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Effect of aperture geometry on the steady flow through the narrow apertures in a pulp screen : numerical and experimental study Mokamati, Satyanarayana V.

Abstract

This investigation examines turbulent flow across a contoured wall with evenly-spaced slots and a series of flow bifurcations, as found in industrial pulp screening. The contoured wall and slotted apertures are formed from an array of 'wires', with cross-sectional geometry characterized by contour height and wire width. Four complementary studies were conducted to examine the velocity and turbulence characteristics of this complex flow. In the first study, a Computational Fluid Dynamics (CFD) model was developed to theoretically examine the effect of wire cross-sectional geometry on the flow field. The model shows that separation of the viscous layer occurs upstream of the aperture, and a vortex occupies the slot entry. As contour height increased, turbulence intensity near the wall increased. Further, turbulence intensity near the wall increased with decreasing wire width. It was shown that the ratio of contour height to wire width controls the boundary layer thickness and turbulence intensity near the wall. In the second study, the velocity field near the slot entry was experimentally measured using Particle Image Velocimetry (PIV) and was compared with theoretical predictions. In general, the vortex size and shape were similar to that predicted by CFD. In the third study, the velocity and turbulence intensity distributions above the wall were experimentally measured using Laser Doppler Velocimetry (LDV). The velocity near the wall was shown to decrease with contour height and increase with wire width. Further, the velocity near the wall was shown to increase as the flow through the slots increased. A correlation for velocity above the wall was determined as a function of the ratio of contour height to wire width, upstream velocity and flow through the slotted apertures. The correlation can be used to estimate shear stress at the wall. The maximum turbulence intensity near the wall was shown to increase with contour height and decrease with wire width. In the fourth study, the motion of 2 mm long nylon fibres moving near the apertures was experimentally observed using High-speed Video (HSV). Fibres were shown to interact with both the wires and the vortices during passage. It was observed that fibres only passed through the slots after impacting the wire and being pulled back into the slot by the vortex. Further, fibres were shown to pass through the slots with high contour wires more readily.

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