In water mist fire suppression, where the attempt is to reach a fine mean drop size and size distribution, there is a vital demand to produce a controllable and non-pulsating spray. Effervescent atomization has better performance in terms of smaller drop size and lower injection pressure. In order to study the effects of the internal flow patterns on the droplet characteristics, the regimes of two-phase flow inside the final discharge orifice of the effervescent atomizer are investigated using computational fluid dynamics. In this paper, the volume of fluid techniques in the ANSYS Fluent software package are used to capture the gas liquid interface. The internal flow patterns and the characteristics of the water film near the final discharge orifice are studied over a range of gas-to-liquid mass flow ratios (GLRs). Several 0.41 mm circle final discharge orifices with length/diameter (L/D) ratios of 5, 10, and 15 are test. The simulation results compare well with the experimental data. Results show that the GLR and L/D are two key control parameters. At low inject levels where bubbly or Taylor flow patterns are observed in the final discharge orifice, increasing GLR will increase the water film thickness gradually. With further increase in GLR, the water film thickness finally tends to a certain limitation. The L/D also is an important parameter that influences the pressure drop of the final discharge orifice but has little effect on the internal two-phase flow patterns and the film thickness. Based on the results of the simulation, recommendations have been made for an effective design of an effervescent atomizer and a technology choice for the water mist fire suppression system.