A Concept for Fibre Flocculation in Turbulent Flow

Abstract

A concept for the fibre flocculation process in turbulent flow is presented including both rupture and aggregation of fibre flocs. Based on results from the literature, a mechanism is proposed for the breaking-up and the building-up of flocs in interaction with turbulent eddies of different length scales in the turbulence spectrum. The mechanism has been formalized into a two-way hierarchical concept with analogy to the decay of a scalar fluctuation. Deformation and break-up of flocs by turbulent eddies with a length scale similar to the floc scale dominates the rupture process. Eddies of a smaller scale agitate the floc causing weakening and activation of the network. Floc aggregation happens due to floc collisions when the flocs are transported by turbulent eddies of a larger scale. The smaller scales activate the floc surface, and increase the probability of forming a floc. Erosion and deposition of single fibres takes place in the turbulent fine structure. This concept finds support when tested and compared with the results in the literature. Experiments have been conducted in a vertical pipe flow with a varying flow rate, fibre length and consistency, taking measurements at various positions downstream of an orifice . Results were obtained using high speed movie film, image analysis of still photos and measurements of flocculation and turbulence spectra using a laser . Increasing the turbulent kinetic energy or decreasing the macro scale gives lower floc intensity at large length scales.