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Artikel

Streak Formation and Fibre Orientation in Near Wall Turbulent Fibre Suspension Flow

Författare: Kvick, M., Håkansson, K.H., Lundell, F., Söderberg, D., Prahl Wittberg, L.
Dokumenttyp: Artikel
Tillstånd: Publicerad
Tidskrift: ERCOFTAC bulletin
Volym: 84  
År: 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 experimentally 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 rp = 7; 14 and 28 are used. The friction Reynolds number is varied between 50 and 230 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 fibre streaks are analyzed by a correlation. 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 friction and particle Reynolds number is there an occurrence of fibre streaks. In the case when the streakiness is low, most of the fibres have an orientation aligned with, or close to, the flow direction.