The present work focuses on numerical investigation of the transient spray characteristics for n-octane liquid fuel issuing from a pressure-swirl atomizer into the ambient atmosphere. The computations were carried out using ANSYS FLUENT 14.0 for axisymmetric spray. The continuous phase was simulated using RNG, k-ε model. Primary atomization and secondary atomization were modelled according to the Linearized Instability Sheet Atomization (LISA) model and the Taylor Analogy Breakup (TAB) model respectively. Three different values of Particle Streams (PS) and Breakup Parcels (BP) are used for a given set of injector operating conditions to recognize the respective consequence on spray characteristics. Initially droplet collision and evaporation models are disabled. Near the leading edge of the spray, a vortex cloud is formed causing the small droplets to move upward towards the injector, which can be explained by the motion of the entrained air flow with a recirculating flow pattern in order to conserve momentum. The Sauter Mean Diameter (SMD) for the overall spray was found to reduce with time due to production of smaller droplets. Both of the above trends agree with literature. No significant difference in the result is observed for different PS and BP considered though the results are more sensitive to BP values for SMD. As a next step, the droplet collision model (O’Rourke algorithm) was implemented for a particular grid size. Due to collision, considerable reduction in overall SMD occurs right after injection, however, as the spray develops it approaches the SMD values for the case without collision. Considering SMD at an axial location far away from the nozzle, initially higher value SMD is observed indicating droplet coalescence at the spray tip though for later time SMD is slightly smaller than the non-collision case. The present study includes comparison of the simulation with droplet measurements by PDA, detail discussion on the effect of droplet collision on droplet size, size-velocity correlation, leading edge vortex clouds, and, importantly, the effect of grid size on the results. Also, the influence of droplet evaporation on the spray dynamics and dispersion has been considered. © 2015 International Conference on Liquid Atomization and Spray Systems. All rights reserved.