Particle dispersion enhancement in the near region of a forced jet

The present study examines the effects of periodic and random excitations o n 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 d ynamics of vortex rings in an unforced axisymmetric jet, and obtain the dom inant frequencies associated with the vortex rings. These frequencies are t hen used for a periodic forcing of the jet to examine the effects of forcin g amplitude and frequency on particle dispersion. A randomly forced jet is also considered to investigate whether exciting all of the dominant frequen cies simultaneously can provide greater particle dispersion compared with t he single-frequency excitation. Results indicate that external excitation g enerally leads to higher particle dispersion, with the pain in dispersion i ncreasing 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, ma ximizing near a Stokes number of unity, Comparison of randomly forced and p eriodically 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 now visualization result s, we attribute this behavior to the fact that the preferred-mode forcing m akes the second-pairing interaction become more organized and occur closer to the nozzle. This suggests that an effective method of actively controlli ng particle/droplet distribution in a combustor is to control the attribute s of vortex rings, such as their size, frequency, and locations of pairing interactions.