A coupled 3-D computational fluid dynamics population balance model (CFD-PBM) is developed to simulate the behaviors of Al2O3 inclusions in a Rheinsahl-Heraeus (RH) reactor. This work attempts to integrate the transport phenomena including steel flow as well as the aggregation and removal of Al2O3 clusters. The inhomogeneous multiple size group (inhomogeneous MUSIG) model is first employed to investigate the inclusion behaviors in steel. Compared to experimental data, numerical results show good agreement for the number density, Sauter diameter, size distribution, and mass fraction of inclusions in steel. Meanwhile, the application of the frequently used homogeneous MUSIG model in inclusion behaviors is also evaluated, and it is observed that this method may not be suitable for modeling the inclusion behaviors in steel where large and small inclusions are likely to segregate because of different momentum fields.