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Numerical study of dense solid-liquid flow in hydrodynamic vortex separator applied in recirculating biofloc technology system

Shi, MingMing, Ruan, YunJie, Li, JianPing, Ye, ZhangYing, Liu, Gang, Zhu, SongMing
Aquacultural engineering 2017 v.79 pp. 24-34
algorithms, aquaculture, biofloc technology, computer software, engineering, equations, hydrodynamics, simulation models, turbulent flow
Hydrocyclone separation technique has commonly been applied in engineering for solid-liquid separation. In aquaculture, since Hydrodynamic Vortex Separator (HDVS) is able to control flocs concentration effectively, the Recirculating Biofloc Technology (RBFT) has been gradually acknowledged (Zhu et al., 2016). In order to operate RBFT system at maximum performance, the removal efficiency of HDVS at different Hydraulic Retention Time (HRT) must be fully predictable. The aim of this work is to investigate the relationship between separation efficiency and HRT of HDVS applied in RBFT system, through Computational Fluid Dynamics (CFD) method. For this, a three-dimensional unsteady transient model was developed to simulate the hydrodynamics in HDVS based on the commercial software Workbenching 17.0. A Two Fluid Model (TFM) using kinetic theory of granular flow (KTGF) has been developed to describe the dense solid-liquid (flocs-water) flow field of HDVS in RBFT system. Additionally, the Reynolds averaged Navier-Stokes (RANS) equations with Reynolds stress turbulence model (RSM) are solved by use of the finite volume method based on the SIMPLE pressure correction algorithm in the computational domain. Finally, pilot-scale studies were conducted to evaluate the accuracy and precision of simulation models applied. The results indicate that the flocs removal efficiency of HDVS obtained from tests were less than that from the simulation results at three different HRTs. But it decreased with the decline of HRT both for experiment and simulation. Additionally, the CFD model proposed can enhance the fundamental understanding of the effects of HRT on separation efficiency and at the same time, probably coupled with physical experiment whenever necessary, offer a more reliable way for optimum control of HDVS under varying conditions. Based on the simulation results, the flocs management in RBFT system is briefly discussed.