A twin-fluid pneumatic injector system with coaxial liquid and air streams mixed slightly upstream of the outlet, is studied in a cross-flow arrangement to understand its working principle. The gap between the orifice and the liquid tube, where the air and liquid cross-flow occurs, is known to determine the modes of working of the atomizer for the given operating flow rates of air and liquid. The two distinct modes, namely the flow focusing (FF) and flow blurring (FB) are observed in the present study, and the transition from FF to FB mode is explored using optical visualization. The characteristics of the internal two-phase flow mixing that occurs due to backflow recirculation in FB mode are also investigated by performing large eddy simulation (LES) using the volume of fluid (VoF) approach. The results of the numerical simulation are validated with experimental observations for the length of backflow recirculation observed in the liquid tube. The present study demonstrates the development of multiple microjets, which explains experimental observations of the lack of core jet and the almost uniform distribution of radial Sauter mean diameter (SMD) reported in the literature. The high-resolution backlit image of a scaled-up transparent injector with a 6 mm orifice diameter validates the numerical results that describe the internal two-phase flow. © 2020 Elsevier Masson SAS