Numerical simulation of mechanofusion system for dry particle coating process
Department of Mechanical Engineering
Doctor of Philosophy
Dave, Rajesh N.
Rao, I. Joga
Discrete element method (DEM)
Dry particle coating
A numerical simulation of the Meclianofusion device for dry particle coating is carried out using Discrete Element Method (DEM) technique. In dry particle coating, tiny sub-micron guest particles are coated onto larger micron sized host particles by using mechanical forces, which the Mechanofusion device provides. It consists of a rotating cylindrical chamber that contains the powder mixture, a stationary inner piece (which has a cylindrical surface at the end), and a fixed scraper blade, which prevents powder from caking against the chamber wall. The simulation studies are performed on two scales: system scale to investigate the particle-particle and particle system interactions, and micro scale to study the degradation of agglomerates prior to the dry particle coating process.
The system-scale simulation is based on a mono-dispersed system where only host particles are taken into consideration. The particles are assumed to be frictional, elastic-plastic spheres. A widely used, non-adhesion Walton-Braun. contact model has been applied in two-dimensional configuration. Two simplified geometric models of the Mechanofusion chamber with and without the scraper have been studied. The visualization of the particulate patterns inside the system and the diagnostic analysis derived from the numerical simulations clearly demonstrate the effect of scraper on the system. The forces acting on the inner piece are calculated and compared with the experimental results available from the literature. Average forces on particles due to interactions with other particles and vessel parts are also calculated and categorized into four regions. The effect of particle properties on coating level is examined through a simple deformation analysis. In addition, minimum coating time is estimated. Furthermore, the important parameters that affect the system performance are studied.
Another important contribution of the dissertation lies in the dimensional analysis of the Mechanofusion system carried out on the basis of kinetic theory under the assumption of collisional flow, verified qualitatively by simulations. An equation for average force on particles inside the system is derived to establish the correlation between a simulated system and a real system. Major kinetic theory modeling based similarity results, verified by simulations and in part from available experimental data, include: (1) Inter-particle (host-host) forces vary linearly with the rotation speed; (2) Force exerted on the particles within the inner-piece zone is inversely proportional to the gap-size; and (3) Force on the inner-piece varies linearly with the square of the rotation speed.
Based on the results from system simulation, the fracture/ fragment at ion of ail agglomerate during normal interactions with host particle and with system wall is examined in detail by the micro-scale simulation. The numerical study is implemented using a DEM code (developed by Prof. Thornton's group at Aston University) in two-dimensional mode, which enables the simulation of auto-adhesive particles. The study shows that single agglomerate may fracture or even shatter inside the system as a result of interactions with the host particles and system boundaries. The fracture pattern of the agglomerate is in agreement with reported three dimensional simulation results. Results show that higher impact velocities lead to higher local damage and debris formation. However, impact velocities as low as 0.1 m/s lead to fracture in some case. In most cases, impacts at velocities of I m/s and higher lead to shattering of the agglomerate.
In summary, the work presented in this dissertation, which is one of the first reported work on DEM simulation of dry particle coating system, shows that DEM technique can be used to model important aspects of the Mechanofusion system, such as the salient pattern of particles inside the system and the overall system performance as well as the agglomerate fragmentation prior to the dry particle coating process.
njit-etd2002-015 (159 pages ~ 8,556 KB pdf)
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Created March 8, 2003