# Optimizing the Auxiliary Air Channels of a Vortex Atomizer by 3D Printing Using the Taguchi Method
## 2 Fabrication of Auxiliary Air Channels of the Vortex Atomizer by 3D Printing  
This study adopted 3D printing to fabricate the vortex atomizers for its fast prototyping and low cost. Three-dimensional printing is a versatile, additive manufacturing technique. For economic parts, thermal plastic materials, such as environmentally friendly PLA, are always used. The structural strength of 3D printed parts can be increased by various methods. For example, [PERSON] and [PERSON] (2022) [21] incorporated nylon and fiberglass materials in their 3D printed parts and showed that the fracture resistance could be improved. Alternatively, printing directions also play a role as illustrated by [PERSON] et al. (2022) [22]. They showed that the fracture toughness increased by printing the fibers vertically instead of horizontally. Similarly, [PERSON] et al. (2017) [23] showed that the orientation, layer thickness, and feed rate of PLA samples affected the mechanical propertiesof 3D printed parts. Their results indicated that the upright orientation leads to weaker parts while those of on-edge and flat orientations were stronger and stiffer, yet increasing layer thickness and feed rate decreased ductility. That is, the mechanical properties of 3D printed parts can be enhanced by various techniques depending on the need. In a different perspective, more recently, for two phase flow in complex structures, [PERSON] et al. (2023) [24] successfully altered the surface wettability of stereo-lithography based 3D printing by incorporating ultrasonic-atomized fine spraying. In other words, with the developments of functional materials and printing techniques, the versatility of 3D printing is enormous and is able to fit different applications as reviewed by [PERSON] et al. (2018) [25]. For optimizing the performance of the present vortex atomizer, the key parameter considered was the auxiliary air holes which were fabricated by 3D printing. The physical pictures of the present atomizing nozzle, the disk with the auxiliary air holes, the air flow shield, and the corresponding assembly are shown in Figure 1. In this study, the focus is on the auxiliary air holes. The goal is to achieve a uniform spraying distribution across a larger distance under high pressure by changing the geometry of the auxiliary air holes. The air and liquid flows in the atomizer are shown in Figure 2. The disk with the auxiliary air holes in it was attached to the air flow shield (diameter of 46.4 mm), and together they were fixed to the atomization nozzle by a fixing seal. As shown in Figure 2a, liquid water flowed into the pipe (diameter of 5 mm) at the center of the atomizing nozzle from right to left, branched into two streams, and entered into the vortex swirler in a counterclockwise direction. Inside the vortex swirler, the flow was swirler and exited from the nozzle center hole (diameter of 2 mm) with an increased speed of 6.25 times due to area restriction, reaching 63 m/s. In the meantime, the air flowed into the passage between the air flow shield and the atomizing nozzle (also from right to left) and passed through the holes in the disk downstream of the nozzle with a speed of 4 m/s. The air flow was also swirled in a counterclockwise direction to interact with the already swirled vortex water jet and exited from the shield, as illustrated in Figure 2b. That is, both rotations of the air and water jets are counterclockwise. Thus, droplet atomization was greatly enhanced by rotation to improve the spraying performance. Both the auxiliary air holes and the air flow shield were designed by a KISSlicer CAD tool and made by 3D printing for fast prototyping and lower cost. Five disks having the designed auxiliary air holes were fabricated as shown in Figure 3. The air holes were evenly distributed circumferentially and oriented in such a way that their resulted swirl rotation was the same as that of the liquid jet. The designed data were transformed into the STL format by the KISSlicer software tool for 3D printing.