Abstract
The mechanisms of interfacial instability due to gas-liquid shear and liquid ligament acceleration that occurs in the near-field of a coaxial two-fluid atomizer play a determining role on the spray characteristics in the far-field, making understanding of these near-field physics key for spray modeling. Several metrics for these physical processes, with an emphasis on the liquid core length, are characterized in detail, using high-speed shadowgraphy, over a wide range of gas-to-liquid momentum ratios, comparing the fluids’ dynamic pressures at the exit of the atomizer. A method that does not require arbitrary thresholds is proposed to rigorously define the initial spreading angle of a spray. The effect of adding azimuthal momentum to the gas co-flow (swirl) on the spray near-field is analyzed, and the possibility of periodic oscillations of the swirl is explored. Increased spreading angle is expected when swirl is present, but a decrease of the liquid core length average and standard deviation is also observed, with variations happening over timescales shorter than the actuation period. The overall effect is a wider, more dynamic spray, that presents a dramatic decrease in characteristic droplet sizes. At high momentum and swirl ratios, while the effects on the liquid core length follow similar trends, the temporal evoluation is largely modified by the development of long-time dynamics, attributed to a vortex formed by recirculating gas downstream of the liquid core on the spray centerline, that becomes unstable and penetrates inside the liquid nozzle. Oscillations of the swirl have little effect on the mean spray structure, as the oscillation frequency is decorrelated from the spray timescales. While the liquid core length may seem to present quasi-steady dynamics, the maxima never exceed the lengths obtained for a steady case, when the swirl equals the value at the trough of the oscillations, while the minima are below what would be observed for the spray under steady conditions equal to the peak of value of the swirl and momentum ratios. Variations of the intact length around the quasi-equilibrium state present similar magnitude and timescale as the steady cases.
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