Turbulence in pneumatic flow focusing and flow blurring regimes.

An important paradigm in pneumatic atomization is the production of droplet sizes in the micron and submicron range, while achieving high energy efficiency by means of simple atomizer designs. Flow focusing (FF) and flow blurring (FB) [A. M. Gañán-Calvo, Appl. Phys. Lett.86, 214101 (2005).] are advancements toward this goal. Both FF and FB feature a fundamental macroscopic soft length scale, e.g., the diameter of the liquid stream formed at a discharge orifice by conversion of pressure into liquid kinetic energy. Droplet diameter distribution data compiled from many experiments reveal that turbulent flow regimes occur in both FF and FB. In FF, like in other jet-based droplet generation techniques, the jet breakup becomes asymmetric for Weber numbers over a transitional one (approximately 20 in FF), becoming turbulent through nonlinear interactions with the gas, downstream of the discharge orifice, for large enough Weber numbers. In FB, the liquid and gas phases interact inherently in a turbulent manner: air accelerates radially and implosively toward the liquid exiting a feeding tube, and mixes with it in a region immediately preceding discharge into ambient air. In our model, droplets form by the action of turbulent pressure fluctuations present in both phases, and a resulting droplet diameter distribution is obtained when coagulation and breakup events of the liquid blobs equilibrate. When the large scale of the turbulent inertial range is taken to be the fundamental soft scale, the model predicts a lower bound to the experimentally determined droplet volume median diameters. On the other hand, the histograms reflect the existence of additional hard length scales imposed by the atomizer outlet geometry.