Laser aided deposition quality largely depends on the powder stream structure below the nozzle. Modeling of the powder concentration distribution rarely relies on the numerical approach partially due to the complex phenomenon involved in the two-phase turbulence flow. In this paper, a numerical model is introduced to predict the particle-gas flow precisely and economically in order to meet the practical requirement for coaxial nozzle design optimizations. This model is able to quantitatively predict the powder stream concentration mode under different outer shielding gas directions and inner/outer gas velocity ratio. The numerical simulation results are compared with the experimental study using prototyped coaxial nozzles. The results are found to match. This study shows that the particle concentration mode is influenced significantly by the outer gas direction and gas flow settings.