Flow phenomena and paper forming

R. J. N. Helmer, G. H. Covey, W. D. Raverty and N. Vanderhoek; Appita 2000

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SUMMARY

The channel flow phenomena of the University of Melbourne stationary wire laboratory former were investigated to further the understanding of the effect of flow conditions on sheet properties. The flow phenomena above the wire in the z-MD plane immediately prior to and during de-watering were investigated using images collected with a 16mm high speed ciné camera (200 frames per second). The flow regimes were observed to be consistent with pipe flow studies in that a plug flow with a turbulent boundary layer was evident. The mechanism of forming was found to be via a boundary layer filtration process. The hydraulic behaviour of the system was characterised the Froude number.

INTRODUCTION

During the last century there was a significant effort to understand the flow phenomena of pulp suspensions in pipe flow (1,2, 3), channel flow (4), headboxes (5) and on the wire on a commercial paper machine (6, 7). The flow regimes of pipe flows which were identified included, plug flow, mixed flow, and turbulent flow, which were dependent upon the flow geometry, suspension type and consistency, and driving force. In many of these flow investigations the use of direct observation and image analysis of the flow was crucial to identifying the various flow regimes. To date there has been no direct investigation of the z-MD plane flow structure on the wire during paper forming. This can be attributed to difficulties posed by the high operational speeds, the nature of pulp suspensions, the risk of damage to the machine, and the inconvenience to papermakers.

Current understanding of the flow structure on the wire is largely restricted (8) to the mechanisms identified by Parker (9) which include drainage forces, oriented shear, and turbulence, as shown in figure 1.

flow_phenomena_figure_01

Figure 1: Flow on the Wire, a). Drainage, b). Oriented Shear, and, c). Turbulence.

Parker (9) also proposed two drainage mechanisms; filtration and thickening. Filtration is widely considered to be the mechanism by which commercial paper is made (10). Whilst these concepts have made a significant contribution to the understanding of the effect of process conditions on paper properties, the understanding is still largely qualitative and the sequence of events which culminate in fibre deposition is still unclear. Furthermore, this lack of knowledge has hindered the development of laboratory formers which accurately simulate the commercial papermaking process.

The devices which have been available to form sheets in the laboratory are quite varied in their design and operation (11-26). The fine structure of sheets produced on many of these laboratory formers differs quite markedly from paper which has been made on a commercial paper machine (20-22). Further problems with grammage profiles and fibre fractionation have also been reported (16-18). The devices which have produced paper similar to a commercial machine have been reported to be bulky, difficult to operate and hard to control (19). None of these devices has been deemed to be a truly satisfactory predictor of what may be expected on a commercial machine from a given fibre stock. Furthermore, they are generally unsuited to investigating the mechanism of fibre deposition and the effect of process conditions on the structure of paper formed commercially. Thus, there is no device which predicts the papermaking potential of particular fibre sources with a degree of precision which approximates the achievements of pilot paper machines, and without the expense and logistical complexity of a pilot scale trial.

The flow above the wire in the forming region of a commercial Fourdrinier former has been classified as being equivalent to a high aspect ratio, rough channel flow and a new laboratory former, the University of Melbourne (UoM) former has been developed (27) as a tool for studying the behaviour of pulp stocks in a system equivalent to a commercial papermachine. In this study, the flow phenomena above the wire on the UoM former has been investigated to gain insights into the commercial forming process.