The two phase flow in the premix duct of a LPP combustor is computed using a Lagrangian droplet tracking method. To reproduce the characteristic spray structure of an air-assisted pressure-swirl atomizer, a sheet spray model is de-rived from measured sheet parameters and combined with an advanced concept for modeling secondary atomization effects. The sheet spray model is used for the discretization of the initial sheet fragmentation, whereas modeling of droplet deformation and breakup during trajectory integration accounts for air-blast effects on size distribution and dispersion of the spray. Droplet drag increase due to subcritical deformation is approximated by a semi-empirical correlation. Breakup modeling addresses the three distinct mechanisms bag, multimode and shear breakup which are typically encountered in fuel-air mixing processes of combustion engines. The models include mechanism-dependent deformation and breakup times, correlations for drag increase as well as size and velocity distribution functions of droplet fragments. Results of the simulation are compared to Phase Doppler measurements of the spray providing local distributions of droplet sizes, velocities and size-dependent liquid volume flux.