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With the aim to improve the separation efficiency of hydraulic vortex separator (HDVS) in recirculating biofloc technology (RBFT) system, three HDVSs with different structural parameters were studied by using numerical method of computational fluid dynamics (CFD). The simulations of the solid-liquid phase flow in HDVSs were conducted based on an Eulerian-Eulerian multi-phase turbulence 3-D model combined with the kinetic theory of granular flow, which specifically analyzed the velocity contours, vector contours, streamlines of liquid phase, inner and outlet distribution characters of solid phase. The simulation results showed that with the increase of α, the velocity field below the inlet part and surrounding area of the sleeve had small difference at the inlet velocity of 0.36m/s. But the difference of results of inner region of sleeve was stark and the turbulence was enhanced in this regions. In addition, some swirls occurred in the inner zone of the sleeve, which added energy waste and had negative effect on the flocs separation. The separation efficiency of HDVS had a negative correlation with the increase of α. When α was 1.5, the removal efficiency was 27%, which was more effective than 17% (α was 2.0). But when α was increased from 2.0 to 2.5, the HDVSs almost had the same solid phase volume fraction at the outlet and the separation efficiencies only had a small change. To validate the accuracy of simulation results, the simulation values and experimental data were compared. The good agreement of the flow velocity and change law of the separation efficiency proves that the mentioned two fluid model (TFM) can be used to optimize the structure of HDVS.