Research On Hydraulic Collecting Methods And Fluid-particle Dynamic Characteristics For Deep-sea Mining

Seabed mineral resources are precious for exploitation because of their wide variety of metal elements,huge reserves,high grade,and abundance.Among them,deep-sea polymetallic nodules are widely distributed in surface sediments of the seabed in the form of large-sized particles.Moreover,the mining of cobalt-rich crusts and polymetallic sulfides requires cutting and excavating,followed by the collection of ore particles that are crushed into small pieces.However,the surface sediments in polymetallic nodule mining areas are susceptible to suspension and diffusion and difficult to settle.The benthic density in cobalt-rich crusts and polymetallic sulfides mining areas is high.Rough collecting methods will pose biodiversity threats.One of the critical technical challenges in designing a deep-sea mining system is how to collect seabed ore particles with low disturbance and high ore particle pick-up efficiency.Hydraulic collecting is one of the most potential ore particle collecting methods.It is of great academic and engineering value to research hydraulic collecting methods and fluid-particle dynamic characteristics for deep-sea mining.At present,domestic and foreign scholars mainly carry out relevant research on the operation performance of hydraulic nodule collectors.The effect of the geometrical parameters,bottom clearance,and flow rate of the collector on the nodule pick-up rate has been analyzed.However,due to the limitation of the previous technical means,the critical scientific issues,such as the “complex geometries-fluid-particle” interaction and the dynamic characteristics of particles and fluid in the nodule collecting flow field,are still not thoroughly understood.On the one hand,most of the previous studies were limited to discussing the nodule collection results,such as the nodule collection rate,rather than studying the physical processes from the perspective of the flow field and the forces and motions of the particles.On the other hand,the current research means are relatively single.There is a lack of prediction methods for nodule collection performances and design means for the nodule collection equipment.In this case,physical experiment and numerical simulation methods are developed further to study the problem of hydraulic collecting in deep-sea mining.The flow field characteristics and the force and motion features of particles of different hydraulic collecting methods are studied;the interaction among wall,fluid,and particles is revealed.A comparative study is carried out to investigate nodule collecting characteristics of models with different principles and innovative design and construction of the nodule collecting flow.New nodule collecting methods are proposed and demonstrated.This paper firstly studies the experimental measurements,analysis technology,and numerical simulation methods of hydraulic collecting.Various experimental techniques for measuring the collecting flow field and forces and motions of particles are developed and validated.A mobile scanning dynamic measuring method is proposed.The technique combined with high-speed image acquisition technology and computer vision is used to measure and analyze the particles’ trajectories and moving speeds in the collecting flow field.The data assimilation method is applied to the numerical simulation of nodule collecting turbulence flow.The experimental data measured by particle image velocimetry(PIV)technology are used to calibrate the turbulence model constants,thereby improving the accuracy of numerical simulation.Secondly,a simplified mechanical model of suctioning a near-wall sphere by a circular pipe is constructed from hydraulic collecting engineering problems.The influences of geometric dimensionless parametrs(Dd,hd,θ)and dynamic dimensionless parameters(Rep,νp/Vt)on particle force coefficients(Cfv,Cfr)and flow field characteristics are systematically revealed.Suppression of near-wall suction flow on the flow separation of a spherical boundary layer is found and explained.Based on the experimental results,a formula for predicting the forces and movements of a single spherical particle in the collecting flow field is proposed.Predicting methods for nodule collection performance are developed and used to guide the design of the geometric parameters of nodule pick-up devices and the adaptive environmental adjustment of the control parameters(h,Q,Vt).Based on the physical experiment and numerical simulation method,a comparative study on the mechanism and characteristics of three existing nodule collecting models,i.e.,suck-up-based model,Coand(?)-effect-based model,and hydraulic sluicing model,is carried out.A preliminary evaluation of the operational performance of the three collecting models is carried out in terms of the collection flow demand and near-bottom flow field disturbance under the same collection capacity.The Coand(?)-effect-based nodule collecting method is further studied by applying experimental techniques such as PIV,high-speed camera,target monitoring,tracking,and the numerical simulation method based on data assimilation.Firstly,the limitations and shortcomings of the previous research method — using the semi-empirical and semi-theoretical formula of two-dimensional curved wall jets to solve the velocity and pressure distribution of the collecting flow field and particle forces are demonstrated.Then the existing nodule collecting model was innovatively designed in geometry and jet flow pattern,respectively.A double-sided circular curved surface is designed to eliminate the reflux zone that hinders particle lifting.Inspired by the experimental phenomenon that a sizeable sweeping jet and transient flow field can significantly improve the particle collection rate,a new nodule collecting method based on the pulsating jet characteristics parameters is proposed and justified.The effect of different jet characteristics(jet frequency and jet amplitude)on the nodule collection rate is investigated.The mechanism of significantly improving the nodule collection rate based on the pulsating jet characteristics is explained.Based on the phenomenon observed in the experiment that particles are easier to be lifted after doing circular motion,a new hydraulic collecting method based on spiral flow characteristics is proposed.The active and passive flow control technologies are used to separately design the embedded spiral deflector and the jet-induced vortex sucking disc type collector.The spiral flow is induced near nodules in the nodule collecting flow field.The particle force,nodule pick-up rate,flow velocity,and pressure distribution under different flow patterns(non-spiral and spiral flow,weak spiral flow,and strong spiral flow)are studied and compared based on physical experiments and numerical simulations.The advantages of using spiral flow collecting nodules in increasing particle force and improving nodule pick-up rate are demonstrated.In addition,a prototype of an underwater mining vehicle is designed and manufactured.Connected by hoses,the nodule collector and the main body of the mining vehicle are separated.The performance of mobile mining operation of the jet-induced vortex sucking disc type nodule collector in vast water space at 10 m depth is tested.In summary,based on the research background of seabed nodule mining,this paper studies the mechanism and the engineering application of the hydraulic nodule collecting method,the mechanism of the flow field,and the dynamic characteristic of fluid-particle from the perspective of revealing the law of the wall-fluid-particle interactions and the design and verification of the collecting flow field.The thesis provides some valuable references for revealing the hydraulic nodule collecting mechanism,and some of the new findings and results have filled the blank in the field of fluid mechanics.Various experimental measurement,analysis methods,and numerical simulation methods are developed and validated in this thesis.Those methods can be used for the quantitative study of hydraulic nodule collecting problems,including predicting the nodule collection performance,innovating the geometric parameters of the nodule pick-up devices,and optimizing the nodule collection control parameters.