Department of Mechanical Engineering,Shiraz University
A computer code for axisymmetric modeling of nano- and micro particle motions in an aerodynamic particle beam focusing system was developed. The effectiveness of the focusing system, consisting of several lenses, a nozzle and the chamber downstream of the nozzle, was analyzed. The code included an accurate model for the Brownian diffusion of nano-particles in sharply varying pressure fields in the aerodynamic lens system. Assuming an axisymmetric condition, the compressible airflow and thermal field in the lens were evaluated. A Lagrangian particle trajectory analysis was performed, assuming a one-way coupling model. The particle equation of motion used included drag and Brownian forces. Trajectories of different size nano- and micro particles in an aerodynamic lens were analyzed and the particle beam focusing process was studied. The numerical results for particle velocity, collection fficiency and beam diameter were compared with the experimental data and good agreement was observed. The importance of the accuracy of the Brownian diffusion model, for predicting the focusing performance of aerodynamic lenses in the focusing of nano-particles, was discussed. The simulation results showed that for particle diameters less than 30 nm in air, the Brownian force could significantly affect beam focusing and particle collection efficiency.