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Fibre Suspension Flows

Fibre Suspension Flows

Fibers in a turbulent wall layer. Flow is from bottom to top.

Purpose and focus of SIG

Fibrous particles are transported by a carrying fluid in many industrial applications. In papermaking processes the cellulose fibres are mixed in water. There are also dry processes, dry forming of absorbent products such as diapers and napkins, for example, where fibres are transported by air flows. Other examples where fibres are present is in polymer flows in melt-blowing extruders or processing of certain food projects (ketchup is one example). Production of textile and carpet fibres aalso involve fibre suspension flows.

The economical importance of fibre suspension flows is huge. For example, the pulp and paper industry is one of the biggest sectors in Europe, especially in Nordic countries.

The length-to-diameter aspect ratio of the fibres is typically in the order to tens or hundreds. It makes modelling of the fibre suspension flows more challenging than that of spherical particles. At low concentration, the fibres rotate and align in a flow. Turbulence makes their orientation more random, whereas mean velocity gradients tend to align them. At higher concentrations, fibre-fibre interactions become important, and fibres form agglomerates, often called flocs. Due to the elongated nature of the fibres, flocs can form already at 1 % concentration. Depending on the fibre concentration, different phenomena are dominating. Fibre orientation and dampening of turbulence are characteristic for dilute fibre suspension flows. Furthermore, optimal measurement techniques like PIV are possible for very dilute (less than 1 %) fibre suspension flows.

When the concentration increases up to 1-2 %, fibre flocculation and water-fibre-turbulence interactions become important. At high concentration it is impossible to use optical measurement techniques whereas ultrasound offers potential, at least to study time-averaged velocities. If the concentration increases towards 10 %, fibres form a continuous fibre network. High concentration fibre suspension flows are modelled by using rheological models, shear-thinning viscosity models, for example. As a whole, fibre suspension flows include many typical aspects of other multi-phase flows, but fibrous particles makes the flow even more complex.

SIG43 aims at bringing together experts from different industrial sectors to share their knowledge on fibre suspension flows, as well as to develop computational methods (CFD) and experimental methods for them.