AERODYNAMIC FOCUSING OF PARTICLES IN VISCOUS JETS

The use of particle inertia to bring particles seeded in a viscous, in compressible jet to a sharp focus is considered. Earlier work on aerod ynamic focusing in seeded supersonic jets has involved relatively larg e Reynolds numbers Re (where viscous effects due to the nozzle walls a re negligible), and has reported sharp focal points for particles char acterized by a Stokes number exceeding a certain critical value S. He re a rapidly converging nozzle geometry is shown to yield similarly sh arp aerodynamic focusing of particles in sheathed subsonic aerosol jet s at Re as low as 15. The diameter d of the particle deposits collecte d on a greased impaction plate placed normally to the aerosol jet is m easured as a function of the distance L between the nozzle and the col lector. A numerical solution of the steady, axisymmetric, incompressib le Navier-Stokes equations along with extensive particle trajectory ca lculations complements the available experimental data. In the range o f Re for which experimental data are available (15 < Re < 300), the me asured particle deposit diameters agree reasonably closely with the ca lculated values, and only a mild Re effect on d is observed. In this r egime, 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 foc al region, comparable to that previously obtained in supersonic seeded jets. Numerical calculations performed for Re < 15 show that aerodyna mic focusing arises even at Re = 3. The presence of the collector plat e is shown numerically to have little effect on the particle aerodynam ic behavior in the focusing regime (i.e. S > S).