The Tribojet is a two fluid atomization unit for liquid melts. It is made of a prefilming nozzle, with primary and secondary gas injection and an internal as well as an external mixing area. A lot of experimental data atomizing water in a model plant are available, but several near nozzle effects are still undiscovered. To gain a deeper understanding of the flow phenomena within the Tribojet Computational fluid dynamics (CFD) simulations were done with Fluent 6.1. The gas flow through differently shaped nozzles as well as the interaction of the air stream through the wire tube, the primary spray and the runout was investigated. The volume of fluid (VOF) method is applied to study the influence of different operation condition on the thickness and formation of the liquid film. It was found that the application of the VOF model is restricted for the given problem to a two dimensional treatment due to the enormous computational effort. A discrete particle model (DPM) is used to explore the behaviour of water droplets in the spraying region. The results are compared with particle image velocimetry (PIV) measurment and yield a good qualitatively agreement. Some optimisation potential of the Tribojet layout was identified and the successful application of CFD to near sonic two fluid atomization processes was demonstrated.