Internally mixing twin fluid Y-jet atomizers are widely used in coal-fired thermal power plants for start-up, oil fired thermal power plants, and industrial boilers. The flow through internally mixing Y-jet atomizers is numerically modeled using the compressible Navier-Stokes equations; wall modeled large eddy simulations (WMLES) are used to resolve the turbulence with large eddy simulations whereas the Prandtl miring length model is used for modeling the subgrid scale structures, which are affected by geometric and operational parameters. Moreover, the volume-of-fluid (VOF) method is used to capture the development and fragmentation of the liquid-gas interface within the Y-jet atomizer. The numerical results are compared with correlations available in the open literature for the pressure drop; further results are presented for the multiphase flow regime maps available for vertical pipes. The results show that the mixing point pressure is strongly dependent on the mixing port diameter to air port diameter ratio, specifically for gas to liquid mass flow-rate ratio (GLR) in the range 0.1 < GLR < 0.4; the mixing port length moderately affects the mixing point pressure while the angle between mixing and liquid ports is found not to have an appreciable effect. Moreover, it is found that the vertical pipe multiphase flow regime maps in the literature could be applied to the flow through the mixing port of the twin fluid Y-jet atomizer. The main flow regimes found under the studied operational conditions are annular and wispy-annular flow.