Electrostatically Assisted Atomization of a Liquid Jet in the Rayleigh Regime
Creators: Bharadia, Bilal
Publisher: name: University of Alberta Library
Year: 2022

<p>The increased demand for metal powders in the additive manufacturing industry requires reliable, economical and efficient methods for producing metal powders with consistent mechanical, chemical and physical properties. Conventional techniques such as gas and water atomization are commonly used to produce metal powders. However, these techniques have many limitations such as particle agglomeration, low particle sphericity, large size distributions, high porosities, and the high cost of inert gas required for atomization. This thesis is thus focused on designing and testing an electrostatically assisted atomization technique, to overcome these limitations. The atomization method is built on uniform liquid jet breakup using mechanical disturbances, and an electrostatic field to stress the jet into a smaller diameter before breakup. </p>

<p>Several experiments were conducted in this study, using water, ethylene glycol and Tin, with orifice sizes ranging between 0.07 – 2 mm. Using the data collected from these experiments,<br>
first the liquid jet formation was tested to ensure that the jet is operating in the laminar Rayleigh regime. Following this, a correlation between the Ohnesorge number and the Reynolds number was generated, allowing the operation velocity to be determined as a function of the thermophysical properties of the liquid and the orifice diameter. The effect of the applied vibration was analyzed next, and it was found that monodispersed droplets were produced with the addition<br>
of a vibration of optimal frequency. Drawing from the first principles of liquid jet break up in the Rayleigh regime, and depending on the liquid flow rate, a formulation has been established correlating the droplet size to the jet velocity and required mechanical vibration frequency. During<br>
each experiment, three different voltages were applied to create an electric field around the jet. It was found that increasing the voltage applied led to a decrease in the droplet size. However, this effect was not very significant. Therefore, to better understand the effect of the electric field (i.e., applied voltage) on reducing the jet diameter, a Bernoulli – Electrostatic model has been created, based on the Bernoulli energy balance, the Young – Laplace surface energy, and the electrostatic pressure balance in electrohydrodynamic atomization. This model shows that increasing the applied voltage (i.e., the electric field strength) leads to a reduction in the jet diameter, and the electric field strength required to constrict the jet diameter increases as the orifice diameter<br>
decreases. The work done in this study was also used to present a design of a functioning electrostatically assisted atomization unit. While the application of an applied vibration will successfully provide monodispersed droplets, the effect of the applied electric field will be insignificant in reducing the droplet sizes when using smaller orifices (&lt; 100 μm), even if a high voltage of 100 kV is used.</p>

<p>Therefore, a better approach to produce monodispersed metal powders would be to operate with smaller orifice sizes (e.g., 0.025 – 0.05 mm), and apply the optimal vibration frequencies<br>
using the correlation generated and presented in this study. With this, droplets of uniform size with low porosities can be created. Furthermore, it would not require a constant supply of inert gas (i.e., low operation cost), and neither will it require expensive pumps or a large atomization unit (i.e., low capital costs).</p>