2005 Volume 1
Cambridgepp 19-34The effect of fibres on laminar-turbulent transition and scales in turbulent decayAbstractPDF
Two physical phenomena which determine fundamental possibilities of paper forming are studied. The two phenomena are (i) laminar/turbulent transition and (ii) decay of turbulence. At first, the relevance of the processes to paper making is reviewed and discussed. The state of the boundary layers (laminar or turbulent) on split vanes and the decay of turbulence in the free stream are found to be of uttermost importance for the control of layer purity, formation and other properties of the final paper. Experiments in which these two processes are studied by visualisations are presented. The experiments emphasize the impact of fibres on these processes, as compared to what is found with pure water. All experiments are performed in model experiments were the structures in the flow are visualised by the addition of small, flake-like particles. It is shown that the addition of fibres radically change the physics of the flow. In a water table experiment, the addition of fibres is seen to promote the production of turbulent spots. At high enough fibre concentrations, the flow of water and fibres is fully turbulent even if a flow of pure water is laminar. In decay of turbulence, the fibres are seen to radically change the energy transfer between different scales so that intermediate and small scales remain active for longer times. It is concluded that fibres have large effects on laminar-turbulent transition and turbulence
decay and that improved knowledge of these effects are a corner stone in the understanding of head box flow and its relations to the resulting paper quality.
The dynamics of fibre suspension flow, especially breakup and reformation of fibre flocs in a closed channel flow past a forward facing step was studied experimentally using fast CCD camera imaging and image analysis techniques that allow for simultaneous measurement of floc size and turbulent flow field of fibres. The recirculation eddy downstream of the expansion step was found to exists only if the step height exceeds mean fibre length. When existing, the behavior of the eddy is similar to that of Newtonian fluid flows. Experimental correlations between floc size and turbulent flow quantities were found indicating that the ratio of the minimum floc size found at the end of the recirculation region and the floc size upstream of the step is strongly correlated with the size of the largest scales of the turbulent field and less directly with the total turbulent intensity of the flow immediately after the step. In addition, an approximate power law scaling behavior of the floc size with the turbulent intensity was found within the decaying turbulence region downstream of the recirculation eddy.
Cambridgepp 51-81Fluid mechanics of the headbox nozzle: Coherent flow structures, instabilities and disturbances close to the nozzle exitAbstractPDF
In this study two sources of flow instabilities in the headbox nozzle are presented. Both of them appear close to the nozzle exit and therefore are easily conveyed to the slice jet. This makes their control critical in the view of the jet quality. These instabilities may lead to so-called “small scale faults” in sheet quality, which appear as cockle, fiber orientation streakiness and other small-scale dimensional stability problems. The first source of instability is the nozzle exit itself. The geometry of the nozzle exit is highly asymmetrical due to the slice bar in the upper lip. This results in sudden acceleration and streamline curvature. The results show that remarkable alterations in the structure of the turbulent boundary layer on the lower lip are observed due to the acceleration generated by a slice bar model. However, compared to the boundary layer turbulence, the flow structures evolving in the slice bar shear layer are an order of magnitude stronger. In essence, the slice bar model utilized in the present experiment creates strong streamwise vortices. The other instability is related to the tip of the vane. Recent experimental work has revealed the reason for stable MD-aligned streaks in the mean-flow field created by some vanes at certain speeds. The streaks results from a fluid-structure interaction, in which the flow excites the vane to respond in a characteristic vibration mode. This paper presents the effect of
flow rate, vane tip thickness and vane material on this kind of streaking problem. Also the fundamental nature of the fluid-structure interaction, responsible for enhanced vortex shedding which is the mechanism the streaks are generated, is explained.
Cambridgepp 83-105Reduction of layer mixing in stratified forming through hydrodynamic controlAbstractPDF
STFI-Packforsk has recently patented the “Aq-vanes”, a new technology for stratified forming headboxes. In this new solution, a thin passive liquid layer (a liquid vane or “Aq-vane”) is injected
in the headbox between neighbouring pulp streams through a narrow hollow channel, thereby preventing mixing between the layers.
One of the most interesting features of the Aq-vane technology is that layer purity and separation can be controlled externally by tuning a set of process parameters. This opens the field for a widespread industrial application of stratified forming, a paper maker’s quantum leap that may reduce energy and raw material consumption, lead to improved product properties and possibly even to the development of new grades. In fact, although the basic concept of producing an engineered layered structure is not new in papermaking; its application has been extremely limited to a few selected grades, such as high grammage paperboard or multi-layer tissue.
The Aq-vanes have been implemented on EuroFEX, STFI-Packforsk’s research paper machine, for extensive pilot scale trials. Thereby a number of technical solutions for the injection of the liquid layer has been tested and evaluated. In a parallel project, a method for measurement of layer purity in stratified forming has been developed. It is based on sheet splitting using a heat-seal pouch lamination technique. An image analysis method is then used to identify the colour of the fibres and thus the layer
Cambridgepp 109-128Modelling of mechanical dewatering in contact with rough permeable surfacesAbstractPDF
A model of the mechanical dewatering process during wet pressing was formulated that includes a non-uniform compaction at the interface between web and felt. This non-uniform compaction is assumed to be caused by the surface roughness of the felt. The model of the simultaneous deformation and flow in the fibre network was implemented in ABAQUS, a general finite element system. The dewatering behaviour of different rough permeable surfaces, representing model-felt surfaces, was investigated. Results obtained with the model showed the influence of the felt model surface on dewatering, especially at lower grammages. Here, the non-uniform compaction leads to a considerable decrease in obtained dryness. This decrease was more pronounced for rougher felts. In addition, each felt had an optimal dewatering behaviour at a certain grammage. In general, finer model felts performed better at lower grammage, which is in agreement with practical experiences in wet pressing.
