International Journal of Multiphase Flow, Vol.82, 119-142, 2016
Periodic structure of the dispersed phase in a forced jet and their effects on the particle dispersion
This paper presents an experimental study on an acoustically forced particle-laden jet. The acoustic disturbances cause a train of strong periodical vortex due to the selected frequency and its high excitation level. The jet Reynolds number is not small (Re = 11,900) and the particle Stokes number is about one, responding partly to the forcing. The flow was tested using a Phase Doppler Anemometer (PDA). The paper includes measurements of both gas and dispersed phase over the whole forcing cycle. An external post-processing (developed by the authors) carefully corrects the bias inherent to the operation principles of the PDA in all supplied averages (including the phase-averaged values). This post-processing gives also some variables which were defined ad-hoc to characterize the periodic structure of the flow. Such information is never given in the previous literature. This work continues a previous study done by the authors. Measurements detect three axial zones. The strong periodic gas vortices control the flow in the area close to the nozzle exit. They generate highly concentrated clusters of particles as well as tongue-shaped structures of radially ejected particles (or radial streaks). Downstream, the gas vortices vanish and inertia plays a central role in the development of the dispersed phase. The particle clustering ends here. Finally, all periodic motion disappears and flow degenerates into an unforced two-phase jet. Radially, the inertial zone of the particulate phase covers the outermost layers. The influence of the particle size is also discussed. The radial dispersion of particles across certain section is quantified by means of a suitably defined parameter. This dispersion radius was measured at the end of the area disturbed by forcing for both the forced and unforced jet. Thus, the comparison assesses the total effect of forcing on the transversal dispersion. The dispersion of the whole size distribution and of each particle size is quantified. Results show that forcing enhances the dispersion and it is controlled mainly by the periodic streaks while turbulence has a secondary role. The streak shape is accurately computed from the measurements and its extension has been successfully related with the particle's history and it size by means of a suitably defined Stokes number. Finally, this study supplies a set of high quality data useful to validate inherently unsteady numerical models. As stated by other authors, there is a lack of periodic well-characterized experiments for validation purposes which mimic the interaction between the particles and the large scales of turbulence. (C) 2016 Elsevier Ltd. All rights reserved.