Visualization study of annular sheet breakup dynamics in sonic twin-fluid atomizers

The experiments in the present work have been performed to examine the effect of high-speed air-jet, ejected from the converging or converging–diverging (CD) atomizer on the annular sheets of 70 µm and 280 µm thickness. Different orifice diameters (2.0 mm, 4.0 mm and 5.0 mm) for the airflow were utilized. Two imaging techniques were employed- shadow imaging to visualize the air jet and backlight shadow imaging to study the primary breakup of the sheet. The difference in the sheet breakup mechanics is discerned due to the peculiar flow dynamics of the converging and converging–diverging (CD) jets, as in the former case, initially Prandtl–Meyer expansion waves were formed, while an oblique shock pattern was observed in the latter. The interfacial contact strength is the governing factor in the breakup dissimilarity in the location of the neck formation region. The stripping mechanism is observed in both converging and converging–diverging (CD) atomizers. The bursting phenomenon was also observed, and the bursting frequency was measured and non-dimensionalized using sheet thickness and sheet velocity. Strouhal number (St) showed an increasing trend with the increase in the air-to-liquid ratio (ALR) for both types of atomizers. The bursting phenomenon is more pronounced in converging–diverging (CD) atomizers, corroborated by the larger Strouhal number (St) values. Graphical abstract