Drying 2004 – Proceedings of the 14th International Drying Symposium (IDS 2004) São Paulo, Brazil, 22-25 August 2004, vol. A, pp. 341-349 341 VALIDATION OF THE LAGRANGIAN APPROACH FOR PREDICTING TURBULENT DISPERSION AND EVAPORATION OF DROPLETS WITHIN A SPRAY Justin J. Nijdam 1 , Baoyu Guo 1,2 , David F. Fletcher 1 and Timothy A.G. Langrish 1 1. Department of Chemical Engineering, University of Sydney, NSW 2006, Australia E-mail: nijdamjj@chem.eng.usyd.edu.au 2. School of Materials Science and Engineering, University of New South Wales, NSW 2052, Australia Keywords: spray dryer, computational fluid dynamics, particle drying ABSTRACT The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modelled reasonably well, although the downstream velocity decay of the larger droplets is under-predicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly towards the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analysing particle trajectories and drying within the more complex spray dryer system. INTRODUCTION Spray drying is the process of atomising a liquid containing dissolved solids within a flow of hot air, where moisture is progressively evaporated from the droplets until dried particles are produced. This process is normally carried out within a drying chamber, which is specially designed to provide sufficient droplet/particle residence times for a given heat load so that the powder collected at the exit point of the