A Numerical Investigation into the Effectiveness of Multi-Element Pressure Screen Rotor Foils

Pressure screening is an efficient way to remove unwanted debris from a pulp stream, which improves the quality of the end product paper. Past work has found that increased foil camber and angle-of-attack improve the performance of pressure screen foil rotors by increasing the magnitude and width of the negative pressure pulse on the screen cylinder while at the same time reducing the magnitude of the positive pressure pulse on the screen cylinder. Too large an angle-of-attack or too much camber leads to separation of the flow over the foil and a loss in rotor performance, however. This study therefore investigates, using computational fluid dynamics, the ability of multi-element rotor foils to delay stall over the foil and improve upon the performance of an existing pressure screen rotor foil. In this study, the effect of foil angle-of-attack, flap angle, the geometry of the trailing edge of the main foil, and the positioning of the flap relative to the main foil were studied. A multi-element foil was developed based on the NACA 8312, a foil used in industrial pressure screen rotors. In general, stall was delayed and a larger angle-of-attack was obtained than the single-element foil, and increased camber was added to the foil by deflecting the flap. Positive pressure pulse on the screen cylinder approached a negligible value with both increasing angle-of-attack and increasing flap angle, while the negative pressure pulse increased in magnitude with both increasing angle-of-attack and flap angle before the foil began to separate and the suction was lost. The $x$-positioning of the flap was shown to have less of an effect on the foil performance than the $y$-positioning. All told, the magnitude of the negative pressure pulse was increased by 15% while at the same time eliminating the positive pressure pulse.