Özgür Ertunç

In pressure swirl atomizers, a two-phase swirling flow occurs when liquid is introduced through tangential ports into the swirl chamber. Due to the interactions among jets from neighboring inlets, flow instabilities and non-uniform spray patterns can occur. These three-dimensional effects raise concerns about the accuracy of two-dimensional axisymmetric simulations. The authors investigate whether a purely axisymmetric ring-like inlet surface can eliminate flow non-uniformities, although such designs often require exceptional inlet configurations. This study focuses on open-end pressure swirl atomizers, as closed-end designs are more prone to separation downstream of contraction. High-fidelity three-dimensional flow simulations using the Volume of Fluid and Quasi Direct Numerical Simulation methods are performed. While the primary performance parameters—such as discharge coefficient and spray angle—show no significant differences between two-dimensional and three-dimensional simulations, the three-dimensional simulations reveal Görtler vortices within the liquid film. Although these vortices lead to slight differences in velocity profiles, they contribute to a vortical non-uniform velocity field at the injector exit, resulting in wrinkled waves in the spray film. Furthermore, simulation with axial swirling film inlet condition indicates that Görtler vortices can develop without flow separation, still causing spray film wrinkling. These findings suggest that Görtler vortices can negatively impact atomization and lead to non-uniform spray distribution, regardless of the inlet quality.

This paper proposes a drop-on-demand (DOD) system that can produce single droplets of highly repeatable size in the order of 2 mm. This system utilizes an on-the-shelf solenoid injector used in automotive applications. The design methodology is explained along with the necessary measurements and numerical simulations of droplet generation. The invention consists of a solenoid injector that produces monodisperse single or in-series droplets with the help of a developed pulse width modulated signal generator. Mass per injection is measured over a range of supply pressures and injection durations to find the operation window to generate 2 mm droplets. Later, various nozzle geometries are designed and tested by flow simulations. The contracting nozzle is found suitable for generating single droplets, so the design is implemented at the tip of the solenoid injector. The effects of different opening times, pressures, and nozzle’s orifice diameters were tested to observe the operating window of the newly designed DOD system and the repeatability of generated droplets by utilizing a coherent circular Hough transform image processing algorithm to measure droplet sizes. The standard deviation of measured diameters is less than 5% of the mean droplet diameter, which is in the range of 1.68–2.07 mm. Next, the voltage and current signals are measured per injection, and exact instants for the initiation and ending for both opening and closing are determined to construct transient mass flow rate functions for flow simulations in which the dependence of droplet formation on the speed of closing is revealed. The numerical and experimental results indicate the repeatability and consistency of the invention.

Pressure-swirl atomizers (PSAs) are commonly used in aerospace engines and various industrial settings. Even though the design of PSAs is simple, the internal flow dynamics can be quite complex due to the presence of swirling motion, turbulence, and an air core. While certain parameters such as spray angle and droplet distribution have been widely studied and modeled, the circumferential distribution of the atomizer flow is not well understood. The objectives of this study are to identify the cause of non-uniformity in the spray produced by PSAs and to develop a uniformity model to predict the level of circumferential patternation of spray. To accomplish this, 3D numerical simulations were run on a range of PSAs, and three atomizers were fabricated using laser-lithography to ensure maximum manufacturing precision. Of the fabricated atomizers, two exhibited uniform distributions, while the other had deficiencies in both internal and external flow fields. LIF/Mie experiments on a radial plane and high-speed images on central axial plane of the manufactured atomizers were conducted.The results indicated that non-uniformity in the internal flow greatly impacts downstream droplet distribution. The root cause of non-uniformity was determined to be the interaction of inlet flows as they enter the swirl chamber. A new semi-empirical uniformity model, based on experimental data, was developed and found to be more accurate than existing models.