NUMERICAL SIMULATION OF AERODYNAMIC AND ELECTROSTATIC EFFECTS IN A POWDER COATING FLUIDIZED SYSTEM

The work described in this thesis was carried out for a better understanding of the phenomena in an electrostatic fluidized bed for powder coating. The publications in different areas of science (Computer Science, Chemical Engineering and Electrical Engineering) have been of great help to obtain a final written computer code that combines novel computational methods which aim at simulating the physical effects that occur in any electrostatic fluidized system. One of the main purposes of existing industrial fluidized beds is the coating of metallic pieces. The process to achieve this includes using insulating powder of a particle size appropriate for fluidization. The metallic piece to be coated has to be pre-heated substantially above the melting point of the powder coating material in order to obtain enough enthalpy to cure the required mass of coating insulation. Once this appropriate temperature is reached, the piece is dipped in the fluidized bed for the length of time necessary to acquire a uniform coating. This way of coating in industrial fluidized beds is very inefficient since energy is spent pre-heating the metallic piece and because usually once the dipped piece has been coated, it is placed in an oven again for a new heating period, to finally obtain a uniform surface coating. All these procedures involve substantial increases in energy costs. A new method to obtain uniform powder coated pieces with minimal energy costs was thought of. This idea implies the electrification of the insulating powder particles. In this case if a cold metallic electrically connected to ground piece is dipped in a fluidized bed,\nthe electric field generated by the space charge in the bed will propel the charged particles towards the piece to be coated and remain attached to it. Once taken out of the fluidized bed, the charged particles remain attached (due to the Coulomb attraction force) while the piece is taken to the oven for curing. In 2004, experiments were carried out at the Applied Electrostatic Research Centre (A.E.R.C.) at the University of Western Ontario. An experimental fluidized bed with powder particles was used which included a suction pump system. Through this suction system fluidized powder particles were sucked from the system, and forced to travel around a Teflon tube. Due to the difference of work functions the particles acquired a high positive charge. Furthermore, the particles were injected back into the system, increasing the total net charge of the system. Thus, so when a metallic electrically earthed piece was dipped, due to the Coulomb attraction forces between the charged powder particles in the system and the electrically earthed piece a good surface coating would be obtained. Results from these experiments showed that during injection the total net charge of the system increased for a certain period of time and then gradually decreased. It was concluded that the electrification (injection of highly positive charged particles) of the bed powder had to be carried out only during the time that the piece to be coated was dipped in the bed; otherwise a good coating was not possible. These unexplained findings are the basis for this thesis. In this project, a numerical complex simulation of a fluidized system was performed, for a system with three different size particles (60 pm, 80 pm and 100 pm). Throughout this thesis snapshots of these particles’ positions are shown to better visualize their trajectory paths. After a certain period of time the system acquired a total net charge due to the tribocharging process, at this point a new set of highly positively charged particles (120 pm) was injected. It was expected that the total net charge of the system increases accordingly to the total charge of these new injected particles, however, the total net charge increased up to a certain level and then it seemed to fade away as first stated by the results of the A.E.R.C.’s experiments. The simulations performed in this work tried to reproduce the conditions set in the experiments done in the A.E.R.C. (2004). One of the main achievements was the visualization of the trajectories of all the particles in the system. Appendix 1 presents snapshots of this visualization. Additionally, Chapter 6 shows other insightful results. As mentioned earlier, one of the main objectives of this thesis was to find out why the most efficient coating occurred only when the metallic piece and the positive charged particles were dipped and injected simultaneously. Analyzing the video and graphs obtained the conclusion was that if the injection of charged particles had been done prior to the dipping of the metallic piece, part of the total net charge would have gradually discharged into the electrically earthed walls. Therefore, by the time the earthed metallic piece was dipped the total net charge of the system was not going to be as high as expected which consequently would have ended up in a poor coating.

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