On the formation and disintegration of particle clusters in a liquid—solid transport bed

Abstract

Experiments are conducted in a two-dimensional, liquid—solid transport bed to study the mechanisms of particle cluster formation and disintegration. The transport bed is 20 cm in width, 1.2 cm in thickness and 121 cm in height. 2.5 mm nylon spheres and glycerin aqueous solution (45 to 61 wt% of glycerin) are used as a solid and liquid phases, respectively. The operation range of the particle Reynolds number ( Re = ϱ U td p /μ) is from 0.662 to 10.88, achieved by varying the glycerin concentration. The liquid superficial velocity is operated in the range of 2 to 8 times the particle terminal velocity. A video camera is used for the visual study of continuous formation and disintegration of particle clusters. It is found that when a cluster forms, particles organize and gather in a vertical direction. On the other hand, when a particle cluster disintegrates, the cluster rotates to a horizontal alignment and gradually disperses into individual particles or/small clusters. The formation of a doublet cluster is most commonly observed. The probability of observing a larger cluster decreases drastically as the cluster size increases. Within the present operation range of solids holdup (0.0056–0.028), it is found that the particle clustering is more pronounced as the solids holdup is higher. As the Reynolds number increases from 0.662 to 10.88 at a given solids concentration of 0.019, the degree of particle clustering first increases, reaches a maximum around Re = 3.16 and then decreases. It is also found that the slip velocity of a cluster depends on the cluster size as well as the particle alignment within the cluster. More specifically, the cluster slip velocity is higher when the cluster is larger or when it is better aligned vertically. The present analysis also indicates that there exists a certain degree of similarity between the cluster formation in a liquid—solid system and that in a gas—solid system.