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Numerical Study to Investigate the Effect of Lens and Nozzle Geometry on Aerodynamic Focusing Lens Performance
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During the sampling of atmospheric aerosol particles from ambient environments, particles must be separated from bulk fluid flow into a narrow flight path in order to perform accurate mass spectrometry under vacuum. To establish and maintain a low-divergence aerosol flight path, an aerodynamic focusing lens system (AFL) is used. A series of aerodynamic lenses are employed to moderate the pressure drop along the length of the system and separate nano- to supermicron-particles from the bulk fluid. AFLs require a steady laminar flow to enable particle convergence along the center axis, and thus the generation of turbulent eddies is to be altogether avoided. The objective of this thesis is to enhance the performance of AFL inlets over a wide range of particle sizes (from 10 nm to 10 µm) by (a) investigating the effect of aerodynamic lens and nozzle geometry on the particle flow and (b) investigating the early signs of laminar-to-turbulence transition in the AFL inlets. Three geometric modifications within the selected AFL inlet were analyzed: varying half-angles of a divergent lens, varying the length and diameter of the capillary step of the accelerating nozzle, and varying the nozzle length. The Reynolds number threshold that leads to turbulent structure generation in the selected AFL inlets is also studied. The simulation results can be used to enhance the design of AFL inlets for a particle size range of approximately 10 nm to 2.5 μm, with the possibility of extension to 10 μm.