Hydraulic collecting is the key technology in deep sea mining and dredging engineering. It determines economic benefits of the project and environmental issues. However, mechanistic studies of hydraulic collecting are rarely described. In this study, the mechanism of collecting spherical particles is researched by dimensional analysis and experimental study. The experimental system is established to carry out three kinds of tests including 253 different test cases. The empirical model of collecting performance prediction is established by the tests of vertical force characteristics and vertical incipient motion characteristics of particles in suction flow field. The results show that the vertical suction force coefficient (C_vs) decreases exponentially with the ratio of bottom clearance to diameter of the particle (h/d), increases linearly with the ratio of diameter of the suction pipe to diameter of the particle (D/d), and is nearly independent of Reynolds number (Re). The empirical formula of vertical force and criterion-formula of vertical incipient motion of particles are obtained with the maximum tolerance less than 15%. The phenomenon that the vortex could help strengthen the suction force was observed in the tests. In addition, the characteristics of suction flow field were obtained by flow visualization tests, and applied to explain the force characteristics of particles in the suction flow field.

Turbulent suction pipe flow around a near-wall ellipsoid nodule, as basic research of polymetallic nodule hydraulic collection, is investigated numerically and experimentally in this paper. Seven ellipsoids with axe ratios (a/b) ranging from 1 to 2 are considered as nodule models to reveal the shape effect on the characteristics of suction forces and suction flow field. Methods of particle image velocimetry (PIV) and dye tracing were used to visualize the suction flow field. The results indicate that (1) the suction force coefficient increases with a/b and its growth rate decreases with the ratio of bottom clearance to semi-thickness of the ellipsoid (h/c); (2) the weak vortex shedding in suction flow results in small-amplitude fluctuations of time-history suction forces; and (3) the detached eddy simulation (DES) method based on shear stress transport (SST) model is validated to be accurate and feasible for predicting the suction forces and suction flow field. It is expected to provide references for the design of nodule pick-up devices and to help us further understand the mechanism of hydraulic collection.

Wall-jet collection has been recognized as an advanced technique for mining polymetallic nodules that has significant potential for practical engineering applications. Optimizing the geometry of the collection device can improve collection efficiency and reduce environmental disturbance. In this study, 24 distinct structures of nodule-collection device were investigated using a computational fluid dynamics–discrete element method, which was validated by comparing with the experimental data. A key parameter, the wall-jet half-width coefficient Cc, was employed to examine the collection performance, including the collection efficiency, collection flow field, and particle trajectory. An assessment indicator derived from energy-consumption and substrate-disturbance metrics was proposed, and this allowed the identification of optimal device structures tailored to various requirements. The results showed that based on collection efficiency–jet flow rate (η–q) response curves, the collection performance can be categorized into two distinct patterns. When Cc ≤ 1.56, induced flow will occur, and η can reach 1.0; when Cc > 1.56, a moving vortex that disturbs the particle trajectories is generated, and the jet escapes rightward, resulting in a decrease in η. The influences of geometric parameters on Cc exhibit coupled relationships, which is particularly noticeable in the relationship between the tangential angle of the jet and its thickness. The optimal device geometry varies for different criteria, and maximum reductions in substrate disturbance and jet energy consumption of 48.46% and 19.64%, respectively, were obtained with different optimization criteria. This study is expected to provide data to support the optimization of the structure of wall-jet collection devices.

The nodule pickup device is a crucial component of a deep-sea mining system. It is widely perceived that leveraging water flow for the separation and retrieval of nodules is a promising approach. A series of experiments and numerical simulations were conducted to examine the impact of non-dimensional parameters on the force characteristics and flow field of the particle in Coandă-effect-based hydraulic collection. The results showed that the lift coefficient of the particle initially increased before subsequently diminishing from the jet nozzle to the rear. Notably, the lift coefficient α reached its maximum at the convex curved surface between x/d = −0.17 and 0.33. Furthermore, a distinct linear correlation was established between the maximum lift coefficient αmax and Froude number Fr across varying h/d, and an empirical formula for predicting the lift coefficient was developed through data fitting. Upon experimental validation, the prediction exhibited a maximum error of less than 20%. Additionally, numerical simulations revealed that the particle significantly influenced the flow dynamics within the collection area, and the flow field characteristics and the particle forces can be corroborated with each other. The findings not only provided a reliable quantitative tool for assessing the particle force but also facilitated precise predictions of collection performance, aiding in the selection of optimal operational parameters in deep-sea hydraulic collection.