Experiment And CFD-DEM Study Of Gas-solid Two-phase Flow By Oscillatory Flow

It is one of the key issues to ensure the continuous and stable conveying of the dilute-phase pneumatic conveying system at low gas-velocity and minimum power consumption.The method of oscillatory flow conveying has been proved to be effective in reducing the pressure drop and the minimum pressure drop velocity.However,the complex multi-scale structure of gas-solid two-phase flow in oscillatory flow field has not been fully guided by theory,which seriously hinders the design,optimization and amplification of the dilute-phase pneumatic conveying system.Therefore,in this paper,the particle imaging velocimetry(PIV)technology and the coupled method of computational fluid dynamics(CFD)and discrete element method(DEM)are adopted to study deeply the gas-solid two-phase flow mechanism in the process of the oscillatory flow pneumatic conveying.It has important academic value to promote the industrial application of the oscillatory flow pneumatic conveying technology and broaden the product upgrading of the dilute-phase pneumatic conveying system.The main research contents and conclusions are as follows:Firstly,the pressure drop,the minimum pressure drop velocity and the power consumption is studied by building a dilute-phase pneumatic conveying system in case of axial flow and oscillatory flow,and the stability of oscillatory flow conveying is analyzed according to the standard deviation of pressure fluctuation.The results show that the pressure drop,the minimum pressure drop velocity and the power consumption of the conveying system are reduced by the oscillatory flow,and the oscillatory flow has the most significant influence on the conveying pressure drop in the horizontal conveying pipeline.In addition,by analyzing the standard deviation of the minimum pressure drop,it is found that the standard deviation of the pressure drop of the oscillatory flow conveying system is decreased first and then increased with the increase of particle mass flow rate.Secondly,the PIV technique is used to study the particle distribution and velocity characteristics in cases of the axial and oscillatory flow at different positions and different gas-velocities in the pipeline.Compared with the axial flow conveying,The results show that the sediment layer of particles at the bottom of the tube is reduced by the oscillatory flow,which the particle axial and radial average-velocity are increased at the bottom of the horizontal pipe,and the particle fluctuating velocity is enhanced in the conveying process.In addition,the energy-saving effect of low-velocity oscillatory flow is weakened because the particles cannot be dispersed in time,while the influence of oscillatory characteristics on particle flow is weakened at high-velocity oscillatory flow due to higher gas velocity.Thirdly,the collision restitution coefficient and the friction coefficient(between the particles or the pipe wall)are measured by experiment,high-speed camera and image processing technology.Based on the particles repose angle measured by experiments,the friction coefficient between particles is calibrated by means of EDEM experiment and mathematical regression analysis.The results show that the calibrated inter-particle friction coefficient can better reflect the flow characteristics of the actual particle group.Finally,the turbulence characteristics of the oscillatory flow field are studied by CFD dynamic mesh technology.And further,on the basis of the established discrete element model of test particles,the microscopic flow characteristics of the oscillatory flow field particles are discussed by CFD-DEM method.The results show that the axial velocity of the oscillatory flow in the pipeline is more uniformly distributed along the radial direction,and the oscillatory flow has a strong tangential component.At the same time,the oscillatory flow has periodic turbulence pulsation,but the turbulence pulsation intensity of the oscillatory flow is decayed along the flow direction of the pipeline gradually.In addition,the CFD-DEM simulation shows that the initial motion trajectory of particles are changed by the oscillatory flow,so that the number of collisions between particles and pipe wall is increased and the number of collisions between particles is reduced.Moreover,compared with axial flow conveying,the drag force acting on particles in oscillatory flow conveying is larger.