Dense medium cyclone (DMC) is widely used to upgrade run-of-mine coal in the coal industry. In practice, different designs of the outlet geometry of the vortex finder are used to achieve different purposes. However, the underlying mechanisms are not well understood. In this work, this phenomenon is studied numerically with reference to the effect of the pressure at the vortex finder. The simulation is carried out by use of a combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) (CFD-DEM). In the model, DEM is used to describe the motion of discrete coal particles, and CFD to describe the motion of medium slurry which is a mixture of gas, water and fine magnetite particles. It is shown that a relatively small change of the vortex finder pressure can cause significant variations of both the medium-coal flow and DMC performance. An important finding is that the flow direction of the axial velocity of the air phase in the "air-core" could reverse (changing from upward to downward) as the vortex finder pressure increases, which results in the downward viscous drag force on coal particles and consequently causes some low density coal to be misplaced to the reject/underflow. This work suggests that the control of the pressure at the outlet of the vortex finder is important for DMC performance. (C) 2011 Elsevier Ltd. All rights reserved.