Study On Gas-solid Flow Characteristics And Separation Efficiency Of Cyclone Separator Under Low Load

With the rapid development of clean energy such as wind and solar energy,thermal power generation systems need deep peaking operation to improve the grid’s ability to consume new energy.In peaking operation,thermal power units are often under low load operating conditions,so it is necessary to study the operating characteristics of circulating fluidized bed boilers under low load.As one of the important components of the combustion system of circulating fluidized bed boiler,the gas-solid flow characteristics,separation efficiency and flow field instability have a great influence on the operation of circulating fluidized bed.In this paper,the high temperature cyclone of circulating fluidized bed boiler of 350 MW supercritical thermal power unit is used as the research object.The gas-solid flow characteristics and the vibration characteristics of the vortex finder in the cyclone are studied by numerical simulation method.Firstly,the Reynolds Stress Model(RSM)is used to calculate the gas flow characteristics of the cyclone,and the effects of load and vortex finder insertion depth on the gas velocity distribution and pressure distribution are analyzed.In the calculation,an isothermal incompressible process is assumed in the cyclone,and the inlet boundary condition takes into account the inlet flue gas velocity distribution along the height direction.To verify the accuracy of the calculation results,the calculation results were compared with the field collected data,and the error is relatively small.The calculation results show that the internal flow field of the cyclone has a double-vortex structure with poor symmetry.With the increase of load,the maximum tangential velocity and axial velocity in the cyclone gradually increase,and the pressure drop gradually increases.When the load is reduced from350 MW to 150 MW,the pressure drop is reduced from 1321 Pa to 226.2 Pa.When the insertion depth of the vortex finder increases,the symmetry of the double-vortex structure becomes better,the maximum tangential velocity and axial velocity increase,and the pressure drop in the separator increases first and then decreases.The pressure drop is the lowest when the insertion depth is 6.16 m,and the lowest value is 1269.4 Pa.Secondly,on the basis of gas phase calculation,the Discrete phase model(DPM)is used to calculate the particle flow characteristics of the cyclone,and the effects of load and vortex finder insertion depth on the separation efficiency,particle movement trajectory and residence time are analyzed.The particle phase model considers the effects of gravity and drag force,and uses the Discrete Random Walk model(DRW)and the Random Eddy Lifetime model(REL)to track the particles.In addition,the actual wall roughness and the sphericity of solid particles are also considered in the calculation.To verify the accuracy of the calculation results,the calculation results were compared with the sampling analysis results of the field return vessel sampling,and the agreement is good.The results show that the separation efficiency increases with the increase of particle size.When the particle size is smaller than the cut-off diameter,the larger the load,the higher the particle separation efficiency;Otherwise,the lower it is.The cut-off diameter of particles under different loads ranges from 26.2μm to 30.8μm,and the separation performance of the cyclone is good.There is no uniform rule of particle separation efficiency with the insertion depth of the vortex finder.The cut-off diameter of the particle under different insertion depth of the vortex finder is between 22.9μm and 34.2μm.The separation performance of the cyclone is good.Thirdly,the size and inlet boundary conditions of the cyclone are modeled by Froude criterion(F_r),Reynolds criterion(Re)and Stokes criterion(S_t).And the gas-solid flow characteristics of the cyclone with 1:1,1:2,1:5,1:10 and 1:15 models are compared and analyzed.The results show that with the increase of the modeling ratio,the airflow structure characteristics remain unchanged,but the tangential velocity gradient and axial velocity stagnation(or backflow)degree increase.The pressure drop,separation efficiency and particle motion trajectory of different modeling ratio models are similar.The flow field of cyclone is pulsating flow field.The larger the modeling ratio and load,the larger the pulsation amplitude and the lower the frequency.Finally,a one-way transient fluid-structure interaction method is used to study the vibration characteristics of the vortex finder from the Angle of resonance induced by fluid excitation,and the influence of load and the insertion depth of the vortex finder is compared and analyzed.The results show that the natural frequency of the vortex finder decreases gradually with the increase of the insertion depth.The deformation mode of the vortex finder is multi-waveform,and the maximum deformation position and the maximum equivalent stress position appear at the entrance.When the insertion depth is 4.16 m and the load is greater than 315 MW,the vortex finder meets the resonance condition.When the load is 350MW and the insertion depth is greater than 3.16 m,the vortex finder meets the resonance condition.This study can provide theoretical guidance for the design and operation of CFB cyclone.