Study On Granular Dynamic Behavior Of During Silo Discharging

Discharging is an essential intermediate link in agricultural and industrial production.Although the structure of the discharge system is simple,it is related to the stability and safety of the whole production process.During the discharging process of granular particles,there will be many problems that seriously affect the continuity and safety of discharging,such as intermittent discharge,blockage,vibration collapse of silo and so on.However,due to the complex dynamic behavior of particles in the discharging process,the above problems cannot be fundamentally solved up to now.Granular matter is the most common fourth state substance in nature except solid,liquid and gas,which is also widely used in many industrial fields.The dynamics of granular matter is closely related to industrial production and natural disasters,including agriculture and food,avalanche quicksand and so on.As a classical physical model with a variety of particle dynamics problems,silo discharging also shows its important academic value.Therefore,clarifying the dynamic behavior of particles in the process of granary discharging has not only important engineering value but also important scientific significance.In this study,based on the methods of theoretical analysis,experiment and numerical simulation,the dynamic behavior of particles in the discharging process is analyzed by the combination of two-dimensional discharging system and three-dimensional discharging system,and the mass flow state in the discharging process is studied.the purpose of this paper is to explore the origin,propagation and evolution of dynamic fluctuation in the discharging process,and to clarify the outflow flow behavior and impact effect.The main research contents and conclusions are as follows:(1)The flow state of particles during the discharging process.By studying the kinematics general law of particles above the outlet center,the velocity discontinuity described by the free-fall arch theory and the central velocity profile of the actual discharging process are compared and analyzed,and the continuity of the particle flow state in the discharging process is confirmed.Moreover the real origin of the transformation of macro and micro motion characteristics when particles break through the bottleneck are further clarified.As a result,the origin of discontinuity in the discharging process can be redefined and characterized.The results lays a theoretical foundation for the application of continuous hydrodynamics in the process of discharging.(2)Evolution and propagation of dynamic fluctuation in discharging process.The origin of initial disturbance is characterized by the peak transition point of particle falling velocity fluctuation above the outlet,and its identity with the boundary of discontinuous transition of motion state is proved.For the evolution of dynamic fluctuation,the dynamic evolution law and its relationship with blockage are clarified.According to the correlation between fluctuation evolution law and blockage,two different blockage mechanisms in the discharging process are proposed and clarified,which are collision blockage mechanism based on collision dissipation and blockage transition mechanism based on critical packing density.The discovery of different blocking mechanisms and main influencing factors provides reference for the selection of discharging auxiliary equipment and the anti-blocking design of silo.For the propagation of dynamic fluctuations,the correlation analysis method of dynamic fluctuations is adopted to fully reveal the propagation law of dynamic fluctuations in the silo from the correlation of axial space and time scale and the correlation of radial space and time scale respectively.The time scale and wave path of axial propagation of dynamic wave are quantified.The shear action in the process of radial propagation of dynamic fluctuation and the magnifying effect of radial fluctuation in spatial scale are clarified.(3)The flow behavior of particles after outflow during discharging process.The flow behavior of particles after discharge is systematically studied from three aspects: the analysis and modeling of the outer profile field,the formation mechanism of the flow behavior after outflow and the structural characteristics of the internal flow field after outflow.For the analysis and modeling of particle outflow profile field,the particle outflow profile field is divided into core layer profile field and boundary layer profile field,which is defined by 99% of the cumulative fraction of particles.Due to the different profile characteristics of them,the contraction section and divergence section of the core layer profile field,and boundary layer profile field are modeled respectively.Their mathematical models can be dimensionless on the length scale of the outlet radius,and the unified expression is given at last.For the formation mechanism of flow behavior of particles after discharge,the discharge behavior of particles under different discharge conditions is studied comparatively.The results of comparative analysis show that the formation mechanism of discharge behavior of particles is different from that of liquid,which is not affected by environmental fluid pressure and cohesion between particles.It is proved that its real origin is the transition boundary of grain motion state in the silo.In addition,the origin boundary of grain out flow behavior mentioned above is constructed during the discharging process of brown rice grains in three-dimensional silo.With regard to the internal structural characteristics of the flow field after particle outflow,a quantification method of local characteristics of particle outflow based on Voronoi unit division is proposed.The particle distributions in particle outflow field under different outlet sizes,falling heights and cumulative fractions are analyzed according to this method.The results show that this method can numerically calculate the concentration and uniformity of particle distribution in the particle outflow field,which can provide important design parameters for the discharging system.(4)The impact effect of particles after outflow during discharging process.Based on the analysis of the change law of particle velocity and attitude after outflow,the impact form of particles on the side wall and horizontal plane during discharging are revealed,and the prediction model of average falling velocity of particles is constructed.According to the single grain crushing probability model,the critical unit mass collision energy and critical impact breaking velocity of brown rice grains under vertical and horizontal impact posture were obtained respectively.Combined with the variation of particle impact attitude and average falling velocity with falling height,the effect of particle outflow impact on grain self-fragmentation is comprehensively evaluated.Through the form of theoretical derivation,the prediction models of important engineering design parameters such as mass flow rate,impact force and impact pressure are constructed and verified.The results show that the models have good prediction ability.The research results of this study can provide reference for the design of discharging system and the basic theory and research methods of particle dynamics.