Flow patterns and wall stresses in a moving granular filter bed with an asymmetric louvered-wall and obstacles

Abstract

This study investigated the flow patterns and wall stresses in a two-dimensional asymmetric louvered-wall moving granular filter bed with an obstacle placed at each stage of the granular filter bed. The obstacles utilized were: (1) a single vertical plate and (2) a wedge-shaped obstacle comprising a left-vertical-plate and a right-inclined-plate. A digital camcorder was used to record the flow pattern histories of the granular solids in the moving granular filter bed. The instantaneous vertical velocity distribution at a certain height from the bottom of the third stage in a two-dimensional asymmetric louvered-wall granular filter bed was determined from two successive flow images. The static stresses and the variations in the dynamic wall stresses with time were obtained using a two-directional pressure gauge, which could simultaneously measure the normal and shear stresses of the granular solids. The preliminary theoretical study of the obstacle's placement in the asymmetric louvered-wall moving granular filter bed was proposed. The magnitude of the average dynamic normal stress on the convergent section of the louvered-wall greatly exceeds that on the vertical section of the louvered-wall. A wedge-shaped obstacle (or a single vertical plate) placed at a proper location, which causes two asymmetric flowing streams in the granular bed, may increase the velocity and refreshing rate of the filter granules in the quasi-stagnant zone near the louvered-wall and further the overall dust collection efficiency of the filter granules because the central flowing core above exit is not evident in the granular bed. Additionally, a wedge-shaped obstacle installed at an appropriate location, which consists of a left-vertical-plate and a right-inclined-plate, satisfies the following requirements: (1) it keeps the granules in the left part of the granular bed away from the obstacle, preventing interference therewith; (2) it shrinks stagnant zone and declines the normal stress on the convergent section of the louvered-wall in the right part of the granular bed; and (3) it solves the potential problem of a roof-type insert, on which the stagnant zone may sit.