Volume of fluid and continuum surface force methodologies were applied to two- and three-dimensionally model the motion of a liquid jet injected vertically downward from a rectangular nozzle into another immiscible liquid. Grid independent solutions were obtained for a 10?mm2 nozzle with aspect ratios in the range 110. It was found that unlike the 3D simulation, the 2D CFD model was not able to predict the necking and breakup features observed in the experimental system. The 3D model showed that upon exiting the rectangular nozzle the liquid jet underwent a transition before becoming circular in cross-section and eventually reaching an equilibrium diameter prior to breakup into droplets. For a given nozzle geometry it was found that equilibrium jet diameter increased with increasing liquid volumetric flowrate, with good agreement between CFD simulations and experimental observations. The 3D model was applied to rectangular nozzles with different aspect ratios and it was found that for a given liquid flowrate there was an optimum aspect ratio for generating minimum-sized droplets, which was approximately 30% less than for a circular nozzle with the same cross-sectional area. (c) 2011 Canadian Society for Chemical Engineering