Flow Focusing and Flow-Blurring: The In-Between Regime

The so-called two-fluid “flow-blurring” atomizer induces strong interphase constructive interference in a premixing region upstream of the atomizer orifice, resulting in efficient atomization of viscous fuels. Its less energetic counterpart, the “flow-focusing” atomizer, exists with different geometries and/or flow conditions. For a fixed geometry, lower gas flow rates and/or higher biofuel flow rates cause flow-blurring behavior to revert to flow-focusing. In doing so, the premixing region disappears, and atomization quality suffers. With the use of a previously validated volume-of-fluid-to-discrete-phase modeling methodology, a new flow regime is discovered, which lies between flow-focusing and flow-blurring. This intermediate “flow-undulating-mixing-swirling” regime replaces the multiphase premixing region with an oscillating biofuel–gas meniscus driven by concentric radial waves resulting from instabilities at the gas injection annulus. Bubbles form within this oscillating meniscus and remain trapped in the biofuel jet, but there is no orifice exit shattering that is typically associated with flow-blurring. Mixing across lateral axes and swirling are also exhibited in the biofuel jet due to secondary azimuthal and axial waves. While frequencies in the 1000 Hz range characterize primary radial wave formation, a wide range of timescales evolve as a function of distance from the bubbling/meandering meniscus.