Numerical modeling of a particle separation process is carried out to understand the gas-particle two-phase flow field inside a cyclone prolonged with a dipleg and results of the numerical simulations are compared with experimental data to validate the numerical results. The flow inside the cyclone separator is modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow is predicted by solving Navier-Stokes equations using the differential RSM turbulence model with nonequilibrium wall functions. The second phase is modeled based on a Lagrangian approach. Analysis of computed results shows that the length of the dipleg considerably influences the cyclone separation efficiency rather than the cyclone pressure drop, especially for lower inlet velocities in relatively short cyclones, by providing more separation space. (C) 2009 Elsevier B.V. All rights reserved.