The effect of the flow patterns inside the mixing chamber on the atomization performance of large-orifice (> 1.2 mm) effervescent atomizers for the low injection pressure range (< 4 bar) was examined experimentally. The transparent mixing chamber has a rectangular cross section (9 mm x 2 mm). The parameters tested were the air/liquid ratio (ALR), injection pressure, and the nozzle orifice diameter. Three different flow regimes were observed: bubbly, annular, and intermittent flows. In the bubbly flow regime, the discharged air/liquid mixture disintegrates into drops through the processes of bubble expansion and ligament breakup. On the other hand, in the annular flow regime, the liquid annulus disintegrates into fine drops by aerodynamic interaction due to high relative velocity between the core gas and the liquid film. In the intermittent flow regime, the bubble expansion/ligament disintegration and the annulus disintegration modes appear alternately. Transition criteria between the two-phase flow patterns within the mixing chamber are proposed based on the drift flux model. Finally, a drop size correlation encompassing all those flow regimes is proposed.