Research On Dynamic Performance Of Solid-Liquid Two-Phase Flow In Hydraulic Transport In Deep Sea Mining

Deep-sea mineral resources are of vast reserves and high grade,and have great commercial exploitation value.The study of key technologies for a safe,efficient and environmentally friendly deep-sea mining system is an important subject for the exploitation and utilization of marine resources.One of the key issues in the design of deep-sea mining system is how to transport ores from the ultra deep seabed to the sea surface.Vertical hydraulic transport is the most potential ore transportation method,which is widely recognized in the world.The seawater is the carrier of the ores;the upward flow in vertical pipelines impels the ores to move upwards,thereby lifting the ores from the seabed to the sea surface.Owing to the large size,high density of the ores and the long transport distance in deep-sea mining,the research on the dynamic performance of the solid-liquid two-phase flow formed by the ores and the seawater in the pipelines is meaningful and contributes significantly to the design of a safe and efficient vertical hydraulic transport system.The study of vertical hydraulic transport has been conducted by a lot of experiments in the published works;the settling and floating movements of particles,the pressure loss and the transport characteristics have been analyzed,providing a guideline to ore transport.However,the previous studies were mainly carried out from the perspective of engineering application;the researches on the critical scientific issues such as the interaction between the ore,the fluid and the pipe wall and the local characteristics of the solid-liquid two-phase flow have not been well documented.This paper focuses on the dynamic performance of solid-liquid two-phase flow in vertical hydraulic transport in deep-sea mining.On the one hand,numerical simulations on the migration of a small number of particles in the upward pipe flow are conducted to reveal the interaction between the particle,the fluid,and the pipe wall.On the other hand,the local characteristics of particle motion and flow pattern of the solid-liquid two-phase flow in the pipelines are analyzed to provide essential references for engineering applications.Based on direct numerical simulation,the hydrodynamic forces on the freely falling sphere are compared to those in the case of flow past a fixed sphere.The free-fall of a sphere is simulated at different particle Reynolds numbers;four different path trajectories of the sphere and the surrounding flow patterns are analyzed.The mean sphere velocity and the drag coefficient are compared at different pipe diameters,revealing the effects of the pipe wall on the drag coefficient.In addition,a numerical study of the sphere migration in upward pipe flow is conducted;the trajectory,the surrounding wake structure,the slip velocity and the radial position of the sphere are discussed in detail.Different types of path trajectories are observed at different flow velocity,solid-to-fluid density ratio and pipe-to-sphere diameter ratio.Detailed analyses of the path regimes are provided according to the wake structures around the sphere.Based on direct numerical simulation,the migrations of two spheres in upward pipe flow are simulated.The phenomenon of drafting-kissing-tumbling of two spheres in pipe flow is compared to that in quiescent fluid;the flow patterns around the spheres are analyzed in detail to reveal the interaction between the spheres in fluid.In addition,the studies of the sphere-sphere interaction in pipe flow at different flow velocity,pipe-to-sphere diameter ratio,sphere-to-sphere diameter ratio and the initial release position are conducted,providing a basis for theoretical research on solid-liquid two-phase flow.An indoor test system for vertical hydraulic transport of ores in deep-sea mining has been established.A new method for measuring the local concentration and particle velocity has been developed by means of high-speed image acquisition and computer vision methods.The experiments are conducted to analyzed the effects of local flow velocity and local concentration on particle velocity and slip velocity.The local flow velocity has a significant effect on the particle migration.As the increase of local flow velocity,the slip velocity increases first and then decreases,whereas the particle velocity increases obviously.The larger local concentration results in slightly smaller slip velocity and slightly larger particle velocity.Based on computational fluid dynamics and discrete element method(CFD-DEM),a numerical study of vertical hydraulic transport of coarse particles in pipelines is carried out.The present numerical method is verified to be accurate for simulating solid-liquid two-phase flow,providing a new way to analyze the mechanism and engineering application of vertical hydraulic transport.According to the parameters in the initial design of the vertical hydraulic transport system for the engineering application,the time histories of particle concentration and velocity are analyzed;the instantaneous distribution of particle concentration and velocity are provided in both axial and radial directions.The probability density functions of particle velocity and slip velocity are revealed.The instantaneous and time-averaged flow patterns in pipelines are presented.The sensitive analyses of flow velocity,feed concentration,particle density,particle diameter,and particle size gradation are conducted to provide a guideline for parameter selection of vertical hydraulic transport.Based on the numerical simulations,semi-empirical expressions are proposed for calculating particle velocity,slip velocity and local concentration during vertical hydraulic transport.The accuracy of the expressions is verified by comparing the calculated results to the numerical and experimental data.The expressions for calculating the pressure loss and shear stress on the pipe wall are summarized.The different regimes of particle motion at low flow velocity are analyzed based on numerical simulations;the formation and change of plugs are observed in the experiments,contributing to the analysis of plug and blockage in the pipelines.In general,based on the engineering concern of vertical hydraulic transport in deep-sea mining,this paper studies the mechanism and the engineering application of solid-liquid twophase flow in vertical hydraulic transport from the perspective of the solid-fluid interaction of a small number of particles in upward pipe flow and the overall dynamic performance of solidliquid two-phase flow.A new method for measuring the local characteristics of solid-liquid two-phase flow in the experiments has been proposed.Meanwhile,the CFD-DEM method has been introduced to conducting numerical studies of solid-liquid two-phase flow.Semiempirical expressions have been summarized to calculate the particle velocity,slip velocity and local concentration in the pipeline,providing a valuable reference for engineering applications of vertical hydraulic transport in deep-sea mining.