| Pneumatic conveying is a green and widely used bulk material conveying technology.Due to the complex material properties and variable operating system boundary conditions,it is easy to produce flow instability inside the conveying pipeline,which may affect the safety of the pneumatic conveying system in serious cases,and there is still a lack of consensus research findings on the evolution characteristics of pneumatic conveying flow patterns.To this end,this paper investigates the evolution characteristics of pneumatic conveying system flow patterns by combining Newton’s second law,gas-solid two-phase coupling theory,computational fluid dynamics and discrete element coupled numerical simulation（CFD-DEM）and pneumatic conveying orthogonal tests,specifically.Based on Newton’s second law and the coupled CFD-DEM framework of four-sided coupling,the mass conservation equation and momentum conservation equation of gas-solid two-phase are established to clarify the force situation of particles in the pipe,analyze the selection of traction model and clarify the kinetic equation of particle collision contact process,which provides basic theoretical support to study the flow pattern evolution of pneumatic conveying system.Based on the coupled CFD-DEM numerical simulation method,a numerical model of vertical pipe pneumatic conveying process is established to study and reveal the flow characteristics of vertical pipe pneumatic conveying from three aspects,such as volume fraction of particles in the cross section,flow pattern evolution in the pipe and pressure drop per unit pipe length.The results show that in the vertical pipe pneumatic conveying process,the cross-sectional particle volume fraction first increases with time and then tends to stabilize,and the particle volume fraction in the stabilization phase gradually decreases with the increase of apparent gas velocity ug,and the flow pattern in the pipe shows blockage,high concentration annular nucleus flow,medium concentration annular nucleus flow,low concentration annular nucleus flow and suspension flow in order,and the pressure drop per unit pipe length first decreases and then increases,and the numerical simulation experimental conditions The velocity of the lowest point of pressure drop under the numerical simulation is 8 m/s.Based on the coupled CFD-DEM numerical simulation method,a numerical model of the pneumatic conveying process in the horizontal pipe is established,and the particle flow behavior under different apparent gas velocities is investigated from three aspects,including the inlet pressure signal,the particle mass in a standard volume（the volume of a cylinder with a diameter of 80 mm and a height of 4 mm）,the particle distribution and velocity characteristics,and the pipe cross-section is divided into eight investigation areas to evaluate the particle distribution characteristics along the cross-section.The distribution characteristics of the particles along the cross-section were evaluated to reveal the flow pattern characteristics of horizontal pipe pneumatic conveying.The results show that the fluctuation of the pressure signal at the inlet can reflect the change of the flow pattern in the pipe,and when the apparent gas velocity ug is 5 m/s,the pipe shows embolic flow,and the particle mass of the standard volume has a peak,and the particles exist in the whole investigation area;when the apparent gas velocity ug is 6-8 m/s,the pipe shows dune flow and the transport flow pattern between embolic flow and dune flow,and the particle mass of the standard volume is in the range of2.7×10-3～3.8×10-3 kg,and the particles exist in the examination area 1～6;when the apparent gas velocity ug is 9～10 m/s,the tube shows stratified flow,and the particle mass of the standard volume is taken as 1.7×10-3～2.5×10-3 kg,and the particles exist in the examination area 1～4.A multifunctional pneumatic conveying experimental bench was designed and developed to study the effect of the silo pump replenishment method on the pneumatic conveying flow characteristics.Firstly,the experiments of dilute phase and dense phase pneumatic conveying were carried out separately,and it was found that the flow pattern in the tube was closely related to the pressure signal,and the fluctuation range of the pressure signal in the dense phase pneumatic conveying system was much larger than that in the dilute phase conveying,and the pressure signal in the dilute phase pneumatic conveying system was mainly in the low frequency signal,while the pressure signal in the dense phase pneumatic conveying system was uniformly distributed in the high,medium and low frequency bands;secondly,based on the relationship between the flow pattern and the pressure signal obtained in the previous subsection Secondly,based on the relationship between flow pattern and pressure signal obtained in the previous subsection,orthogonal test analysis was conducted with fluidized gas（factor A）,regulating gas（factor B）and pressurized gas（factor C）as factors,and the following conclusions were drawn:the appropriate amount of fluidized gas can improve the stability of pneumatic conveying,the increase of regulating gas flow is conducive to maintaining the stable change of flow pattern in the tube,the increase of pressurized gas flow is not conducive to maintaining the stable change of flow pattern in the tube,and the main and secondary factors affecting the pressure signal in the stable phase of pneumatic conveying are The main and secondary factors affecting the pressure signal in the stabilization stage of pneumatic conveying are:fluidized gas valve opening A,pressurized gas valve opening C,regulating gas valve opening B.The optimal level combination of orthogonal test index:A2B2C1,i.e.fluidized gas valve opening 2/3,regulating gas valve opening 2/3,pressurized gas valve opening 1/3. |
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