Effect of electrostatic forcing on coaxial two-fluid atomization

We present an experimental investigation of the electrostatically assisted primary atomization in a coaxial gas-liquid jet. Shadowgraphy imaging reveals the primary breakup dynamics and allows for a semiquantitative assessment of the resulting droplet characteristics. We describe the statistics of the liquid core in terms of the probability distributions of its length, the spray angle, and the virtual origin. A strong axial electric field reduces the liquid core length over a wide range of gas-to-liquid momentum ratios, whereas a predominantly radial electric field influences the atomization process weakly. We characterize the primary breakup dynamics in terms of two frequencies associated with the liquid mass: the flapping frequency and the droplet-shedding frequency. Using a combination of proper orthogonal decomposition and Fourier transforms in time and space, we derive empirical dispersion relations. Observations of the initial interfacial instabilities' growth reveal shorter wavelengths and higher growth rates in the presence of a strong axial electric field. This enhanced growth of the interfacial instabilities is consistent with a mechanism whereby Maxwell's stresses drive liquid ligaments out radially across the high-speed annular gas jet, thus improving the momentum transfer between the two phases and enhancing the breakup process (producing smaller final droplet sizes). These results illustrate the importance of the electric field configuration to assist two-fluid coaxial atomization and establish electrostatic forcing as a potential feedback control input.