Study On Multi-scale Measurement Of Particle Dynamics In Hopper Flow

Particle matter is a system composed of a large number of discrete solid particles.When the frictional binding force between particles is overcome,the particle system will rupture and begin to flow,forming a particle flow.This process exists widely in nature,industrial and agricultural production fields,such as landslides,avalanches,debris flows,the movement of coarse particles at the bottom of rivers,the mixing and drying of different materials in pharmaceuticals,the pipeline transportation of granular materials in engineering and the rheological process of granular media after impacted,etc.Particle flow is a multi-body interaction system,which constitutes a multi-scale structure and has multiple physical processes and complex mechanical properties.Three scales are involved,that is,the macroscopic scale of the whole particle motion,the mesoscopic scale with the characteristic length of several particle sizes and the fine scale of the unit particle.From a macroscopic perspective,granular media is a typical disordered system,which is usually described as the random movement of particles.From the mesoscopic scale,the motion of particles is not completely random,but a series of ordered mesoscopic structures exist,such as force chains,shear bands and soft zones which are prone to nonaffine displacement.On the fine scale,the translational and rotational motions of the unit particle obey the classical Newtonian mechanics.Different scales of particle motion have different physical mechanisms,and particle movement between different dimensions has a certain correlation.Therefore,the study of multi-scale structure is the key to understanding the mechanical properties of particle flow and achieving precision regulation.The particle flow driven by gravitation can greatly simplify and provide a costless transportation mechanism,so it is generally concerned.The most common particle handling equipment: chutes,funnels and silos are gravity-driven.This paper focuses on the dynamics of solid particles in silos driven by gravity.The connection between particle flow mechanics in the silo in different dimensions is established and the movement of unit particle rotation at fine scale,the vector speed field distribution and particle temperature field distribution at a mesoscopic scale and the granular flow and mass flow rate at macroscopic scale are measured and represented by spherical detector based on inertial navigation technology,particle tracking speed technology based on optical flow method and spectrum-visual spectroscopy measurement technology.The main contents of this paper are as follows:1)A flat-bottomed rectangular silo device with variable orifice size was designed.The particle rheology of the glass beads in the inner fuel in the initial discharge process is measured by image method and synchronous speckle visibility spectroscopy,and the structural characteristics of the regional evolution of particle rheology were characterized.At the same time,the propagation velocity of particles in different directions is calculated according to the starting time difference of particles in different positions,and the relationship between the transformation of macroscopic particle flow pattern and the propagation velocity of mesoscopic particle movement is established.2)By using a particle tracking speed technology and speckle visibility spectral measurement technique,the vector speed field distribution and particle temperature field distribution during steady state flow in the silo is measured,according to the dynamic model to calculate the diffusion of particle movement in different regions of the intensity,analysis the correlation of the vector velocity field and temperature field,found in the radial direction vector velocities was positively correlated with the temperature of particles.However,due to the microscopic diffusion motion between particles,the vector velocity has a negative correlation with the particle temperature in the axial direction,especially at the axis of central symmetry of the silo,the vector velocity has a negative linear relationship with the particle temperature.This finding reveals that the diffusion motion between particles plays a crucial role in the energy dissipation of the silo particle flow.3)The spherical detector was placed as a tracer particle at different positions in the semi-cylindrical silo to measure the translational and rotational motion of the particle in the silo during unloading.By comparing the axial velocities of particles at different heights,it is found that the boundary between mass flow and funnel flow is certain.When the particle rotation angle is studied,it is observed that the variation of the particle rotation Angle satisfies the exponential function distribution,which is independent of the initial position of the particle.4)Speckle visible spectroscopy was used to find a low particle temperature region near the orifice of the silo.By changing the orifice size of flat bottom rectangular silo,it is found that the particle temperature near the orifice is inversely proportional to the mass flow rate.Under the condition of small orifice size,the dependence between the frequency of intermittent plugging and the orifice size of silo was established according to the change rate of particle temperature curve.5)The particle tracking velocimetry technology based on optical flow method is used to measure the vector velocity field distribution near the orifice of the silo.According to the variation of particle velocity at different times,the boundary of the free-falling arch is calculated,and it is observed that the free-falling arch is parabolic in the steady flow process.However,the geometric structure of free falling arch before blocking is asymmetric,which revealed the relationship between mesoscopic free-falling arch and macroscopic mass flow rate.On the other hand,according to the variation rule of particle temperature curve before plugging,the warning of plugging event is found.