# Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers
## 2 Materials and Methods  
Converging and converging-diverging (CD) atomizers are employed, as shown in Figure 1. The exit (throat) diameter for both the converging atomizer and CD atomizer is 3.0 mm. The exit diameter for the CD atomizer is 6.0 mm, thus the area ratio (A/A\({}^{*}\)) is 2.0. The Mach number for the perfectly expanded flow based on the isentropic relation given by Equation 1 (Section 3.1.1) is 2.19. The maximum Mach number or maximum operational pressure employed for the study is 1.80 or 5.79 bar (g), respectively. The high-speed air core and liquid sheet (annular) interaction are shown in Figure 1c. The schematic for the experimental setup for the gas flow study is shown in Figure 2. A red colour led light (3.0-Watt power) was used as a light source, and the light emitted then passed through a collimated illuminator lens (Opto engineering) of 100 mm in diameter. This lens provides uniform illumination and a reduction in edge diffraction effects. Then,  
Figure 1: Experimental sonic atomizer: (**a**) atomizer body with cap, (**b**) assembled atomizer, and (**c**) nozzle schematic depicting core air and annular liquid flow. after penetrating a high-speed air jet, the light was collected by another telecentric lens of the same diameter connected to the CMOS-based Photron SA-Z camera with an adapter at the end. Shadowgraph imaging was performed at 21,000 frames per second with a shutter speed of ~50 us. The schematic diagram for the backlight shadow imaging setup is shown below (see Figure 3). The atomizers tested were of two types--converging and converging-diverging (CD) air-assist atomizers with core airflow (3.0 mm throat diameter) and liquid (water) injected through an annular gap (coaxial arrangement between atomizer body and the cap). The nozzle was connected at the end of a lance mounted onto the traversing system from Bosch Rexroth (Figure 3). To study the breakup process, two different sheet thicknesses, 70 um and 280 um, were employed to examine the sheet velocity (momentum) effects at employed flow rates. A displacement pump (Froster AS company) supplied the liquid after passing through a filter. The liquid mass flow rate was regulated by altering the frequency of the pump, which was recalibrated for uncertainty in flow rates (<1%) for a given mass flow rate. The air was drawn through an in-house installed compressor with a maximum capacity of 100 psi (7.0 bar(g)). Coriolis type flowmeters from Yokogawa Rotamass and Endress Hauser Promass 83 were used for both air and water flow rate measurements. The spray ejected from the atomizer was collected in a box container; then, it was again pumped to the injector through the hose. Figure 3: Schematic of the experimental setup for sheet breakup (in-flow). Figure 2: Schematic of the shadowgraph setup for the gas flow study (in-flow). The backlight imaging method was adopted to provide the necessary insight into the near-nozzle dynamics. The diffuser screen provided a diffused uniform background for image acquisition with two halogen lights (dedolight dedocool), 250 W each. Photron CMOS-based high-speed camera SA-Z model was employed to capture the images with a 135 mm Nikon Micro lens employed to acquire a field of view (FOV) with a dimension of 15 cm \(\times\) 15 cm at the frame rate of 8000 frames per second with a shutter speed of 125 \(\upmu\)s. However, it is not enough to capture instantaneous images at higher flow rates per the [PERSON] sampling criterion, but instantaneous images that are good enough for the primary breakup study must be captured. Water and air were used as working fluids. The properties of fluids are assumed as per the tabled values at STP (20 \({}^{\circ}\)C), such that liquid viscosity (\(\upmu\)) = 1.0 \(\times\) 10\({}^{-3}\) Ns/m\({}^{2}\) and surface tension (\(\sigma\)) = 0.072 N/m in the current study.