THREE-DIMENSIONAL FLOW VISUALIZATION OF DILUTELY DISPERSED SOLIDS IN BUBBLING AND CIRCULATING FLUIDIZED BEDS

Three-dimensional flow structures of dilute suspensions in the freeboard of a bubbling fluidized bed as well as in a circulating fluidized bed of 200 mm internal diameter, roughly a (125)-scale model of a commercial boiler, were observed by the laser sheet technique. With this technique the suspension behavior in an arbitrary cross section can be observed. Furthermore, by applying multiple laser sheets the three-dimensional structure of the suspension can be investigated. In the dilute suspension above a bubbling fluidized bed, the so called “ghost bubbles” were clearly visualized. In the dilute phase transportation regime the existence of clusters was confirmed. The average shape of a cluster was a paraboloid heading downward and enclosing a gas wake in the upper side. From a cluster, particles were shed to the dilute phase continuously and these were again absorbed by other clusters. Cluster sizes and their velocity distributions were determined from the video image analysis. The present data would serve for the future construction of mathematical models on the flow structure of circulating fluidized bed reactors.