Abstract: In this paper, the dynamic forces acting on the particles and the radial movement of particles in hydrcyclone are studied on the basis of experiment and CFD simulation. The study shows that in the radial direction the dynamic forces acting on the particle balance on the particle itself; there is not a socalled trajectory of the particle in which the composition of forces acting on the particles is zero, but there is a trajectory of the particle in which the radial slip velocity of particle is zero, the trajectory of the particle is signality to the separating particle diameter of the hydrcyclone. The formula of the trajectory of the particle in which the radial slip velocity of particle is zero is offered.

Key words: Hydrcyclone, particle, slip velocity, seperating particle diameter, formula