Two numerical models were developed to simulate the sauter mean diameter (SMD) of the droplets during atomization and the growth of particles inside the flame by coagulation and sintering. These models were linked to CFD to simulate flame spray pyrolysis (FSP) process. The effects of reactor geometries and processing parameters on the temperature and velocity profiles, droplet evaporation and particle growth were predicted using the validated computation models. The results show that increasing the oxidant gap size (from 0.1 mm to 0.5 mm) by keeping the dispersion gas pressure drop constant at 1 bar (transonic regime similar to 310-315 m/s) across the nozzle tip increased the gas to liquid mass ratio (GLMR) by a factor of 6. This reduced the flame height and lowered the residence time of the particles in the high temperature zone of the flame, thus, decreased the sintering rate and the growth of nanoparticles. The results also showed that decreasing the oxygen content of the dispersion gas helped to decrease the peak temperature of the flame and reduced the particle size. The simulation results can be used for the FSP equipment design and process optimization. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.