2001 Volume 1

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Paper structure characterisation has been extended to include 2-D formation and periodic marks as well as wavelet techniques. Local fibre orientation anisotropy and misalignment angle analysis, in
combination with tape-splitting layering and image analysis have improved the understanding of the relationship between paper structure and properties as well as increased the possibilities to
trace the forming history from final paper structure.

The Crowding Factor has been introduced, to describe the tendency of fibre suspensions to form fibre flocs. It gives an improved predictability in comparison with fibre concentration, by also taking fibre slenderness into account. The dynamic characterisation of de-flocculating and re-flocculating mechanisms is getting increased attention, partly due to the potential of CCD-cameras and image analysis techniques. It is the opinion of the authors that floc stretching is a more promising way of de-flocculating than turbulent shear. It is doubtful whether turbulence in simple fluids can aim as a model for the flow irregularities in fibre suspensions, due to the damping effects of fibres and flocs.

For headbox CD-profile control, dilution water injection has been introduced. Since this makes slice lip deformations un-needed, it has the potential to uncouple control of grammage and misalignment angle profiles and also to improve CD-control resolution.

The importance of headbox nozzle design for fibre orientation anisotropy is now appreciated. A high nozzle contraction ratio will increase anisotropy and also introduce some deflocculation through floc stretching. Vane insertion helps to decrease fibre orientation anisotropy and is also applied for multi-layer forming. Although applied in tissue forming and in linerboard forming, further improvements have to be made for printing paper applications.

In twin-wire forming, the roll-blade principle has been accepted by the main machine manufacturers. Since its introduction in the STFI-Former in 1991, the blade section design with stationary blades on one side and loadable blades on the opposite side is the main design for printing paper applications, and also increasingly used for different board qualities.

Reference geometries consisting of a constriction block and a secondary pipe were used to provide reference information for estimation of the performance of more complex geometries resembling to real headbox designs. This information included the fluidisation ability curves, i.e. the minimum attained floc size in function of mechanical energy loss, the rate of fluidisation in sudden expansion, the rate of subsequent reflocculation and the level of saturation floc size. The functionality of this approach was illustrated with measured information of a particular complex geometry on several research environments of different scale. By paper samples produced with a similar geometry, the tight connection of the fluidised state of the suspension and of the attained formation was verified. Reduction of the residence time of suspension in the headbox resulted to lower floc size in slice lip area and to better level of formation in produced paper. In addition, both properties revealed a similar form for this dependency. By changes on tensile strength ratio of the produced paper, the operation of the forming section was shown to have an apparent but not controlling effect on the level of formation obtained.

We introduce a novel method for measuring the properties of consolidating wood fibre network during filtration of liquid-fibre suspension. The device consists of a hand-sheet mould equipped with a pulsed ultrasound-Doppler anemometer for measuring the local time-dependent velocity field of the fibre phase during vertical filtration. Simultaneously, the total flux of the suspension and fluid pressure loss through the filtrated fibre layer are measured. Based on this experimental information other relevant flow quantities can be computed. We thus find the space-time evolution of velocity, volume fraction and pressure (stress) fields separately for the fluid phase and for the fibre phase. This method allows us to experimentally study the details of the consolidation process in dynamic conditions. The device can be applied as an advanced laboratory test instrument for measuring relevant physical fibre network properties. As a result we present, e.g., the measured local stress-strain history of the consolidating fibre layer during a filtration experiment.

Fluid dynamics plays an essential role in the paper manufacturing process. The quality of paper is affected by the turbulence properties in the headbox and jet. In this paper the structure of turbulence in the wall area of a two-dimensional converging channel is studied experimentally. The measurements are performed with Particle Image Velocimetry, which provides instantaneous two-dimensional velocity fields. The structure of turbulence is studied by analysing both instantaneous and time-mean velocity fields. As a result several kind of flow structures can be identified close to the channel walls. The most prominent are streamwise elongated structures, which manifest themselves as a spatial modulation of the streamwise velocity component. Substantial activity in the near-wall region is related to the mean-shear close to the surface. The mechanisms of the wall-turbulence are discussed in a short
review of the main concepts found in the literature. The results of this study are expected to improve the understanding of the significance of the headbox slice boundary layers in the paper-making process.

