Development of simplified models for air diffusers is essential for the applicability of CFD-modelling to room air flow simulation. The geometry of a diffuser is usually complex and, therefore, it is not possible to use an exact geometrical model in practical CFD-simulations. The model should be simple enough to enable the modelling of a room with several diffusers. In this study, a simplified model was developed for the nozzle duct diffuser. The diffuser consists of a duct, which is partly covered with small nozzles creating high-velocity jets. A complex circulating flow pattern is created between the jets. The room air enters the flow from the nozzle-free sector and is mixed with the supply air. The flow field near the diffuser was investigated with laboratory measurements in order to determine the boundary conditions for the simplified model. The developed model is based on momentum sources describing the induction of secondary air caused by the jet system. The model was validated with laboratory test cases by comparing the simulation results with measurements. The measured flow patterns in the four test cases with different Archimedes numbers were well reproduced by the simulations. (C) 2004 Elsevier B.V. All rights reserved.