Aerodynamic focusing of particles in viscous jets

Abstract

The use of particle inertia to bring particles seeded in a viscous, incompressible jet to a sharp focus is considered. Earlier work on aerodynamic focusing in seeded supersonic jets has involved relatively large Reynolds numbers Re (where viscous effects due to the nozzle walls are negligible), and has reported sharp focal points for particles characterized by a Stokes number exceeding a certain critical value S*. Here a rapidly converging nozzle geometry is shown to yield similarly sharp aerodynamic focusing of particles in sheathed subsonic aerosol jets at Re as low as 15. The diameter d of the particle deposits collected on a greased impaction plate placed normally to the aerosol jet is measured as a function of the distance L between the nozzle and the collector. A numerical solution of the steady, axisymmetric, incompressible Navier-Stokes equations along with extensive particle trajectory calculations complements the available experimental data. In the range of Re for which experimental data are available (15 < Re < 300), the measured particle deposit diameters agree reasonably closely with the calculated values, and only a mild Re effect on d is observed. In this regime, provided a sheath air fraction of 50% is used, concentration of the particles by a factor on the order of 1000 is observed in the focal region, comparable to that previously obtained in supersonic seeded jets. Numerical calculations performed for Re < 15 show that aerodynamic focusing arises even at Re = 3. The presence of the collector plate is shown numerically to have little effect on the particle aerodynamic behavior in the focusing regime (i.e. S>S∗).