Normally, the main purpose of vanes installed in the slice chamber of a hydraulic headbox is to control the tensile strength ratio by affecting the mean fiber orientation of the suspension. The use
of vanes inherently leads to the formation of peculiar vortex structures, referred to here as coherent flow structures (CS), in the downstream flow. These CS are believed to produce non-homogeneity in paper. Although the CS are geometrically three dimensional, their machine direction (MD) and cross direction (CD) components are dominant, cf. Kármán vortex street, and have distinctive characters of their own. The CD component of the vortex maintains its coherent nature better than the MD component and, therefore, its appearance in a form of MD spatial scale is used to express the inheritance of the flow structures in the paper. By using bluff vanes of different thicknesses, it is shown that the CD component of the vortex street maintains its characteristic appearance from the slice chamber to the slice jet. This made it possible to study the inheritance of flow structures in paper with more realistic vanes. By marking particular parts of the flow in the slice chamber with dye streamers the CS were also made visible in the paper. An analysis of these samples reveals that the average scale of ink spots is related to the MD spatial scale of flow structures. Finally, a correlation between the CS of flow and the structural cockling tendency was found. Thus, important complementary information has been created to serve the goal of finding a general linkage between structures in flow and paper. The knowledge of this link would not only allow research and analysis of wet end operations without actually producing paper, but would also provide a means of evaluating wet end conditions in a mill environment by paper analysis.

The essential scientific problems in wet pressing are concerned with water removal from the wet web, its runnability and the effect of pressing on the quality of the web and the paper produced from it. This paper briefly reviews the present understanding of the effect of wet pressing on the web and paper quality and discusses some questions concerning the runnability of the web through the press section. The main emphasis is placed on water removal. A short historical review of the development of our present understanding of wet pressing fundamentals is presented. The modelling of wet pressing is also discussed.

The water removal from the fibre cell wall starts at fairly low solids contents of the web, in the range of 20–25%. In modern press sections, the solids content of the web after pressing is about 45–50%. At this solids content, most of the water is in the fibre wall. Thus, when trying to enhance water removal further, it is necessary to understand the mechanisms and controlling factors in cell wall dewatering. Present scientific efforts should therefore be focused on finding as invariant and quantitative  knowledge as possible on the behaviour of the cell wall under wet pressing conditions.

Recent research on cell wall dewatering is reviewed in the paper. Advanced measuring methods such as NMR, solute exclusion, WRV(CCV) and DSC techniques have produced new and to a certain extent invariant information on the cell wall structure and dewatering. As a result, a clearer picture of the differences in the behaviour of mechanical and chemical pulps, softwood and hardwood pulps and different types of fines material has emerged. The effect of hornification and beating has also been clarified. Further development of measuring techniques such as DSC-based thermoporosimetry is most likely to improve our understanding in this area, helping to make it more accurate and invariant.

A number of quantitative models of wet pressing have been proposed in the literature but none of them take into account the observed rate dependence of the structural pressure. This rate dependence, which is ascribed to the flow resistance of the water in the fibre walls, should be incorporated in wet pressing models.

This paper presents a hydrodynamic model of wet pressing that includes the rate dependence of the structural pressure and thus takes into account the flow resistance of the intra-fibre water. The model is general in the sense that it can predict changes in the state of the fibre web due to a pressure pulse of arbitrary shape, under a wide range of web saturation and temperature conditions. The model can predict the effect of high press-roll temperatures.

The predictive capability of the model has been studied with the help of data from the third press of a pilot paper machine. For wet pressing at ambient temperatures, the model gives good predictions of solids content over wide ranges of machine speeds, grammages and linear loads and its predictive capability is quite adequate for most engineering applications.

For high press roll temperatures, the predictions are satisfactory over a wide range of press roll temperatures and linear loads but some significant systematic discrepancies occur. These predictions can probably be improved by using better estimates of the hardening curves at high temperatures. Once calibrated, the model can give considerable information about changes in the state of the web as it passes through the nip and some insights about the mechanisms that are active.

Laboratory drying experiments were done with a standard tensile testing device to study basic mechanisms of moisture induced tension variation. Also, pilot machine experiments were done to compare with the laboratory tests. The results show that the tension variation of paper is generated by a combined effect of moisture variation and straining during drying. The laboratory experiments indicate that tension variation is induced by strain differences in paper web. Tensile stiffness, which is affected most by wet straining, is almost constant between areas of different initial moisture contents. On the other hand, pilot tests showed only small effect of the initial moisture variation. In rewet experiments it is found that dried-in strain is recovered when paper is rewetted.