Particle Dispersion Enhancement in the Near Region of a Forced Jet

The present study examines the effects of periodic and random excitations on particle dispersion in the near field of a transitional axisymmetric jet. The study is motivated by the consideration that particle dynamics in the near-jet region is governed by vortex structures, whose behavior, in turn, can be altered through external excitations. A large eddy simulation model based on a fourth-order phase-accurate scheme is employed to simulate the dynamics of vortex rings in an unforced axisymmetric jet, and obtain the dominant frequencies associated with the vortex rings. These frequencies are then used for a periodic forcing of the jet to examine the effects of forcing amplitude and frequency on particle dispersion. A randomly forced jet is also considered to investigate whether exciting all of the dominant frequencies simultaneously can provide greater particle dispersion compared with the single-frequency excitation. Results indicate that external excitation generally leads to higher particle dispersion, with the gain in dispersion increasing with the forcing amplitude. Not only does the particle dispersion exhibit size-selective behavior for both the unforced and the forced jets, but also the dispersion enhancement exhibits a size-dependent behavior, maximizing near a Stokes number of unity. Comparison of randomly forced and periodically forced cases indicates that a single-frequency forcing is more effective in enhancing particle dispersion compared with multiple-frequency forcing. In addition, the preferred mode forcing has the maximum effect on particle dispersion, followed by forcing at the first-pairing and roll-up frequency, respectively. Based on the spectral and flow visualization results, we attribute this behavior to the fact that the preferred-mode forcing makes the second-pairing interaction become more organized and occur closer to the nozzle. This suggests that an effective method of actively controlling particle/droplet distribution in a combustor is to control the attributes of vortex rings, such as their size, frequency, and locations of pairing interactions.