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Hydrodynamics of blade gap formers

University of British Columbia

In blade gap forming in papermaking, a headbox discharges a pulp suspension into a gap between two moving permeable fabrics under tension. As these fabrics wrap around fixed blades, sizeable pressure pulses are induced in the gap between the fabrics. These pressure pulses cause drainage through the fabrics during which pulp fibres are deposited by a filtration process to form a mat. The objective of this study was to improve our understanding of this process. To achieve this, three aspects of the problem were addressed: fluid flow, fabric deflection, and mat resistance. Simplified one-dimensional and numerical viscous two-dimensional models were developed. The effects of key variables on pressure pulses generated by a single blade were first considered, then the effect of applied suction between two adjacent blades was added. In the final step, the effect of pressure in the stagnation zone at the contact point between a moving fabric and stationary blade was included. It was confirmed that a wide flat blade (> 2 cm) generates two distinguishable pulses, one at the leading edge of the blade and the other at the trailing edge. As blade width decreases to approximately 1 cm, the two pulses converge in a single pressure pulse. The magnitude of the pulses depends on a number of variables, such as fabric tension, fabric velocity, and fibre mat resistance. It was further confirmed that blade curvature exerts a strong effect on the pulses. As a flat surface assumes a curved shape, the pressure pulses at each edge merge into a single pressure pulse over the blade surface. This convergence occurs at rather small curvatures that are within the range of wear of plastic blades found in commercial paper machines. This study also examined the effect of suction in one side of fabric between blades. It was found that such suction induced a negative pressure in the gap between fabrics just downstream of the first (upstream) blade. This negative pressure increased to a positive value at the second (downstream) blade. The net increase in pressure from the negative value corresponded to a pressure pulse that was larger in amplitude than one created in the absence of suction. This was attributed to the larger wrap angle at the leading edge of the second blade induced by the downward deflection of the fabric resulting from suction. Other aspects of blade forming examined in this study were the effect of inertial force on drainage and viscosity on the flow between fabrics. Inertial effects were found to be small, even for modest levels of mat build-up (10 g/m²). Inclusion of viscosity in the analysis gave a boundary layer in the flow parallel of the fabric, but this was very thin owing to the drainage perpendicular to the fabric which drew off the boundary layer. Lastly, the effect of the stagnation zone pressure at the leading edge of a blade in contact with a moving fabric was examined. The size of this zone was found to depend on the radius of curvature of the edge of the blade. Under typical papermachine conditions, the pressure acting on the fabric from the stagnation zone was found to be about 50% of the stagnation point pressure. The size of the zone over which the pressure extended was a few millimeters. This pressure increased the pressure between the fabrics by 30% to 100%, depending on the radius of curvature of the blade edge. The stagnation zone pressure also forced some water back into the gap between the fabrics, but the amount was small only about 10% of the drained water.

Thesis/Dissertation

application/pdf

2009-07-23

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http://hdl.handle.net/2429/11159

Mechanical Engineering

University of British Columbia

UBCV

DSpace