Research On Evolution Characteristics Of Clusters In A Riser Based On The Clustering Algorithm

The complex movement process of particle clusters in the riser results in uneven distribution of the solid phase in space and time,which seriously affects the gas-solid contact process of the process bed and weakens the gas-solid mass transfer efficiency.Scholars at home and abroad have carried out a lot of research on the characteristics of clusters.But at present,on the one hand,the method of discriminating particles clusters is not clear,and different criteria should be adopted for different working conditions.On the other hand,few researchers track and analyze the mechanism of the clusters’ evolution process.Therefore,this paper uses the K-means clustering algorithm to study the characteristics of the agglomeration and the mechanism of the clusters evolution process by analyzing the void ratio field of the riser and the particle velocity field within the clusters.Based on the discrete particle model,the gas solids two-phase flow behavior is numerically simulated in a riser,and the experimental datas of the simulation result Varas et al.,Verify the accuracy of the simulation results.At the same time,gas-solid flow characteristics such as average particle volume,particle velocity,and granular tempreture were analyzed at the same time.The results show that the gas velocity of the riser is gradually reduced in the axial direction,and the length of the side wall is high in the horizontal direction,the center low ring-nuclear flow distribution,the granular temperature is gradually reduced.With the increase of the inlet mass flow rate,the axial distribution of the average particle concentration along the riser increases,and the granular temperature increases.Based on the K-means clustering algorithm and image processing technology,clustering analysis of the void ratio field of the riser,realizing multi-threshold automatic recognition clusters’ shape under different conditions.The influence of different air velocity and mass flow rate on the characteristic parameters of clusters size,solid holdup,cluster frequency and cluster duration were fuether studied.The results show that with the increase of air velocity,the radial and axial sizes and average solid holdup of particle clusters decrease gradually.With the increase of inlet mass flow,the radial size and average solid holdup of particle clusters increase gradually.The cluster frequency was almost linear with the average particle volume fraction.Based on the K-means clustering algorithm,the instantaneous voidage field in the riser was identified,and two complete single cluster motion processes(single cluster dissipation and aggregation)were captured.The secondary clustering analysis of the particle velocity fields in the two processes was carried out.The evolution mechanism of clusters in different motion processes was analyzed by combining the clusters aspect ratio and instantaneous granular temperature in local region and particle velocity vector distribution.The results show that during the dissipation process,the cluster tends to flatten gradually,and the granular temperature above and on the right side of the cluster increases gradually,and a large number of particles gradually concentrate on the right side of the cluster,and gradually break away from the cluster.During the polymerization process,the ratio of width to height of the aggregates changed little,the granular temperature above and on the right side of the clusters decreased gradually,the number of particles invaded the clusters was small,some particles melted into the clusters.Based on the K-means clustering algorithm,the instantaneous voidage field in local region was analyzed to capture the cluster motion process,including the cluster co-collision process and the cluster anisotropy collision process.By analyzing the clusters aspect ratio,fractal dimension,instantaneous granular temperature and particle velocity vector distribution in local region,as well as the clustering analysis results of particle velocity distribution in clusters using K-means algorithm,the evolution mechanism of co-direction and anisotropic collision process of clusters was studied.The results show that in the process of co-collision,the clusters aspect ratio decreases rapidly at first,then increases slowly and finally becomes regional stable.Under the action of clusters at the top,the cluster at the bottom gradually form a half-U-shaped structure close to the wall and then wrap the cluster at the top,until they reach a relatively stable state.In the process of anisotropy collision,the clusters aspect ratio decreases rapidly,then increases sharply,and finally becomes smaller and becomes stable.