Study on Characteristics of Flow Field and Mechanism of Particles Passing through Sieve under Centrifugal Coupled Screening Effect

Author:Wang Chuan Zhen

Supervisor:chen jian zhong


Degree Year:2019





Three products hydrocyclone screen(TPHS)innovatively adopts the centrifugal classification coupled with screening(named“coupled”for short)for particle separation based on size by adding the cylindrical screen.Recently,the experiments were used to consider the structural optimization,process adjustment and performance prediction of TPHS.However,the understanding of its internal multiphase flow is insufficient,and the accurate description of the flow field characteristics and the particle passing through sieve are yet unexplored.Thus,around TPHS,from the multiphase hydrodynamics and theoretical analysis,the coupled flow field was investigated based on the reasonable simulation theory system.The distribution characteristics of centrigufal coupled with screening flow field and the mechanism of particles passing through sieve were investigated to reveal the motion laws of fluid and particle by numerical simulation and experimental verification.Details of research and conclution are shown below:1)PIV technology was used to test the coupled fluid inφ75mm THHS,and the results were analyzed by the Top-hat filtering denoising model and Adaptive correlation algorithm to obtain the exact results.2)The geometric model of coupled flow field was built based on TPHS,and the flow field was modeled using VOF and RANS algorithms.The convergence solutions of coupled flow fields with different boundary conditions can be obtained by SIMPLEC algorithm under the conditions of 10-33 s time step,10-6 iteration residual and 20 s simulation time.The stable macroscopic flow pattern can be gotten by the time-averaged calculation from 15 s to 20 s.3)The verification and validation of numerical results show that:the medium grids(about 2 million)can provide the economical grid-independent solutions;the mesh generated by the initial grid height as shonw in Table 4-2 can produce the rational y+;the comparisons of CFD simulation and PIV experiment under the same boundary conditions reveal the coupled flow field can be solved economically and accurately by combining VOF multiphase flow model with Linear Pressure-Strain turbulence model.4)The coupled flow field was thoroughly explored used the CFD simulation:(1)The high-speed fluid feeding along tangential direction swirls downward and flows through the sieve at the cylindrical screen as the screen downflow;after reaching the cone,partial fluid flows via the spigot as the underflow;the rest fluid gradually fills the flow field and discharges from the overflow outlet as the overflow..(2)Besides external swirl,internal swirl,central downflow and secondary circulation flow,the coupled flow field presents screen downflow and screen backflow with the absent air column and short-circuit flow.(3)Compared to CH,the coupled flow field presents lower tangential velocity and higher radial velocity,where the value first increased and then decreased with the inceasing axial depth.Moreover,the LZVV was created by the axial velocity in the latter.(4)The distribution of static pressure was axisymmetric,where the value first decreased and then increased and finally decreased with the increasing axial depth.Furthre,the pressure difference between the inner and outer sides of the cylindrical screen(△p)decreased from positive to negative with increasing axial depth.(5)The turbulent intensity I,turbulent kinetic energy k and turbulent dissipation rateεshow the similar distributuon:the area around the inlet column and the screen underflow outlet>the clylidrical screen>the conical area,and the inner side of the screen>the outer side of the screen.(6)The flux of screen underflow and screen backflow are proportional to the inlet velocity,and the net from the inner to the outer side of the cylindrical screen gradually changed from positive to negative as the axial depth increased.5)The flux of screen underflow and its ratio to feed were modled by the simplified sieve model,while the pressure difference between two sides of screen was the fundamental reason.Finally,the flow equation of screen underflow in the coupled flow field(?) was deduced by the dimensional analysis and Darcy’s law.6)The optimization of the coupled flow field points out:(1)the association of single inlet and double screen underflow outlets can enhance the symmetry of coupled flow field without reducing the centrifugal strength of conical section;(2)the reasonable cylindrical screen(sizes of sieve slots and sieve height)can completely eliminate reflux phenomenon.7)CFD-DEM was used to research particle motions in the coupled flow fluid:(1)The combination of particle dynamics and Re similar amplification criteria was adopt to model the momentum equation of particles in the 1/2 scaled geometry.The distributed strategy was used for simulation,wherein the results reveals the similar trendency to that in the literature.(2)Different from CH,particles are separated in TPHS by the combined centrifugation with screening.The classification process can be divided into the pre-stratification in feeding column,the coupled classification in cylindrical screen and the centrifugal classification in cone.TPHS presents the better performance than CH under the same simulation conditions.(3)The process of particles passing through screen occured mainly in the radial and tangential motion,while the axial motion can be basically ignored.The particle size passing through sieve can be described as d p<(?).(4)during particle passing through sieve,drag force was the dominant force,while virtual mass force and pressure gradient force hinder particle screening movement,however gravity can be ignored,and contact force was randomly affected.There are 78 figures,17 tables and 230 references in this paper.