The flow behavior in hydrocyclones is quite complex. The computational fluid dynamics method was used to simulate the flow fields inside a hydrocyclone in order to investigate its separation efficiency. In the computational fluid dynamics study of hydrocyclones, the air-core dimension is a key to predicting the mass split between the underflow and overflow. In turn, the mass split influences the prediction of the size classification curve. Generally in hydrocyclone simulations, assuming low particle volume fractions, the discrete phase effects on the continuous phase have been excluded; therefore, one-way coupling method has been used. Due to high particle consistencies, regions in some cases, especially in underflow areas, excluding discrete phase effects on continuous phase may be ineligible. In this study for an example case by consisting discrete phase effects and using two-way coupling method, simulation accuracy noticeably has been improved. Three models, the model, the Reynolds stress model (RSM) without considering air core, and Reynolds stress turbulence model with volume of fluid multiphase model for simulating air core, were compared for the predictions of velocity, axial, and tangential velocity distributions and separation proportion. Results by the RSM with air-core simulation and two-way coupling model, since it produces some detailed features of the turbulence and discrete phase mode effects, are clearly closer in predicting the experimental data than the other two.