm that the power spectrum analysis is applicable. Ca 1/5of the flocs were broken into two by the contraction. The meanfloc size increases in MD while in CD it decreases during theflow contraction. Both the floc aspect ratio and the flocorientation in MD increase during the flow contraction. The netfibre floc area is decreased and the fibre flocs areconcentrated by the flow contraction, which is confirmed by anincrease of gray value of the flocs in the light transmissionimages. The dewatering of fibre flocs may thus have alreadybeen started in the suspension before reaching the wires in theforming zone.

Some physical influences on fibre suspension flocculationhave been investigated. The results confirm that fibreconcentration and fibre length are the dominating factorsaffecting fibre suspension flocculation. Increasing absoluteflow velocity has an insignificant effect on fibre flocs in theflow contraction. Suppressing turbulence, by increasingsuspension viscosity via a decrease of medium temperature,shows a clear effect on reducing fibre suspensionflocculation.

Some chemical influences on fibre suspension flocculationhave also been investigated. A retention aid, flocculant,cationic polyacrylamide, C-PAM, increases fibre suspensionflocculation by a bridging mechanism, and a formation aid,class II, anionic polyacrylamide, A-PAM, decreases fibresuspension flocculation by suppressing turbulence. Fibresuspension flocculation can also be reduced by surfacemodification with carboxymethyl cellulose, due to a reductionof the friction between fibres. The amount of fibre dispersiondepends on the ionic form of the grafted CMC, due to theelectrostatic repulsion between negatively charged groups onthe grafted CMC moieties. Xyloglucan, a non-ionic polymer,which is strongly adsorbed on cellulosic fibre surfaces, showsa similar influence on reduction of fibre suspensionflocculation by decreasing the friction between fibres.

The fibre flocculation data in the flow loop system werealso compared with the corresponding paper formation data inthe sheets produced on a pilot paper machine, both with andwithout chemical additives. The results show that the fibresuspension flocculation is well correlated with the paper sheetformation: when the fibre suspension flocculation is increased,the corresponding paper sheet formation deteriorates,especially in the large scale range.

A new model for simulating non-Brownian flexible fibres suspended in a Newtonian fluid has been developed. Special attention has been given to include realistic flow conditions found in the industrial papermaking process in the key features of the model; it is the intention of the author to employ the model in simulations of the forming section of the paper machine in future studies.

The model considers inert fibres of various shapes and finite stiffness, interacting with each other through normal, frictional and lubrication forces, and with the surrounding fluid medium through hydrodynamic forces. Fibre-fluid interactions in the non-creeping flow regime are taken into account, and the two-way coupling between the solids and the fluid phase is included by enforcing momentum conservation between phases. The incompressible three-dimensional Navier-Stokes equations are employed to model the motion of the fluid medium.

The validity of the model has been tested by comparing simulation results with experimental data from the literature. It was demonstrated that the model predicts the motion of isolated fibres in shear flow over a wide range of fibre flexibilities. It was also shown that the model predicts details of the orientation distribution of multiple straight, rigid fibres in a sheared suspension. Model predictions of the viscosity and first normal stress difference were in good agreement with experimental data found in the literature. Since the model is based solely on first-principles physics, quantitative predictions could be made without any parameter fitting.

En ny modell för simulering av rörelserna hos icke-Brownska böjliga fibrer dispergerade i en Newtonsk vätska har utvecklats. Eftersom det är författarens avsikt att modellen skall kunna tillämpas vid simulering av arkformning under de förhållanden som råder i en modern pappersmaskin, har särskilt omsorg givits till att inkludera motsvarande flödesvillkor i modellens giltighetsområde.

Modellen hanterar fibrer av varierande form, massa och styvhet, som växelverkar sinsemellan via normal-, friktions- och smörjkrafter. Deras växelverkan med den omgivande vätskan sker via hydrodynamiska krafter vid finita Reynolds-tal. Den så kallade tvåvägskopplingen mellan fibrerna och vätskefasen har tagits i beaktande genom att kräva att rörelsemängden bevaras vid interaktionen mellan faserna. Vidare har Navier-Stokes ekvationer för inkompressibla vätskor använts för att beskriva mediets rörelser.

Modellens giltighet har undersökts genom att jämföra resultat från simuleringar med experimentella data från litteraturen. Det har påvisats att modellen förutsäger rörelsen hos ensamma fibrer i ett skjuvflöde, för vitt skilda fiberflexibiliteter. Det visades också att modellen förutsäger detaljer hos fiberorienteringsdistributionen i suspensioner utsatta för skjuvflöde. Det kunde också konstateras att modellens förutsägelser av fibersuspensioners viskositet och första normalspänningsdifferens under skjuvning väl överensstämde med experimentella data i litteraturen. Kvantitativa förutsägelser har kunnat göras utan någon parameteranpassning, då modellen bygger uteslutande på väletablerade fysikaliska samband inom klassisk mekanik och strömningslära.

Turbulent mixing is present in many pulp and paper processes. It is a particularly important factor in the design and improvements of the paper machine headbox, influencing the final paper structure. During this project, experimental methods to quantify the effect of fibers on turbulent suspension flows have been developed, and then used for studying turbulent mixing in fiber suspensions. A technique that uses microprobes to measure passive scalar mixing of salt for the characterization of turbulent fluctuations in a fiber suspension flow has been developed: Conductivity micro-probes have been built and turbulence measurements have been performed in simple jet and wake flows, studying turbulent mixing between the two streams of pulp suspension, of which one has been doped with salt. A relatively new technique to measure fluid velocity non-intrusively in opaque fluids has also been tested. The technique makes use of ultrasonic pulses to obtain velocity information through the Doppler-shift of reflected pulses. The main efforts reported on in the thesis are focused on method design and development as well as method evaluation.

The fiber suspensions in the production line from wood topaper are subjected to many types of chemical and mechanicalprocesses, in which the flow of the suspension is of vitalimportance. The flow of the suspension determines the degree ofuniformity of the fibers through the processing, which inreturn affects the properties of the fiber suspension. In orderto optimise the process, thorough knowledge of the suspensionflow is necessary, both on the level of suspension, fibernetworks and individual fibers. Knowledge of the fibersuspension behaviour combined with commercial CFD simulationprovides an efficient design method for any unit operation inthe papermaking process.

This work concentrates on macroscopic modeling of thebehaviour of fiber suspensions from 0.5-5% dry content, purefiber suspensions without fillers or additives. Any mechanismscausing the characteristic behaviour of the pulp suspensionhave not been included, they are only included through theirinfluence on the suspension parameters. Excluded mechanismsare, for instance, the fiber-fiber coupling mechanisms that arethe reason for the formation of fiber networks and parts offiber network, flocs.

By combining a rheology model for the bulk suspension, awall function that accounts for the slip layer and finallyintroducing turbulence, a model has been created that is ableto simulate the flow of most fiber suspensions. The flow of thesuspension is not constrained to any particular flowconditions; the models discussed in this work aim at describingthe behaviour of the suspension for all flow rates and flowtypes. The models are developed under simple flow conditions,where all variables can be controlled, but the models areintended for usage within the industry-based flows in real pulpand papermaking applications.

Keywords:rheology, fiber, suspension, CFD, model, wall,slip, turbulence