NUMERICAL-ANALYSIS OF PRESSURE PULSES INDUCED BY BLADES IN GAP FORMERS

Pressure pulses between forming fabrics in twin-wire blade formers aff ect paper formation. These pulses have been modeled using four rigorou sly derived equations (mass conservation, momentum, top-fabric equilib rium, and bottom-fabric equilibrium) that describe the behavior of the fiber suspension and fabrics in a gap former. Results of this numeric al analysis-particularly the pressure pulse generated by a forming bla de-were compared with the approximate analytical solution derived by Z hao and Kerekes in a previous work (here-in after, ZK). While most of the assumptions of that ZK model produced minimal errors, two produced consequential errors: The assumed constant permeability of the wire a nd associated fiber mat caused the peak pressure to be underestimated by about 15%. Moreover, the assumption that the stagnation point assoc iated with the doctoring effect of the blades is negligible caused the ZK model to underpredict peak pressure upstream of the blade by 5-25% . For certain paper-machine operating conditions, the ZK model predict s the existence of an oscillatory pressure distribution upstream of th e blade. The computational model presented here supports this predicti on and shows that tension in the upper and lower fabrics plays a role in the development of the oscillatory pressures.