Dispertion And Dust Characterstics Of The Free-falling Particle Flow

There is bulk material transport,convey,loading and unloading process in metallurgy,cement,chemical,pharmaceutical,food and other industries.Normally the material is free to fall to the receiving port or the conveyor driven by gravity.The dust produced by these processes is one of the most important pollution in industrial environment.Now more and more production processes transfer from outside to inside and the dust will accumulate in the enclosed space,resulting in a serious pollution of the whole building and extremely affecting safety of the production and the health of workers.Dust pollution is harmful to the workers exposed to the bulk material especially.However,the actual operation of the system still cannot solve the problem of dust pollution on the basis of the existing ventilation technology and methods.It is one of the key factors to solve the problem of dust pollution reasonably and efficiently by studying the transfer characteristics of falling dust.Therefore,on the basis of discussing the mechanism of dust production,this article studies the falling process of particle flow and the transfer characteristics of dust laden air obtained by high speed camera,and analyzes the dust generation characteristics of granular flow,which has important practical significance on realizing highly efficient control of dust and achieving the fine design of efficient control.In order to obtain the above research objectives,the characteristics of objects were analyzed firstly,due to different materials used in different production processes,four materials used in industrial production commonly are chosen for analysis.The results show that the distribution of alumina and aluminum fluoride is similar and the mean diameter is 56.02 ?m and 61.32 ?m respectively,the sphericity ? of alumina and aluminum fluoride is about 0.83~0.96;the mean diameter of 1# SiO2 and 2# SiO2 are 312.5 ?m and 133 ?m,respectively,and the spherical degree of silica is about 0.543~0.691;the mean diameter of 1#,2#and 3# glass beads are 303.7 ?m,127.7 ?m and 81.84 ?m,respectively,and the sphericity of the glass beads is 1.The mean diameter of alumina and aluminum fluoride are the same,and the particle size of 1# SiO2 and 1# glass beads are the same,and the particle size of 2# SiO2 and 2# glass beads are the same.Then the axial and radial flow laws of particle flow are studied by high-speed camera.The free falling particle stream could be divided into 3 regimes along the falling direction,named as stable regime,transitive regime and dispersion regime successively,and dust generation is analyzed quantitatively.The change rules of the dimensionless dispersion diameter(d/d0)and half width of the boundary layer(?/d0)during free falling follows a relationship with(Z/d0)are get,the first two matching the Boltzmann model in Sigmoidal distribution,And the last one presents the power function.It is shown that the axial velocity of particle flow in cross section is a Gauss distribution with self-similarity.The variation of axial velocity shows different change laws in different flow regimes.The axial velocity of the particle flow increases gradually and nearly equal to the velocity of the particle flow in vacuum when the flow comes into the transitive regime from the stable regime.Besides,the axial velocity of dispersive regime decreases gradually with drop height and close to the settling velocity finally.And the axial velocity between the two regimes increases at first and then shows a tendency of decreasing.The radial velocity is basically linear distribution.Three processes are divided for the movement of particle flow after impacting on the underlying surface,which can be summarized as: the formation of the first circle granular jump,the contraction accumulation process of the circle granular jump and the increasing process of the material pile.The radius of the first particle jump shows a linear growth with drop height for stable regime and transitive regime.As radial displacement increasing,the accumulate height of circle granule jump accords with the power exponent and shows an attenuation trend.The drop height has the greatest impact while the particle diameter has little effect on it.However,the hopper outlet diameter is basically making no difference.After the entrainment air is carried by the granular and then impacting on the wall,the particle flow will dispersion outward along the horizontal direction and the radius of vortex(yv)increases exponentially as the increase of horizontal displacement and nearly not effected by drop height and particle diameter in the range of this study.The share of dust production of falling and collision in different regimes are quantified by using the ground surface and the water surface as the underlying surface.When the particle flow is in the stable regime and transitive regime,the share of the collision dust is dominating,and the dust production rate decreases with the increase of the diameter,showing linear relation with drop height;while in the dispersive regime,the impact of dust production is less,and the dust production rate shows exponential relationship with drop height as the diameter of the outflow increases.The collision dust should be focused when the collision between the particle and the underlying surface occurs in the transitive regime,and the dispersion dust should be noticed when the collision occurs in the dispersive regime.