Numerical Analysis of Pressure Pulses Induced by Blades in Gap Formers, 1996 Engineering Conference Proceedings

Pressure pulses generated between forming fabrics in twin-wire blade formers affect the paper formation. These pulses have been modelled. Four equations (mass conservation, momentum equation, top fabric eqilibrium and bottom fabric equilibrium) that describe the behaviour of the fibre suspension and fabrics in a gap former have been derived rigorously. The equations were solved using Runge-Kutta methods. The results of this numerical analysis - particularly the pressure pulse generated by a forming blade - were compared with the approximate analytical solution derived by Zhao and Kerekes [l] (hereinafter, ZK). Most of the assumptions of the ZK model produce minimal errors. However, two assumptions of the ZK model produced much more consequential errors. The assumed constant permeability of the wire and associated fibre mat caused the peak pressure to be underestimated by about 15%. A second assumption of the ZK model, that there is negligible effect of the stagnation point associated with the doctoring effect of the blades, caused the peak pressure upstream of the blade to be underpredicted by the ZK model by between 5% and 15%. For certain papermachine operating conditions, the ZK model predicts the existence of an oscillatory pressure distribution upstream of the blade. The computational model supports this prediction, and shows that both the upper wire and lower wire tensions play a role in the development of the oscillatory pressures. The model presented here is an additional step in our understanding of twin-wire blade forming. Ultimately, improvements in our understanding will allow us to design better twin-wire formers.