Convection And Separation In Granular Systems Under The Combined Effect Of Vibration And Airflow

Under the coupling of vertical vibration and counter-gravity airflow,granular matter forms convection motion.In the process of particle convection,different types of particles are separated or mixed in both vertical and horizontal directions.Convection and redistribution behaviors of mono/binary-size particles have been investigated by systemic experiments under the combined effect of vertical vibration and counter-gravity airflow.Varied patterns of particle convection and distribution have been observed and phase diagrams of particle convection and distribution have been provided.The control methods and conditions of particle convection pattern,particle distribution pattern and particle separating degree are obtained.With the increase of vibration acceleration(I'=2?7),the increase of air velocity(w = 0.00?1.82 m/s)or the decrease of vibration frequency(f= 15?45 Hz),the convection with a pair rolls is gradually changed to the convection with a single roll.An increasing I'or a decreasing f/tends to cause the transition from Brazil nut separation(BN)to the mixing distribution(MIX),with the appearance and disappearance of Horizontal separation(HS).When u is increased gradually,BN is changed to Reverse Brazil nut separation(RBN),with the appearance and disappearance of MIX and HS.Based on the experimental results of particle convection and distribution,the mechanism that particle convection causes particle separation is proposed.The specific process of the particle redistribution is given.It's found that theseparation behavior of particles is mainly depended on the intensity of particle movement,namely,the convection intensity.The specific pattern of particle distribution is mainly determined by the convection pattern,which includes the relative position of convective regions of large and small particles,and the state of particle movement in convective regions.By analyzing the effects of external excitation and particle poperties,the vertical separation criterion is presented to discuss the separation behavior of binary particles under under the combined effect of vertical vibration and counter-gravity airflow:(1)The binary particles5 acceleration difference component ?e is proposed to represent the influence of the specific external-excitation condition on a certain granular system.With a low u,the effect of vibration is dominant and ?e>0,large particles move up relatively to small ones,causing a Brazil nut separation(BN).With a high u,the effect of airflow is dominant and ?e<0,small particles move up relatively to large ones,causing a Reverse Brazil nut separation(RBN).(2)The separation response coefficient } is proposed to evaluate the separation response of the granular system itself.For values of ^>1,the system has a weak separation response where it inclines to separate weakly or mix uniformly.For values of?<1,the system with a strong separation response exhibits the mixing state,weak separation,or strong separation depending on the specific condition of the external excitation.With the increase of binary particles' size ratio ?d or density ratio ?p,or the decrease of total-mass ratio?M,the separation response coefficientis reduced,the separation response of the granular system is weakened.