CFD-DEM Simulation of the Fluidization Behavior of Particles during Coating in a New Developed Spouted Bed Apparatus
In this work, we studied the particle dynamics during a coating process in a self-developed spouted bed apparatus, consisting of a conical and a cylindrical part, which is operating at negative pressure. The apparatus consists of an annular gap through which the gas enters with a high velocity. The base plate can be adjusted by a fine threat in height and thus the gap height can be varied. With the help of a two-fluid nozzle the liquid was injected into the apparatus. As coating substance, water with coloring agent was used. The coating process was calculated during 3D CFD-DEM simulation in order to obtain additional information about the fluidization behavior of the particles and the influence of the injected liquid on the particles.
For the calculation of the particles contacts, a novel contact elastic-plastic model was implemented. In the model, the restitution coefficient depends on the load level and number of collisions due to contact hardening. The drying rate of the water droplets for the process parameters was determined by experimental investigations. For that, droplets were applied on a substrate with a known contact angle and the time to complete evaporation was measured under different humidities, temperatures and gas velocities. The size distribution of the droplets formed by the used nozzle were obtained by static light scattering. Furthermore, the wetting of the particles with the injected droplets was implemented in the simulation and determined. The additional attractive forces due to the existence of liquid layers after droplet deposition were taken into account based on the model of Tsuazawa et al. (Tsunazawa et al. 2016).
The results were used to determine the boundary conditions under which a successful fluidization and coating of particles in the self-developed spouted bed can be realized. With the help of the simulations the particle movement in this spouted bed was analyzed. Another focus was on the behavior of the particles during the coating process as well as the different contact scenarios (particle-particle or particle-wall contact). The numerical results were compared with the conducted experiments.
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