Research On The Influence Of Cone Structure Of Three-Stage Cone Water-only Cyclone On Its Internal Flow Field

As a new type of Water-only Cyclone(WOC),the Three-stage Cone Water-only Cyclone(TWOC)has three stage cone with different cone angle that decreases gradually from the beginning of cone to apex,it had improved the fact that the Single-stage Cone Water-only Cyclone(SWOC)has poor separation performance and had been used in coarse coal slime separation widely.As is well-known,the three dimensional flow field of cyclone dominates its separation,therefor,in order to explore and research the separation differences and the reason of separation differences caused by the special three-stage cone structure,this paper,on the basis of laboratory separation experiments,used CFD numerical simulation and PIV technology to compare and analyze the three dimensional flow field(flow streamlines distribution,air core,speed field,pressure distribution)of TWOC and SWOC and utilized flow field differences to explain the reason of separation differences between two types of cyclone.Meanwhile,the real flow field in cyclones obtained by PIV technology also verified the accuracy of CFD numerical simulation.Via coarse coal separation experiments,the separation performance differences had been acquired:the yield and ash content of SWOC overflow were higher than TWOC,while the yield of underflow was lower than TWOC,and ash content was higher than TWOC.The separation accuracy of TWOC was obviously higher than that of SWOC.The difference of separation performance between two types of cyclone would not be changed by the difference of coal sample and feed pressure.Stereo PIV technology was used,adjusting the appropriate camera sampling frequency,laser frequency,exposure time and the number of photographs,to get the three-dimensional velocity field of two types of cyclone under 0.06 MPa,0.10 MPa and 0.14 MPa pressure gradient.Meanwhile the appropriate compute field was drawn and the discrete method,appropriate operation model,definite solution conditions and solution method were selected,CFD numerical simulation was carried out for the clean water flow field of two types of cyclone under 3.3m/s,5.5m/s and 6.8m/s inlet velocity(converted by pressure)gradient.The streamlines distribution,velocity value and distribution of the flow field inside two types of cyclone obtained by two methods were in good agreement,which had proved the accuracy of CFD numerical simulation.CFD numerical simulation was used to compare and analyze the flow field of two types of cyclone in detail,the results showed that:the fluid motion and the instability of the air core in TWOC cone were all stronger than those in SWOC.The flow layer near the wall of TWOC cone was disordered and two secondary vortexes were formed.the secondary vortex increased the tangential velocity and accelerated the axial velocity change;the tangential and axial velocities of the TWOC cone were smaller than those of SWOC.A peak value of radial velocity was formed at the bottom of SWOC cone,and the cross-section area of internal spiral flow was larger than TWOC,which resulted in a large number of fluid returning to the inner spiral flow at the bottom of SWOC cone,but TWOC did not have this phenomenon;the pressure drop of SWOC cone was larger than that of TWOC.In addition to energy dissipation,the static pressure energy was transformed into fluid kinetic energy,which resulted in that the fluid velocity of SWOC cone was larger than that of TWOC;the cone structure only affected the flow field of the cone of cyclones,but had little effects on the cylinder.The differences of the flow field in cone part only related to the difference of cone structure,the inlet velocity had no effect on the distribution of streamlines and velocity of two types of cyclones,also could not enlarge or narrow the difference of tangential velocity and axial velocity between two types of cyclone.According to the difference of flow field between the two types of cyclone,the influence of fluid movement mode,tangential velocity and axial velocity on separation performance was analyzed,and the reason for the diference of separation performance between two types of cyclone were explained:the increase of tangential velocity would reduce the separation density of cyclone,but also restrained the looseness of materials;the increase of axial velocity would reduce the separation time of materials,which was unfavorable to separation;the turbulence of the near wall flow layer and secondary vortex had the function of loosening materials,in addition,secondary vortex could also reduce separation density and increase separation time.Therefore,for SWOC,coarse coal slime could not be fully separated in downward movement process,a large number of unseparated coarse coal slime went into inner spiral flow at the bottom of the cone,and coarse coal slime could also not be fully separated in inner spiral flow,an obvious part of the coarse coal slime that did not meet the real separation density would go into overflow,resulting in poor separation accuracy of SWOC.The actual separating density of SWOC was lower than that of TWOC under the effect of large tangential velocity,but due to the effect of the reentry flow,apparent separation density shown in its distribution curve was significantly higher than that of TWOC.For TWOC,coarse coal slime was fully separated in downward movement process.and there would not has more reentry flow at the bottom of cone.The separating accuracy of TWOC was obviously higher than that of SWOC,and its actual separating density was basically similar to apparent separating density.This paper verified the accuracy of CFD numerical simulation,obtained the influence of special three-stage cone structure for TWOC internal flow field,revealed the reason why the separation performance of TWOC was better than SWOC,enriched the separation theory of TWOC,and provided a solid theoretical basis for its practical application.