Design and Multiphase Flow Simulation of Centrifugal Pump Negative-pressure Fish Suction System

Abstract

Aiming to address the challenge of efficiently harvesting live fish from large-scale aquaculture platforms in deep sea, where the yield per individual platform has exceeded 100 tons, a high-capacity fish suction system utilizing a centrifugal water pump to create a stable negative pressure was designed. Based on the ideal gas state equation, it was found that a pressure suitable for fish suction could be formed when the liquid level change was within 10.3% of the total height of the fish collection tank. The key dimensions of the fish collection tank device were designed accordingly. A fish barrier grid model was constructed by using a porous medium model, and a three-dimensional transient numerical analysis of the fish suction process was performed based on the coupled VOF and CFD-DEM method. The numerical results showed that the liquid level remained basically constant during the fish suction process, with pressure fluctuations ranging from -37.5kPa to -26.5kPa, and the maximum pressure change rate was 0.037kPa/s, which was far below the fish damage threshold. The change in working pressure relative to the initial value of -35kPa was less than 7%. The distribution of fish schools in the collection tank showed significant differences with increasing flow rate: at 150m3/h, the fish initially clustered densely from the inlet to the outlet, then gradually dispersed;at 225m3/h, the higher flow rate caused faster dispersion to the tank periphery due to strong vortex effects, with more pronounced diffusion at higher velocities. The fish aggregation behavior at the fish barrier grid exhibited nonlinear changes with increasing flow rate: when the flow rate was 150m3/h, the number of fish in contact with the fish barrier grid remained relatively stable;when the flow rate was 200m3/h, the number fluctuated intensely, with a peak of 55;when the flow rate was 225m3/h, non-steady-state fluctuations were significant, with 63 fish in contact with the fish barrier grid, accounting for 32% of the total number of contacts, indicating that an increase in flow rate significantly increased the risk of blockage. In order to further increase the suction capacity, it was necessary to optimize the structure of the fish interception grille by designing a curved grille, expanding the opening size, and adding fish guide channels to reduce the risk of blockage.

Keywords: aquaculture platforms in deep sea, fish suction device, pressure fluctuation, clogging prediction, three-phase flow, discrete element model