Numerical Study On Flow Field And Separating Process Of Hydrocyclone

Hydrocyclone is a typical device based on complex force field of centrifugal force and gravity. Hydrocyclone has been widely used in many areas of separation, lie in its low operating cost and maintenance, high throughput, low floor space requirement etc. Just in mineral industry, hydrocyclone is widely used for the work of classification, desliming, thickness and separation. So there is huge industrial value to deepen the research of hydrocyclone.Now a commonly accepted wisdom is that the separation characteristics of the hydrocyclone including classification efficiency and cut size depend on the flow field, which has been a hot area of hydrocyclone research in recent years. But most of the studies in this problem are partial and monotonous, systematic studies of flow field and particles movement behavior inside hydrocyclone are necessary and urgent.Under the validation of PIV (Particle Image Velocimetry) measurement and physical classification experiments, numerical study method based on CFD (Computational Fluid Dynamics) and CFD software package ANSYS/Fluent were used in this paper, RSM (Reynolds Stress Model) model was used to calculate turbulence, VOF (Volume of Fluid) model was used to capture the interface of water and air, DPM(Discrete Phase Model) was used to calculate the particles’ movement. For a Φ50mm hydrocyclone, systematical studies were carried out to investigate the flow field and separating process of hydrocyclone.(1) Based on numerical method, conversion of pressure energy and kinetic energy was investigated. The formation of air core and the flow situation in this duration were investigated. In a radius of a hydrocyclone, hydrostatic head droped to 0 because of the conversion of pressure energy, negative pressure zone appeared, and air core formed. For the basic parameter hydrocyclone, both physical and numerical study results showed that the air core formed when the time was about 0.6s, the air core presented as a twisted rope, the air of the air core entered the hydrocyclone from the underflow outlet and exhausted from the overflow outlet. The largest swing of the air core occurred in the region that is close to the underflow area, and a protuberance resembling the Adam’s apple appeared when the air core entered the overflow pipe. Unnecessary energy was consumed, but no extra perturbation was brought into flow field by air core.(2) Pressure distribution, pressre drop,3D velocities and movement of particles with different densities were studies systematically. In radial direction, pressure droped from wall to center, in axial direction, there was not obvious pressure gradient. Distribution of pressure drop was just the opposite with pressure. The good agreement of flow velocity obtained by numerical and physical study methods proved the reliability of numerical study method again. Distribution of tangential velocity can be regarded as composition vortex motion, the relation of tangential velocity ut and hydrocyclone radius r can be written as utrn=C. LZVV was the turning surface of axial velocity, higher part of the LZVV was cylinder and lower part was cone, the cylindrical LZW diameter of the basic parameter hydrocyclone was 34.55mm. Radial velocity was the smallest but most complex one of the velocity component, there were opposity radial velocity nearby the air core, radial velocity inside air core was 0.(3) Influences of operational and structural parameters were studied to make sure the separation mechanism of hydrocyclone. All of feed rate, cylindrical section height, inlet height, cone angle, underflow outlet diameter, overflow outlet diameter, vortex finder depth and vortex finder wall thickness had impacts on flow field of hydrocyclone. For the fixed structural parameters hydrocyclone, there was minimum feed rate. Only overflow outlet diameter had obvious impact on the position of LZVV, and LZVV becamed from surmace to region when vortex finder depth more than 0mm, that was beneficial to the more effective particle separation.(4) Based on the existing gas-liquid two-phase flow, particles, with different size and density, were injected. The good agreement of particle separation results obtained by numerical and physical study methods showed that the numerical study method can predict the movement trajectory of the particles exactly. The staying time of particles in hydrocyclone was different, there were not particles inside air core, the volume concentration of particles increased from center to wall. All of paramenters had impacts on particle separation of hydrocyclone too. The yield of the products, the cut size and separation efficience can be obtained by numerical study method conveniently.(5) Studies of actual classification of 075mm hydrocyclone were performed by using the numerical study method for an Anshan low-grade hematite ore, the results showed that, for the basic parameter Φ75mm hydrocyclone, the air core diameter was about 8.54mm, it formed at about 1.9s, the cylindrical LZVV diameter was 61.99mm, and flow field characteristics were similar with Φ50mm hydrocyclone. Cone angle 11°, feed rate 43L/min and underflow outlet diameter 6mm were recommended by numerical study method, and separation results were still in good agreements with physical study method. Therefore, the numerical study method used in this paper is reliable, it can at least partly replace physical study method, thus experiment cost can be saved and efficiency can be increased.