Kenji Kubo

Mutual relation between the streamlines and the residence time curve in orderly bead-packed beds

Abstract Axial dispersion of a soluble tracer along the flow channel through orderly packed beds of spheres was investigated by means of a step response. Streamline observation made it possible to explain the shape of the residence time curve. The streamlines of water flowing in bead-packed beds (cubic and rhombohedral arrangements) were viewed by means of a dye tracer. The packing consisted of thin glass spheres filled with water. The following observations were made: (1) In the very slow flow (laminar) region, creeping flow along the surface of the beads can be seen. With increasing flow rate, the difference between flow velocities observed at the centre of a broad flow channel and in the neighbourhood of the bead surfaces becomes noticeable. (2) In the transition region, two fluid flow regimes can be observed: pockets where the fluid shows a slowly whirling motion, and the central part of the flow where the fluid is passing through at high velocity. This may explain the sudden breakthrough of the residence time curve and its long tail. (3) The rhombohedral arrangement, consisting of staggered layers of beads which lead to less fluid stagnation, shows less tailing of the residence time curve than does the cubic arrangement.