Experimental Measurement of Pressure Pulses From a Pulp Screen Rotor

Abstract

Pressure screens are the most industrially effective way to remove contaminants from a pulp stream, improving the strength, smoothness, and optical qualities of both new and recycled paper. Pressure screens are comprised of two main components: a screen cylinder with narrow slots or small holes and a rotor. The main function of the rotor is to prevent the narrow cylinder apertures from becoming plugged by pulp and debris. In this study, the pressure pulses generated by a novel multi-element foil (MEF) and a single-element foil rotor in a pressure screen were measured at various foil configurations, rotor speeds, and flow rates. The experimental measurements were compared to the results from a computational fluid dynamics model (CFD). Experimental measurements showed that increasing both the angle-of-attack and the flap angle of the MEF increases the magnitude of the negative pressure pulse and reduce the magnitude of the maximum pressure pulse generated by the rotor. At the optimum configurations, the MEF was shown to produce a 126% higher magnitude negative pressure pulse and a 39% lower magnitude positive pressure pulse. It was also found that at higher tip speeds the magnitude of the pressure pulse varies with tip speed squared and the non-dimensional pressure coefficient is Reynolds number independent. Similarly, at higher tip speeds increasing the velocity of the flow through the slots had no effect on the pressure pulse generated by the rotor. At lower rotor speeds, however, the dimensionless pressure was increasingly depending on Reynolds number as slot flow velocity was increased. This is likely due to the increase in slot flow velocity causing the onset of flow separation over the foil. Finally, the numerical model was shown to accurately predict the pressure pulses generated by the MEF at low angles-of-attack and flap angles. However, the model predicted that the foil would stall at lower angles than what was shown experimentally. This is probably because the CFD model used a solid wall boundary condition rather than modeling the slots in the cylinder, preventing low momentum fluid from re-entering the domain.