| Granular solid materials are widely used in industry,and their danger increases with the degree of subdivision.As a metallic dust with high activity and high energy density,aluminum powder is highly susceptible to explosive accidents during production,transportation,storage and use,causing significant damage.The generation of explosive dust clouds is determined by the physical properties and dispersion process of dust particles.In order to better understand the formation of real dust clouds,this study was based on the computational fluid dynamics software ANSYS Fluent and applied the Euler-Lagrange method and discrete phase model to trace the trajectories of discrete phase particles dispersed in the flow field according to the 20 L spherical explosion test device and the test process,and established a three-dimensional non steady-state numerical model.The model reliability was verified by comparing with the data of dust dispersion tests conducted by previous authors,and the formation of aluminum dust clouds under different particle size,particle size distribution and shape factor conditions were simulated.The effects of particle geometric characteristics on the flow field changes,particle aggregation degree,dust concentration and turbulence distribution in suspended dust during the dispersion of aluminum dust in 20 L spheres were discussed,which provided some reference for subsequent aluminum dust explosion modeling.The numerical simulation results show that the dispersion process of aluminum powder in the 20 L ball can be divided into four stages: rapid injection,diffusion,stabilization and settling.For aluminum dusts with different particle sizes,the particles are more likely to stay in relative suspension when the particle size is small,and the aluminum powder with large particle size shows obvious settling phenomenon;the degree of flow field shift of dust-containing gas is enhanced with the increase of particle size.Due to the settling and inhomogeneous distribution,the actual concentration value near the ignition electrode at 60 ms is about 15-32% of the nominal concentration,and the dust involved in the reaction may be much lower than the expected nominal concentration.The peak turbulent kinetic energy is positively correlated with particle size,and large dust particles can increase turbulence by creating an unstable wake.Compared with a single particle size,aluminum dust with multiple particle size distribution has high dispersion quality,the particle settling phenomenon is not obvious,the irregularity of flow field motion is enhanced,and the actual concentration value near the ignition electrode at 60 ms is higher,about 28%-59% of the nominal concentration.The presence of small particles of aluminum powder wrapped by the airflow is an important factor affecting the velocity change of dust particles for the blockage of large particles of aluminum powder movement,meanwhile,the regulation of turbulence by large particles is limited by the influence of small particles,so the particle size distribution only affects the evolution of turbulent kinetic energy in a certain range.Compared with spherical particles,the dispersion quality of non-spherical aluminum powder is poorer,and the actual concentration near the ignition electrode at60 ms is only 1.2%-21.6% of the nominal concentration.The larger the shape factor,the closer the particle shape is to spherical shape and the better the dispersion;aluminum dust with lower shape factor is subject to higher resistance and is more likely to accumulate on the wall.Non-spherical particles affect the velocity variation mainly based on the weakening effect on the kinetic energy of airflow,and the velocity maximum increases with the increase of shape factor;the degree of offset of turbulent kinetic energy distribution is stronger,and its symmetry,peak value and variation amplitude are enhanced with the increase of shape factor. |
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