Optimization of the inlet pipe layout of circular recirculating water aquaculture tank based on STAR-CCM+

Abstract: An industrialized Recirculation Aquaculture System (RAS) has been widely used in an efficient, intensive, and environment-friendly way for modern aquaculture. A large amount of feed needs to be added to the system during the breeding process. Some solid residuals can be easily deposited at the bottom of the breeding pond, such as the uneaten feed and feces produced by fish. These residuals can then be decomposed and produce harmful substances to pollute the water body, while consuming the dissolved oxygen, if they cannot be discharged in time. As such, a great threat has been posed to the health of fish. Therefore, it is very necessary to effectively and timely remove these solid particles in the RAS tank. In this study, a solid-liquid-gas three-phase flow model was constructed to optimize the layout of inlet pipes in a RAS tank using Computational Fluid Dynamics (CFD) technology. A STAR-CCM+ software was also selected to systematically simulate the common layout angle of inlet pipe (θ=0° and 45°, θ was the deployment angle of inlet pipe), under different layout positions (d=0, 1/8r, 1/4r, 3/8r, and 1/2r, d was the distance between the jet pipe and the tank wall, r is the radius of the aquaculture tank). The error of cumulative removal efficiency at each monitoring time was less than 5 percentage point, indicating a high calculation accuracy in the numerical simulation, compared with the experimental. A systematic investigation was made to explore the layout influence of water inlet pipes in the circular RAS tank on the flow field distribution in the tank, as well as the removal efficiency of solid particles. The results show that the position of inlet pipes outstandingly determined the removal efficiency of solid particles. The lowest removal efficiency of solid particles was obtained, when the layout distance was set to be d=0 and θ=0° under the rapid circulation (low hydraulic retention time). Furthermore, the removal efficiency was much higher (>90%) with small difference in the rest of the layout distance. Consequently, the maximum removal efficiencies were achieved in 94.8% and 94.3%, respectively, where θ=45°, d=3/8r, and d =0, whereas, the lowest removal efficiency of solid particles was found, when the layout distance was set to be d=1/2r. Therefore, it can be recommended not to be close to the side wall of the breeding tank in practice, when the inlet pipe was arranged at an angle θ=0°. By contrast, it can be recommended not to be too far from the side wall of the breeding tank, when the layout angle of the inlet pipe θ=45°. Anyway, the optimal collection efficiencies of solid particles with the inlet pipe layout angle of 0 and 45° were achieved similarly to be 95.0% and 94.3%, respectively, when the inlet velocity was 0.46 m/s (large water circulation velocity). At this time, the layout distances of the inlet pipe were d=1/8r and d=0, respectively. The findings can provide a strong reference to optimize the layout distance of inlet pipes in an industrialized RAS tank, thereby improving the comprehensive performance of circulating water aquaculture.