Numerical simulation of cohesive powder behavior in a fluidized bed

A numerical simulation model was developed for wet powder fluidization in the scope of investigation on cohesive powder behavior. The model was developed based on the discrete element method (DEM) with the inter-particle cohesive interaction due to liquid bridging. To take into account the liquid bridge force between particles and between a particle and a wall, a regression expression for the liquid bridge force was developed as a function of the dimensionless liquid bridge volume and the separation distance based on numerical solutions of the Laplace-Young equation. The critical rupture distance, i.e. the maximum separation distance where the liquid bridges rupture between particles or between a particle and a wall, was also correlated in the similar manner. For gas motion the Anderson-Jackson local averaged Navier-Stokes equations were integrated by the SIMPLE method employing the staggered grid system. For the particle motion the Newtonian equations of motion of individual particles were integrated, where the Hooke's repulsive and damping forces for particle collision, the gravity force, the drag force and the particle cohesion force were taken into account. Example computations were conducted for a two-dimensional bed of spherical particles of 1 mm in diameter, density of 2650 kg/m(3), moisture content of 0.27 wt% and gas velocity of 1.2 m/s. Both dry and wet powder behavior in a fluidized bed was successfully reproduced. It was observed that such a wet powder was fluidized forming agglomerates. The fluctuations of the pressure drop and the minimum fluidizing velocity for wet powder were higher than those for dry particles in the present computer experiment. The granular temperature was obtained from particle velocity. The high granular temperature area below a rising bubble and above a forming bubble could be found for both dry and wet particles. (C) 1998 Elsevier Science Ltd. All rights reserved.