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,

The deviations of the glass transition temperature (T(g)) in thin films of an amorphous conjugated polymer poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) are reported. Monotonic and nonmonotonic T(g) deviations are observed in TFB thin films supported on Si-SiOx and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), respectively. A three-layer model is developed to fit both monotonic and nonmonotonic T(g) deviations in these films. A 5-nm PEDOT:PSS capping layer was not found to be effective to remove the free-surface effect in Si-SiOx supported TFB films.

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A canonical co-axial round-jet two-fluid atomizer where atomization occurs over a wide range of momentum ratios: $M=1.9 - 376.4$ is studied. The near field of the spray, where the droplet formation process takes place, is characterized and linked to droplet dispersion in the far field of the jet. Counterintuitively, our results indicate that in the low-momentum regime, increasing the momentum in the gas phase leads to less droplet dispersion. A critical momentum ratio of the order of $M_c=50$ , that separates this regime from a high-momentum one with less dispersion, is found in both the near and far fields. A phenomenological model is proposed that determines the susceptibility of droplets to disperse beyond the nominal extent of the gas phase based on a critical Stokes number, $St=\tau _p/T_E=1.9$ , formulated based on the local Eulerian large scale eddy turnover time, $T_E$ , and the droplets’ response time, $\tau _p$ . A two-dimensional phase space summarizes the extent of these different regimes in the context of spray characteristics found in the literature.

We demonstrate a novel implementation of real-time feedback control on the structure of the spray produced by a two-fluid coaxial atomizer. The ratio of angular to longitudinal gas flow rates, called swirl ratio, as well as the total amount of gas coflow are used as the actuation at the nozzle. The swirling and swirl-free gas flow rate are individually set by the control algorithm, with the control objective set based on an optical absorbance radial profile that is related to the liquid volume fraction across the spray. We analyzed the liquid volume fraction profiles measured in open loop by means of singular value decomposition and principal component analysis (PCA) and found that the different states of the spray across a wide range of operating conditions can be described with fidelity by three principal components. The control algorithm maps the resulting state PCA projections to the control variables. Real time control of the spray is achieved over a wide range of operating conditions (gas-to-liquid momentum 1 − 20 and swirl ratios 0 − 1).

Interfacial instabilities play a major role in breakup events in turbulent multiphase flow. Their role has been clearly identified for two-fluid atomization, and is of paramount importance in spray formation. In planar geometries, Kelvin-Helmholtz instabilities are the main mechanism of creation of a two-phase mixing layer, and information such as wavelengths and frequencies is available in the literature. In cylindrical geometries, the instabilities quickly become three-dimensional and thorough characterization is lacking, despite a wide range of applications using coaxial atomization. We conduct an experimental study of how the interfacial instabilities of a liquid jet surrounded by a turbulent gas co-flow accelerate and develop, before break-up and spray formation. We use high-speed shadowgraphy over a wide range of gas Reynolds numbers to compute the velocity of interfacial perturbations, using Lagrangian tracking, followed by a Eulerian conditioning to obtain local statistics. We identified two regimes of the gradient of the longitudinal mean velocity as a function of the gas Reynolds number: a quadratic scaling at low gas Reynolds numbers and a linear scaling at higher gas Reynolds number. In contrast, the transverse velocity gradients show a linear scaling with gas Reynolds number throughout the studied range.

Electrostatic actuation is used for real-time multiphysics feedback control of two-fluid coaxial atomization. This actuation is added to the modulation of the axial and angular momentum of the turbulent coaxial gas stream, for a total of three actuators to control atomization. We characterize the spray real-time response through optical scattering measurements of radial liquid distribution. We apply principal component analysis on the scattering radial profiles and correlate the first three principal components to the three control inputs. The control algorithm continuously adjusts the three inputs to minimize the difference between a goal radial profile representing the desired spray state and the profile observed in real time. This real-time multiphysics (gas momentum plus electrostatic repulsion) control on the liquid distribution in a two-fluid coaxial spray is a novel contribution to the archival literature on this technology.