Fluid dynamics and flow structures of wood fibres suspended in gas flows

Doktorsavhandling, 2006

The flow of wood fibres suspended in gas plays a major role in the MDF (Medium Density Fibreboard) production process. Development of machinery in this industry has traditionally been based on experience. Only recently has the use of numerical methods such as CFD (Computational Fluid Dynamics) become an important part of the development process. The ratio between fibre and gas density is large in fibre-gas suspensions. There are large separation effects, which make it necessary to treat the flow of fibre-gas suspensions as multiphase flows. The measurement of concentration is important in multiphase flow. For single phase flows, measurements of pressure and velocity generally provide sufficient information about the flow field but, for multiphase flows, the volume fraction is also an important part of the flow field. In the present work, a method for simultaneous measurement of the concentration and velocity of wood fibres suspended in air was developed. The velocity was measured using PIV (Particle Image Velocimetry) and the images obtained with the PIV equipment were used as input data for the concentration measurement. The concentration was evaluated using an image processing procedure, where the area fraction of fibres in each image was evaluated. The area fraction was subsequently converted to volume fraction using a mathematical relationship based on the assumption of isotropic fibre orientation. Good results were obtained for low concentrations of wood fibres; for higher concentrations, however, the measured volume fraction was too low. The use of a standard two-fluid model to describe the flow of fibres suspended in gas was evaluated. The drag force acting on the fibres was modelled using a drag model capable of predicting the drag on fine filaments orientated at different angles to the air stream. Turbulent dispersion of the fibres was modelled using a dispersion force. There were large discrepancies between simulations and experiments, especially for volume fraction. The discrepancies were attributed to real physical effects not included in the two-fluid model that was used. It is argued that the lift forces acting on the individual fibres due to their orientation and shape have a significant effect on the dispersion of the fibres in the air flow. When a dispersion force approach, based on gradient diffusion, was used to model the effect of lift forces on fibre dispersion, the agreement between model and measurement data improved significantly. Fibres in flows have a tendency to flocculate. The mathematical models used in this work to describe the flow of wood fibres suspended in gas are only valid when the fibres are free to move relative to each other. In order to establish the limiting conditions for the models to be valid, the flow regimes of air-wood suspensions were mapped for a variety of concentrations and Reynolds numbers.