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Licentiate seminar

On the Fluid Mechanics of Partial Dewatering during Roll Forming in Paper Making


Defendant Main Advisor Extra Advisor Date
Richard Holm Henrik Alfredsson B. Norman 2002-06-14

Opponent
Alessandro Talamelli, KTH, Mekanik

Evaluation committee

Abstract

Abstract The present work deals with some aspects of the fluid mechanics of paper-making,more specifically partial dewatering during roll forming. The study is mainly experimental. Pressure and wire position measurements have been carried out in an experimental facility, the KTH-Former,which models the roll forming zone of a paper machine. Measurements are carried out with pure water for three different wires (fabrics): A non-permeable,a semi-permeable and a conventional wire. Although not used in paper making,the non-permeable wire is useful when trying to understand the fundamental mechanics of roll forming.The semi-permeable wire with finite but low permeability is used to model the effects of the fibre web on the drainage. Tests have mainly been carried out for different wire tensions and different jet speeds. It is shown that the local curvature of the wire is strongly correlated to the dewatering pressure. The conventiona wire shows a single pressure peak causing complete de- watering in the first part of the dewatering zone.The pressure distributions for the non-and semi-permeable wires are found to show two consecutive pressure peaks followed by a suction peak where the wire is taken of the roll.This oscillating behaviour is due to capillary waves where the wire tension plays the role of surface tension on a free surface.The wavy behaviour of the wire is recovered from an analytical model and the effect is governed by a dimensionless Weber number. The measured wave lengths correspond well to those given by the theory. When the wire tension is high,i.e.a high dewatering pressure,the flow in the impingement region collapses when the dynamic pressure of the headbox jet is about half of the dewatering pressure. It is shown experimentally that the local drainage shows a correlation to the dewatering pressure and hence to the wire curvature.
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