# Experimental Study of Primary Atomization Characteristics of Sonic Air-Assist Atomizers
###### Abstract  
The present study compares two twin-fluid atomizer concepts based on the airflow (shock waves) pattern obtained through shadowgraph imaging for atomization of water with a low air/water pressure supply. The research work was conducted using the backlight imaging technique for converging (sonic) and converging-diverging (supersonic) air-assist atomizers with a 3.0 mm (throat) diameter. An annular sheet of thicknesses 70 \(\upmu\)m and 280 \(\upmu\)m with a high-speed air-core was employed to study the breakup dynamics for different water mass flow rates (100-350 kg/h) and air mass flow rates (5-35 kg/h). Different sheet breakup patterns were identified as the function of the ALR ratio (air-to-liquid mass flow), liquid Weber number (\(\
u_{\text{HeL}}\)), and Reynolds number (Reg). Different breakup modes extend from canonical Rayleigh bubble breakup, ligament-type breakup, to the pure pulsating breakup via annular sheet disintegration. The sheet breakup dynamics were studied in terms of spray angle and breakup length. With higher ALR values, breakup length showed a decreasing trend, while spray angle showed an increasing trend in the converging and converging-diverging (CD) air-assist atomizers, respectively, owing to the drastic difference in the jet flow dynamics.  
sonic air-assist atomizers; sheet breakup; breakup morphology; annular sheet +
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sheets, especially in an annular sheet case such as that investigated by [PERSON] et al. [8]. [PERSON] et al. [3; 9] thoroughly studied the breakup based on air-liquid momentum and found modes such as Rayleigh, bubble, christmas tree, and pure pulsating. [PERSON] et al. [10] discerned the breakup modes--bubble, christmas tree (cellular), and fiber breakup--based on morphological differences. Recent studies [11; 12] also employing the central air core demonstrated that the swirling of annular liquid sheets does not significantly influence the sheet breakup dynamics. However, these studies were performed without considering the air-assist mechanism. [PERSON] et al. [13] and [PERSON] et al. [14] first investigated the sonic atomization concept to study effective atomization with a liquid jet using shock wave dynamics in underexpanded or overexpanded flows. Though the Sauter mean diameter decreases after the advent of shock patterns, it still questioned the use of a transonic or supersonic jet with the aim of optimal atomization. Lately, some researchers [15] have studied the gas flow field distribution effects of transonic flow on the liquid metal atomization, finding that the higher gas flow velocity in converging-diverging (CD) atomizers results in smaller particle sizes.  
This study aims to discern the various breakup modes or patterns in light of effective atomization using sonic and supersonic flow, as they depict different shock dynamics. This paper contains the experimental findings using a novel concept of sonic atomization employing an annular liquid sheet, whereas earlier studies employed a two-dimensional sheet [13] or jet [16]. Different breakup modes of the liquid sheet with co-flow air were observed and subjected to different air and water flow rates (air-to-liquid ratio (ALR)). Spray breakup dynamics was investigated in terms of spray angle and breakup length with different sheet thickness using converging and converging-diverging (CD) atomizers.