Industrial cyclones, such as gas, hydro and dense medium, are widely used in chemical, mineral and process industries to separate particles from fluids or classify particles by size, density or other solid properties. It is well known that the loading of particles can significantly affect fluid flow in such cyclones but the specific effect can be confusing due to a limited fundamental understanding of the working mechanisms involved. In this work, the problem is partially tackled by analysing the influence of particles of different size on the medium flow in a dense medium cyclone (DMC) by using a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) (CFD-DEM). In the CFD-DEM model, CFD is used to model the motion of slurry medium by numerically solving the local-averaged Navier-Stokes equations at a computational cell scale, facilitated by the Volume of Fluid (VOF) and Mixture multiphase models. DEM is then used to model the motion of discrete particles by applying Newton's laws of motion, facilitated by a coarse-grained (CG) method to model small coal particles. It is found that both the particle and medium flows are sensitive to the particle size. The most notable finding is that spatial distributions of solid flow patterns form due to particles of different size have different trajectories in the cyclone. These lead to significant different distribution of volumetric particle-fluid interaction forces which cause different effects on the fluid flow. The findings are useful for developing a better understanding the working mechanisms of solids loading effects in DMCs and also in other similar swirling multiphase flow systems. (C) 2017 Elsevier B.V. All rights reserved.