NUMERICAL-SIMULATION OF PARTICLE FORMATION AND GROWTH IN RAPIDLY EXPANDING AXISYMMETRICAL FLOWS

Results are presented on a systematic investigation of particle formation and growth by homogeneous nucleation and condensation in axisymmetric free jets. The model is validated insofar as possible by comparison with available experimental data. Most of the previous models for particle formation in supersonic jets have been one dimensional. It is shown that one-dimensional models do not completely represent the physics of the flow and therefore cannot display a variety of phenomena found in condensing axisymmetric free jets. It is demonstrated that, in axisymmetric free jets, particle formation occurs initially off-axis due to the cooling in the radial expansion at the nozzle exit. The region in the jet with the largest average particle diameters can be off-axis or on the centerline depending on the stagnation conditions or, equivalently, the dominant particle growth mechanism. At higher stagnation temperatures, largest diameters occur on the centerline, while for lower stagnation temperatures average particle diameters are highest off-axis. These observations on particle growth characteristics in the jet are from this model study; experimental data for confirmation are not available. An increase in stagnation pressure is equivalent to an increase in nozzle diameter with respect to the temperature field in the expanding jet. In scaled coordinates, the location where nucleation is initiated is invariant with nozzle diameter and the subsequent flow is not self-similar.