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Licentiate seminar

Modelling of the Pressure Distributions in Twin-Wire Blade Formers


Defendant Main Advisor Extra Advisor Date
Claes Holmqvist Anders Dahlkild B. Norman 2002-10-16

Opponent
Staffan Toll, Chalmers University of Technology

Evaluation committee

Abstract

During papermaking, the internal structure of the fibre network constituting the paper is to a dominating extent determined in the forming zone of the paper machine. This thesis is aimed at studying the pressure distribution in blade forming sections, which is commonly considered to be a key quantity of the process. Previous work has provided insight into the physics of different devices employed in blade forming. However, there has been a lack of models enabling studies of the effects of the interaction between different components on the pressure distribution. In the thesis, a model is presented for a generic blade forming section consisting of three blades. The positions of two of the blades are fix, and in between them is located a suction box. The third blade is applied by a prescribed force to the opposing wire, in a position facing the suction box. The model admits the study of the interaction between the pulses from the different blades in the blade/counterblade configuration, and between the pulses and one-sided suction. The wires are modelled as tensioned and perfectly flexible Euler-Bernoulli beams of negligible mass. The suspension is treated like an inviscid fluid. Consideration is taken to the influence of fibre deposition on the permeability of the fabrics. By assuming the ratio between the length scales in the thickness direction and the machine direction to be small, a quasi one-dimensional model is obtained. For maximum flexibility, the model domain is divided into modules. Each module is solved individually using a finite difference based discretisation. The solutions for the different modules are matched with each other iteratively. A comparison with published results for a single blade indicates that the model can be used to obtain qualitatively correct predictions of the pressure distribution. New results include a series of calculations showing the non-trivial interaction between the pressure pulses when the blades are positioned successively closer together, the effects of suction on the pressure pulse generated by a blade applied to the opposing wire, and how blades of modest curvature do not necessarily stay in contact with the fabric along their full width and the implication of this on the pressure gradient.
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