Fluid flow through and deformation of wood fiber networks

Abstract:

Cellulose-based commercial items such as paper and laminated flooring are vital to the local economy of northern Sweden. These products are commonly manufactured in suspension where fluids such as water or air are drained through the fiber network in some value-added steps in the manufacturing process. It is crucial to gain in-depth knowledge of the fluid-solid interaction between the carrier fluid and the cellulose product as this may lead to advances in products and processes.
The focus of this thesis was to study the flow of a Newtonian fluid through a compressible cellulose fiber network; both air and water were considered. The work was divided into a number of studies in which information was gathered regarding the ease at which water flows through the network under essentially steady-state conditions to define the permeability of the fiber network. The difficulty is that the network may deform under the loading of the flowing water. In these initial studies a number of novel experimental devices were devised in which the in-plane permeability, i.e. normal to the direction of the principle axes of the cellulose fibers, and uniaxial compressive yield stress were determined as a function of solid fraction. The exact mechanism by which the network deforms under load remains an open question. To start with, a flow field study was performed in a unit operation, i.e. "Metso Continuous Press", compared with collected field data in a mathematical model developed in this thesis.
A mathematical model was established to study the mechanics in how the network deforms under load in a one-dimensional filter press. Results from the first part regarding the permeability and the compressive yield strength for the papermaking fibers were used. Simulated prediction was compared with the experimental results. A rate dependent lag was found for the fiber network.
The study was continued to examine pressure filtration in which the cellulose network deforms dynamically under the action of an externally applied load. An open question in this case is the behavior of the water internal to the fiber network. During compression, initially there is a reduction in pore volume and an increase in the number of fibers in contact. At some degree of compression, the number of fibers in contact is such that further compression can only occur by the individual fibers themselves through deformation of the fiber. This process was modeled, and it was considered that the energy required to compress the network must balance the viscous dissipation rate. Closure equations were developed and tested experimentally by comparison to the pressure developed to drive the piston. Good agreement was found. In subsequent work an attempt to confirm the form of this relationship independently through novel visualization methods was performed, i.e. positron emission tomography. The utility of this approach may be discussed.