CLUSTERS AND PARTICLE SEGREGATION IN GAS-SOLID FLOW-THROUGH A VERTICAL TUBE

Based on an analysis of hydrodynamic inter-particle interactions, a mo del describing particle clusters in two-phase, turbulent, gas-solid fl ow through vertical pipes is presented. The initial stage in the evolu tion of a cluster is the formation of a hydrodynamical aggregate of a small number of particles. This aggregate will then grow due to sedime ntation aggregation. The growth law of clusters, i.e. the cluster size and velocity as functions of its age, is found using a 'mean field th eory' approach. It is shown that clusters have large relative Reynolds numbers which leads to the formation of turbulent wakes behind them. Due to the complex turbulent flow structures, partially induced by the wakes of all clusters, the horizontal motion of a specific cluster ca n be considered to be a random walk. This horizontal motion eventually drives the cluster to the wall. The final stage is the destruction of the cluster due to a collision with the wall or with a high speed par ticle. It is shown that the former dominates the latter. Using this th eory the cluster size distribution function and the mean cluster size are calculated. Theoretical predictions are compared to available expe rimental data and good agreement is found. Furthermore an expression i s derived for the effective radial solid flux in terms of the intensit y with which small hydrodynamical aggregates are formed. This then pro vides an explicit microscopic mechanism for the formation of a core-an nulus structure in gas-solid flow through vertical pipes.