Numerical simulation of flow field characteristics for aquaculture raceway and analysis of solid phase distribution in waste settling zone

In-pond raceway system (IPRS) that was introduced from the United States by American Soybean Association in 2014, and it has been rapidly applied in recent years. It has realized the transformation of aquaculture model from traditional low-efficiency culture to high efficient culture. Many problems that limit the further integration of aquaculture management have not been studied in depth due to the short application time. In order to explore the distribution of flow field of raceway and the solid phase distribution characteristics of waste settling zone in the culture unit of in-pond raceway system, in this study, the data were simulated by dense discrete phase model (DDPM) based on computational fluid dynamics (CFD) and then were analyzed to form the liquid phase velocity contours, liquid phase velocity path lines and solid-phase distribution characteristics. Firstly, the mesh of culture units was constructed by ICEMCFD software. Secondly, the velocity of flow and nine groups of total suspended solids with different particle sizes were simulated with DDPM using numerical simulation software of FLUENT. In this simulation, the boundary condition of the air inlet was set to the speed, and the other exits were regarded as the pressure outlet. Three-dimensional n-s equation and uncoupled implicit scheme were used for discretization using finite volume method. A three-dimensional equation and a non-coupling implicit scheme discretized by a finite volume method was used, a standard k-ε two-equation turbulence model was selected, and a phase coupled SIMPLE algorithm was used as a numerical solution method in the separation pressure correction method in this study. The default standard format was selected in the pressure term of the velocity correction equation. The standard format, the diffusion term, the source term, and the convection term were all separated by a first-order upwind difference format. The velocity distribution of raceway in culture unit was detected by current meter, and the image for particle deposition in waste settling zone was scanned by a sonar scanning imager. The root mean square error (RMSE) between simulated value and measured value was 0.013 m/s. The water flow in raceway was in a slow-flow state. The high-speed flow field in the water surface area was pushed by the gravity and inertia along the length of raceway from the liquid surface to the bottom and then tended to be stable after reaching the bottom. During the sweeping process, the water flow formed an inherent backflow region between the front retaining wall and the bottom of raceway and the characteristic length of the backflow region was linear with the average flow velocity of the water inlet. The solid phase particles moved with the flow field along inside wall on both sides of the raceway. The solid phase particles were deposited in a U-shape after entering the waste settling zone which corresponding to each raceway. The deposition ratio of solid particles larger than 1.30 mm was over 85%, while the deposition ratio of solid particles smaller than 0.60 mm was less than 44.13%, and the total deposition ratio was 37.77%. The study showed that the influence of the design of the in-pond raceway system on the solid-phase particle deposition can be evaluated preliminarily by using DDPM, the deposition effect of waste settling zone on solid particles smaller than 0.6 mm in diameter was not ideal in this system.