Modelling of Jet Impingement in Twin-Wire Paper-Machines: Impingement on Once Fabric, 2000 Engineering Conference Proceedings

Barbara Dalpke, Sheldon I. Green, Richard J. Kerekes--Important paper qualities like the formation, fines distribution and two-sidedness are influenced by the geometry of the forming section of a twin-wire paper machine. To be able to control these qualities, it is crucial to have a good understanding of the flow behaviour in the forming zone. In the past, there has been a focus on the roll and blade drainage, but the jet impingement zone may also significantly influence these qualities.

We have studied the fluid dynamics of jet impingement in a roll-former. The computational fluid dynamics (CFD) software FLUENT was used to develop a two-dimensional model that predicts pressure distribution, shear and flow velocities in the jet impingement zone. The VOF (Volume of Fluid) method was used to predict the shape of the pulp suspension jet, and the forming fabrics were represented as permeable media obeying Darcy’s law with an extra inertial term. These equations also model the fibre mat resistance when fibre-mat build-up is included in the simulation.

Preliminary results for jet impingement on a single wire were obtained for typical values of fabric resistance, impingement angle, and jet velocities used in paper machines for printing grades. The influence of fabric rush/drag was also studied. The results show that the drainage force from jet impingement is important only in the vicinity of the impingement point. Approximately 50% of the drainage takes place over a distance comparable in size to the jet thickness. Higher jet velocities lead to slightly higher drainage values. The pressure distribution is mainly influenced around the impingement point and not further downstream. Lower impingement angles lead to less drainage. For the cases of both varying jet velocity or impingement angle, the computed values follow the predicted trend from a simple one-dimensional model. In the case of varying rush/drag, shear stresses are nearly constant downstream of the impingement location. The influence of fibre-mat build-up on drainage was computed by including an average drainage resistance for the fibre-mat.