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- Lindström, Stefan B, 1974-
(författare)
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Modelling and simulation of paper structure development
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A numerical tool has been developed for particle-level simulations of fibre suspension flows, particularly forming of the fibre network structure of paper sheets in the paper machine. The model considers inert fibres of various equilibrium 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 phases is included by enforcing momentum conservation between phases. The incompressible three-dimensional Navier–Stokes equations are employed tomodel themotion 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 well 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. Furthermore, model predictions of the shear viscosity and first normal stress difference were in fair 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. Based on these validations, a series of simulations have been performed to investigate the basic mechanisms responsible for the development of the stress tensor components for monodispersed, non-Brownian fibres suspended in a Newtonian fluid in shear flow. The effects of fibre aspect ratio, concentration, and inter-particle friction, as well as the tendency of fibre agglomeration, were examined in the nonconcentrated regimes. For the case of well dispersed suspensions, semi-empirical relationships were found between the aforementioned fibre suspension properties, and the steady state apparent shear viscosity, and the first/second normal stress differences. Finally, simulations have been conducted for the development of paper structures in the forming section of the paper machine. The conditions used for the simulations were retrieved from pilot-scale forming trial data in the literature, and from real pulp fibre analyses. Dewatering was simulated by moving two forming fabrics toward each other through a fibre suspension. Effects of the jet-to-wire speed difference on the fibre orientation anisotropy, the mass density distribution, and three-dimensionality of the fibre network, were investigated. Simulation results showed that the model captures well the essential features of the forming effects on these paper structure parameters, and also posed newquestions on the conventional wisdom of the forming mechanics.
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| 2. |
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| 3. |
- Lindström, Stefan B, 1974-
(författare)
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Simulations of the Dynamics of Fibre Suspension Flows
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 4. |
- Mattsson, Amanda
(författare)
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Characterisation of Time-dependent Statistical Failure of Fibre Networks : Applications for Light-weight Structural Composites
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The future of a sustainable society requires that materials not only be renewable, but also leave as small a carbon foot-printin the environment as possible. One such product is light-weight composite material for transportation packages. Cellulose fibres have been and will continue to be ideal for this purpose.The strength design of light-weight composites is becoming increasingly important. The challenge is to neither over- nor under-design, but instead to target the right strength underrealistic loading conditions. The question then is: What is right strength? Under realistic loading conditions (e.g., fatigue, random loading, and creep), materials fail differently from what one expects from tests of static strength: materials often fail at much lower stresses than are measured in these tests, the failure is time-dependent, and time to failure is highly variable. Therefore, to answer the above question, we have set up the following objectives: (1) theoretically formulate time-dependent statistical failure (TSF), and examine the validity of the model; (2) define material parameters describing the multi-faceted strength characteristics based on this formulation; (3) develop an experimental method to determine the material parameters; (4) investigate the impacts of fibre properties and network structures; and finally (5) characterise containerboard (the fibre material used in corrugated boxes) samples in terms of the new material parameters. The results for these five objectives are presented below, one by one.(1) A general formulation of TSF, originally proposed byColeman [1] for fibre failures, has been used. We have found that this model is indeed valid, even at the fibre network level, with only two restrictions: the existence of a lower bound onweakest-link scaling and an approximate nature of the Weibull distribution.(2) Accordingly, we have defined three material parameters that characterise different aspects of material strength: shortterm strength, durability/brittleness, and reliability. We call these parameters the new strength metrics.(3) Although the newly defined material parameters are most comprehensive, it takes up to several months to determine them by using creep tests. We have developed a new method, using constant loading rate (CLR) tests, that not only gives values comparable to those from creep tests, but also requires only about one day, allowing a drastic reduction in the testing time.(4) Monte-Carlo simulations of lattice networks have been performed to determine the basic relationships between fibre properties and network failures. The brittleness of an individual fibre (or a breaking element) influenced both brittleness and reliability of the fibre network, the higher the brittleness, the lower the reliability. Reliability, on the other hand, exhibited more intricate relationships with fibre properties and network structures. Several important analytical relationships have been derived.(5) Finally, using the CLR tests, we have characterised commercial containerboards in terms of the new strength metrics. Containerboard, as a cellulose fibre network, is quite comparable to typical stiff polymer-based fibre composites (e.g., glassfibres and aramid fibres). However, the reliability and durability/brittlenessof containerboard varied considerably within the operating windows, suggesting ample opportunities to fine-tune these properties even using current papermaking practices.The fact that the multi-faceted nature of strength can be expressed by three parameters is remarkable, and the implications are profound for how materials are designed and new materials developed. It is the author’s hope that this thesis will be of some use when it comes to redefining materials for a sustainable society, particularly the renewable alternative –cellulose fibres.
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