<p>Effervescent atomizers play a crucial role in various industrial applications, where their performance is governed by internal two-phase flow dynamics. Among the different two-phase flow regimes, annular flow is particularly effective in producing fine and stable sprays, making it ideal for fuel atomization. This study numerically investigates two-phase flow dynamics within an inside-out effervescent atomizer using three-dimensional simulations based on the Volume of Fluid model. Simulations were performed at a constant liquid flow rate of 0.38&#xa0;L/min and high gas to liquid mass ratios to analyze annular flow behavior within the atomizer’s mixing chamber and discharge passage. The results successfully reproduced the annular flow regime observed in experiments, demonstrating the accuracy of the numerical models. To visualize and interpret the internal flow dynamics, nitrogen volume fraction contours and volume renderings were generated at various time intervals for different aeration levels. The findings revealed that increasing the gas to liquid mass ratio reduces the liquid film thickness while intensifying surface instabilities due to enhanced interfacial shear forces, which facilitate the primary breakup of the liquid film into ligaments and droplets. Additionally, the rate of liquid film thickness reduction diminishes with further increases in aerating ratio, suggesting the existence of an optimal gas to liquid mass ratio for achieving desirable spray characteristics at a given liquid flow rate. </p>