Cambridgepp 129-159A hybrid drying process: cylinder drying with through air after-dryingAbstractPDF
A method is described that classifies water in a cellulosic fiber and water system. Thermogravitmetric analysis (TGA) is used to determine hard-to-remove (HR) water from an isothermal drying curve. The HR water content is defined as the moisture ratio (g of water / g of oven dried sample) of the fiber-water system at the transition between the constant rate zone and falling rate zone of an isothermal drying curve. The TGA instrument provides tightly controlled drying conditions that allow one to distinguish small differences in drying behavior. The exact value of the HR water content was found to be influenced by the initial moisture ratio, solid mass, and isothermal drying temperature. Experiments at optimal conditions showed that the HR water content is linearly correlated with the water retention value. This correlation was examined in terms of the similar states of the fibers (i.e. minimum saturation point) when the HR water content and water retention value are measured. The dependence of the HR water content on the solid mass of the sample revealed two constants, y-intercept and slope. The y-intercept is considered to be linked with instrumentation and the slope is to be associated with the fiber. Once the HR water content measurements are adjusted for these constants, experiments may be conducted at any solid mass down to a few milligrams. The dependence of the HR water content on the isothermal drying temperature was linked to differences in the drying rate of the constant rate zone. It is proposed that the higher drying rate for higher isothermal temperatures results in the transport of water internal to the fiber becoming an important factor at higher moisture content. This results in a higher HR water content being observed. It was also found that the pulp yield affected the HR water content. At the same water retention value, the highest HR water content was found for mechanical pulp followed by unbleached chemical pulp, and bleached chemical pulp. These differences may be attributed to differences in chemistry, pore size and volume, and the dynamics of pore collapse during drying. This method can be performed on extremely small samples and provides a convenient and insightful characterization technique for cellulosic fibers.
Cambridgepp 187-212Development of new methods for characterizing uniformity of Yankee coatingsAbstractPDF
An adhesive coating formed on the surface of a Yankee dryer is critical for manufacturing creped tissue and towel grades. Due to the complexity and dynamic nature of the creping process, there has been very limited information available on the structure of Yankee coatings. This paper discusses laboratory methods for preparing Yankee coating films and imaging techniques, atomic force microscopy (AFM) and scanning electron microscopy (SEM) that were used to characterize these films. The effects of
various modifying agents are demonstrated on the structural and compositional uniformity of the Yankee coating films. The applicability of the SEM and AFM data from this study to the actual creping process and the practical aspects of the results are also discussed.
Cambridgepp 213-240The influence of through-thickness variation on the mechanics of paper dryingAbstractPDF
A material model for drying paper is presented. Moisture-dependent material parameters, hygroscopic shrinkage, the elastic and the time-dependent responses of the material to load, and the effect of unloading at a higher stiffness than the load was applied at are modelled. The model is used to determine the effects of a varying moisture ratio through the paper during drying on free shrinkage development and stiffness development at free drying. Simulation results for the stress development during drying and the state of residual stress immediately after drying are also presented. The model predicts a variation of in-plane elastic moduli through the paper, a prediction that is studied by experiments.
Cambridgepp 241-281Predictions of anisotropic multiply board properties based on isotropic ply properties and drying restraintsAbstractPDF
There is a link missing between pulp properties and machine-made paper properties. The aim of this paper is to close a part of this gap by proposing an engineering model which, based on pulp or stock properties, makes it possible to predict the resulting anisotropic material behaviour of a multiply paper or board based on any given fibre anisotropy and drying restraint.
An anisotropic model for the shrinkage and stiffness inter-action between the individual plies in a multiply structure is formulated. The input data to the model is the isotropic restrained dried and free dried stiffness, the free shrinkage strain and the density of each ply in the multiply structure. The basis weight, fibre anisotropy and total strain after drying are variables in the model. This means that besides the standard handsheet procedure only measurement of shrinkage and stiffness for one extra free dried handsheet is needed for the calibration of the model.
The proposed model is validated with a series of anisotropic handsheet trials. Various combinations of single ply anisotropic handsheets were couched together into seven different multiply
boards, which were dried freely and restrained. The isotropic input data of the individual plies were used to predict the free and restrained dried tensile stiffness index and bending stiffness of the multiply boards. The agreement between the experimental and predicted results showed good agreement. The model constitutes a useful tool in engineering predictions and parametric investigations of the mechanical behaviour of multiply boards. As a demonstration of the use of the model, the relation for tensile and bending stiffness versus total strain accumulated during drying was predicted for different board compositions. The basis weight and fibre anisotropy were varied in one of its plies.
As another example the MD and CD stiffness profiles of a multiply board were simulated based on a given MD stretch and CD shrinkage profile.