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Article

Fibre Streaks in Wall Bounded Turbulent Flow

Authors: Kvick, M., Håkansson, K.H., Lundell, F., Söderberg, D., Prahl Wittberg, L.
Document Type: Conference
Pubstate: Published
Journal: 7th Int. Conf. on Multiphase Flow, May 30 - June 4, 2010, Tampa, Florida, USA
Volume:   
Year: 2010

Abstract

In papermaking, the structure and mechanical properties of the final, dry paper are to a large extent determined by the flow of a low concentration (< 1%) cellulose fibre suspension early in the process. A key flow device is the headbox nozzle, a 2D contraction that generates a thin and wide fibre suspension sheet that is jetted out onto permeable forming wires, where the paper is formed. Aiming at understanding how the mass and orientation distributions are affected by the flow along internal surfaces of the nozzle a turbulent fibre suspension flow near a wall is investigated in this study. The experimental setup consists of an inclined open rectangular channel made of glass with reservoirs in the upstream and downstream position. A pump is used to transfer the suspension from the downstream to the upstream reservoir. The suspension flows down the inclined channel driven by gravity. Cellulose acetate fibres with a density f = 1300 kg/m3 and an aspect ratio of rp = 7; 14 and 28 are used. The flow Reynolds number is varied between 500 and 20000 by adjusting the angle of inclination of the channel and the thickness of the water layer flowing down the channel. By analyzing images taken from beneath, through the glass bottom of the channel, on the fully developed flow, fibres are detected using a steerable filter. The position and orientation of the fibres in the flow parallel plane are obtained and the evolution of fibre-streaks in time are visualized. The width of the fibre-streaks are compared with the empirical value of  50l+ for low velocity streaks in turbulent boundary layers, where l+ is the viscous length scale. The result show that the fibre-streaks scale in the same manner as the viscous sublayer streaks in a turbulent wall bounded flow. It is shown that the fibres do not form streaks for all cases studied, only in a certain range of flow and particle Reynolds number there is an occurrence of fibre-streaks. In the case when fibres have not formed streaks, most of the fibres have an orientation aligned with, or close to, the flow direction.