Investigation On Frictional Charging And Electrostatic Effects Of Moving Granular Materials

Electrostatic phenomena of powders or granular materials are pervasive in the energy,chemical,and pharmaceutical industries.Due to continuous particle-to-particle and particle-towall contacts or frictions during the preparation,fluidization,transportation,or storage processes,the particles will inevitably be electrically charged.Excessive static charges on powders or particles can cause agglomeration,particle-wall adhesion,and the electrical discharge even results in fire and explosion hazards.A full understanding of particle charging characteristics and electrostatic effects in the moving particle system has important theoretical significance and industrial guidance value for preventing particle electrostatic hazards.However,the current research on the electrostatic characteristics of particles is still scattered,and the understanding of the contact electrification is still poor.In this thesis,experimental investigations on the impact charging and frictional charging of the single-particle were conducted to refines the robust electrification model for particles;at the scale of the particulate system,both the experiments and discrete element method(DEM)numerical simulations were employed to systematically analyze the multi-particle triboelectrification process and the movement of charged particles.The main results of the thesis are summarized as follows:A particle-plate impact charging system was designed and constructed and the high-speed photography technology was utilized to reconstruct the trajectory of the particle.The amount of charges on the particle before and after the impact with the plate were calculated according to the Newton's second law.The system overcomes the disadvantages of the traditional Faraday cup approach has difficulty in determining the impact charge with the external electric field and the characteristics for single-particle collision electrification under different working conditions are obtained.The impact charge is linearly related to the initial charge of the particle,and the linear slope is determined by the material of the particle.Meanwhile,the impact charge of the particle is found to increase with the impact speed.The polarity and magnitude of the impact charge for the particle-plate collision vary with the material types of particles.With the finite element method,the magnitude and direction of the external electric field at the impact point are determined,and the influence of the external electric field on the impact charging is explored.It is found that the impact charge under the external electric field is not affected by the initial charge of the particle but the external electric field dominates the direction of charge transfer during charging for the particle-plate contact electrification.Results of the particle-toparticle impact charging experiment show that when the particle with a smaller particle size collide with a particle with a larger particle size,it tends to obtain a positive charge.Based on the experimental results and the theory of electronic energy levels,the mechanism of the effects of surface charge on the particle and external electric field on the contact potential difference for the charge transfer is clarified and the physical model for driving charge transfer is proposed.The characteristics of the single-particle frictional electrification are experimentally investigated by changing the initial charge values,the material types,and the frictional distance for the particle.The results show that the average value and standard deviation of the triboelectric charge increase with the frictional distance.The conclusion that the triboelectric charge of the particle decreases with the increase of the initial charge is further established.Furthermore,the triboelectric coefficient is proposed to characterize the efficiency of frictional charging for the particle.The triboelectric charges and the triboelectric coefficient of different types of particles are compared.It is found that both the triboelectric charges and the triboelectric coefficient of the polymer particles are smaller than those of the cellulose particles.For the same material particles and after the same frictional distance,the triboelectric charge and the triboelectric coefficient for rolling friction of the spherical particles are smaller than those of the sliding friction of the cylinder particles.With the improved Greenwood and Williamson model,the relationship between the real contact area of rough particles and the amount of triboelectric charge is quantified for the first time.When the normal load increases,the real contact area of the rough particles increases as well,resulting in a significant rise in the amount of triboelectric charge.The average triboelectric charge is found to be negatively correlated with the average surface roughness and the summits curvatures of the rough peak are responsible for the change of the real contact area and eventually leads to the change of the triboelectric charge.At the scale of the particulate system,the triboelectrification of particles under different physical parameters and operating parameters is systematically studied when particles are in a rotating spherical container.Based on the curve of exponential growth of the electrostatic quantity of the particles with time,the equilibrium charge and the total triboelectric coefficient of the particles are extracted.The effects of rotational speed,particle material,and relative humidity on the equilibrium charge and triboelectric coefficient were analyzed.The relaxation law of the surface charge density on the charged particles is obtained and the charge generation constant and the charge relaxation constant of the particles are obtained.Based on image processing technology,the parameters such as the trajectories and velocities of the particles in the rotating container are accurately collected.To characterize the distribution characteristics of the charged particles,some characteristic parameters such as the minimum circumscribed circles(MCC),the radial probability distribution functions(PDF),and the granular temperature are employed.The influence of the electrostatic interaction on the particle dispersion is clarified.As the charging time increases,the charge values on the particles increase,which in turn leads to the radius of MCC as well.In addition,the growth of the charge value on the particles causes the particles to diffuse outward collectively and results in the curve of PDF becomes flatter.It is also found that the total number of contacts between particles decreases rapidly with the charge on the particles.With the increase of the charge on the particles,the granular temperature of the particles undergoes a sharp rise initially,then decreases to a minimum value,and finally increases again gradually.A discrete element method(DEM)is developed for the first time,in which a robust numerical model for analyzing frictional charging of particles arising from the rolling or sliding are implemented and applied the charge generation and the electrostatic dispersion processes of particles in a rotating container are numerically investigated.The sensitivity of charge transfer to the mesh density is assessed and the DEM model was validated using the experimental data,such as the charge value on the particles,the degree of particle dispersion,and the granular temperature.It is found that these data obtained numerically and experimentally are in good agreement.Meanwhile,the charge accumulation and distribution on the wall surface and its effects on the particle charging and electrostatic interaction were also considered and implemented into the DEM.The existence of surface charges on the inner wall of the container will reduce the charge on the particles,and the Coulomb's attractive force between the charged particles and the charged wall will promote the dispersion of the particles.Using the validated DEM,a systematical study of frictional electrification behavior is performed numerically and discussed in this study.The degree of the particle dispersion in terms of the radius of MCC increase with time,indicating the charge on the particles plays a significant role in the dispersion of the particles.As the rotation continues,the granular temperature for neutral particles starts to fluctuate around a constant value,while for the charged particles the granular temperature gradually increases.It is shown that the coefficient of friction has a limited influence on the charge transfer process but affects the dispersion and the granular temperature of the particle system.