Numerical Study On Flow Field And Seperation Process Of Ballast Water Hydrocyclone

The indiscriminate discharge of ship’s ballast water has caused serious adventitious biological invasion.To this end,the International Maritime Organization(IMO)has developed relevant conventions and guidelines to regulate the treatment and discharge of ballast water.However,due to the characteristics of ballast water treatment,solid-liquid separation subsystems are difficult to meet the processing requirements.The hydeocyclone can be used as a pretreatment unit of the solid-liquid separation system to effectively remove large-sized impurities in seawater and release the pressure of subsequent processing units.In this paper,the numerical calculation of the flow field and separation process inside the hydrocyclone is carried out,and the application scenario of the hydrocyclone is extended to the field of ballast water treatment.A numerical model for flow field calculation and analysis of hydrocyclone is established by using computational fluid dynamics(CFD).In this model,the Reynolds Stress Model(RSM)and the Volume of Fluid model(VOF)are used to describe the gas-liquid two-phase flow field inside the cyclone and the volume distribution of each phase.Based on this,the DPM model is introduced to predict the trajectory of solid-phase particles.Using the above numerical calculation model,the flow field and separation process inside the cyclone are studied.The results are as follows:(1)As the static pressure continues to decrease,a negative pressure zone is generated near the central axis of the cyclone,and air enters the flow field from the underflow port to form an air-core.The tangential velocity distribution can be divided into three regions: air core,combined vortex,and boundary layer.The axial velocity gradually increases along the wall to the central axis,and there is a zero point.Different axial velocity zeros constitute the zero locus of zero vertical velocity(LZVV).(2)The increase of the vortex finder diameter narrows the main separation zone of the hydrocyclone,and the conversion of static pressure decreases,which directly leads to the reduction of the tangential velocity and the centrifugal inertia force of the liquid flow.The change of the equivalent diameter of the inlet is equivalent to changing the inlet flow of the cyclone,directly increasing its inlet speed and centrifugal inertia force.The increase in the depth of the vortex finder can increase the stroke of the downflow and the circulation of the cover,making it difficult for the flow to be directly discharged from the vortex finder without the separation effect.(3)Particles with a small particle size(less than 20 μm)are easily separated from their own orbit,passing through the locus of zero vertical velocity(LZVV)into the internal swirl,and finally discharged from the vortex finder.The movement trajectory of the solid particle with larger particle size(greater than 70μm)or density is close to the wall surface,always in the external swirl flow,and finally discharged from the underflow port;(4)When the diameter of the vortex finder is 16 mm,the diameter of the underflow port is 15 mm,and the equivalent diameter of the inlet port is 16 mm,the overall separation efficiency of the 80 mm cyclone reaches a maximum of 85.2 %.In this paper,the flow state and particle motion of the hydrocyclone are regularly summarized and analyzed,and the influence of structural parameters on its flow characteristics and separation characteristics is proved,which indicates the direction of design and industrial application of the hydrocyclone.