Characterization and Control of Electrostatically Assisted Two-Fluid Coaxial Atomization

Abstract

Multiphysics control of atomization is a promising new area of research that could result in more robust and versatile atomizers, producing a desired range of droplets under a very broad range of operating conditions. This would enable their use in applications that require the atomization process to operate within tight bounds under the influence of a changing environment and external perturbations. This dissertation studies the physics behind two-fluid coaxial atomization and transport of droplets in a turbulent gas jet, combined with the physics of electrostatic forcing. The effects of electrostatic forcing in the primary atomization, the resulting droplet population and the transport in the turbulent spray are studied over a wide range of gas-to-liquid momentum ratios, gas swirl ratios and electric field strengths. Based on the fundamental understanding of this complex multiphysics problem, a practical implementation of real-time electrostatic feedback control, based on sensing of the spray liquid distribution, is also demonstrated. First, the role of the electric field geometry in the primary breakup of coaxial atomization is studied. Evidence is presented for strong electric stresses with an axial component opposing the flow direction that increase the growth rate and decrease the wavelength of the interfacial instabilities initiated by the shear at the gas-liquid interface. This also results in reduced liquid core lengths and smaller droplets. In contrast, a predominantly radial electric field was shown to have little effect on the metrics mentioned above. The electrostatically assisted coaxial atomizer system at the basis of this thesis consists of a canonical coaxial nozzle enclosed between two large metallic plates held at a high electric potential. Atomization of an electrolyte solution revealed a combination of aerodynamic and electrostatic breakup mechanisms. For momentum ratios much lower than the electric Euler number, electrostatic processes were observed to dominate breakup. At high momentum ratios, aerodynamic breakup dominated the large scale features of the spray in the near field, but the electrostatic stresses gave rise to small-scale features, indicative of electrostatic breakup of small ligaments. Interferometric measurements in the mid field revealed substantial decreases in the droplet sizes due to electrostatic forcing. Substantial radial transport that modified the mean diameter and concentration profiles of the spray was achieved for coaxial atomization at many swirl ratios. We observed preferential radial transport of small droplets, consistent with electric charge densities following a power law of the diameter given by the classical Rayleigh limit for charged drop stability. Finally, real time feedback control is demonstrated. Proper orthogonal decomposition was used to characterize optical scattering and attenuation signals identifying the liquid distribution in the spray.