The fluctuating feed solid contents usually cause instationarities in industrial hydrocyclone separation. This demands a controlling mechanism for stabilizing the separation effect. Understanding the transitional/dynamic behaviour is necessary to design such control mechanisms. In this context, the present paper is aimed to understand the dynamic behaviour of a large-scale 250-mm hydrocyclone in terms of air core development and solid concentration distributions using the modified mixture model coupled with large eddy simulation turbulence model. Numerically predicted two-phase flow field data is validated against laser Doppler velocimetry data at two axial positions. A close agreement is found between the modified mixture model predicted particle classification curve and experimental data. Longer residence time is observed with near cut size particle compared to the and fine size particles. Further, the analysis of the equilibrium radius of different solid particles inside the cyclone in the presence of turbulence is compared with the classical Kelsall experiments and observed that the classification of particles influenced by both the turbulent dispersion and the centrifugal force. Finally, the effect of feed solids concentration (10–50 wt%) on the hydrocyclone performance and particle segregation is also undertaken in this study. © 2019 Curtin University and John Wiley & Sons, Ltd.