Study On Dense Powder Flow Of Pneumatic Conveying

Dense powder pneumatic conveying is widely used in various industrial fields due to its low energy consumption,high solid-gas ratio and low pipeline wear.Frequent interactions of particle to particle and particle to wall increase the complexity of flow when transported at higher solid concentration.At present,the research on pneumatic conveying mechanism and some related flow characteristics of high-pressure dense powder is not mature,especially the evolution law of flow pattern,flow pattern recognition mechanism and flow pattern transition prediction are less understood.Based on experiments and numerical simulation,the solid-phase flow characteristics of dense powder pneumatic conveying are systematically studied.Especially the mathematical model construction and the in-depth discussion on the mechanism of pneumatic conveying of dense powder have been carried out.In view of the shortcomings of the existing mathematical models for dense pneumatic conveying,a mathematical model for describing the movement of discrete powder is established by combining Euler and Lagrange methods,which can adapt to the extremely uneven distribution of solid concentration in dense phase pneumatic conveying.In the process of describing the internal interaction between solid phases,the model is divided into two parts,i.e.intra-grid and inter-grid.The particle to particle interaction in the grid is described by particle characteristics,particle concentration and particle velocity.Through statistic of solid phase information in each grid,the shear stress forces caused by the concentration gradient and velocity gradient among grids is calculated by Euler method,so as to make the mathematical model closer to reality.In order to verify the accuracy of the model,the conveying flow pattern and transformation law under different conveying conditions were obtained by establishing a visual dense-phase pneumatic conveying experimental device.By establishing a visual hopper experimental device,the flow pattern of the same kind of powder discharging was photographed and analyzed,and the mixing curve was obtained by mixing experiments of different layers of powder discharging process in the hopper.The qualitative comparison and quantitative analysis of the experimental results and simulation results show that they are in good agreement,which proves the validity and accuracy of the new mathematical model proposed in this paper.The model was used to numerically simulate the pneumatic conveying of high-pressure dense powder horizontal pipe.The evolution of flow patterns under different operating parameters was studied.The physical characteristics of different flow patterns in the conveying pipe were analyzed.The solid phase concentration signal was analyzed by signal analysis method,and the relationships between the solid phase volume fraction,power spectral density function,probability density function of solid concentration,Re number and Ar number and flow pattern were established.The prediction of flow characteristics and flow pattern transition in pipeline was realized.The flow pattern transition rule and criterion of high pressure dense pneumatic conveying process were obtained.The results show that when the superficial gas velocity was increased or the ratio of solid to gas was reduced or the particle size was decreased or the pressure was increased,the flow pattern in the horizontal pipe changed from slug flow to dune flow and then to stratified flow.When the slug flowed through horizontal pipeline,pressure loss increased gradually from the front to the tail of slug.When the particles in the tail of slug were no longer filled with the pipeline,pressure loss decreased sharply.In addition,the velocity of particles in the tail of slug was faster than that in the front of slug,and the concentration of solid phase in the tail of slug was denser than that in the front of slug.The pressure loss from the front to the tail of the dune increased first and then decreased,while the particle velocity decreased first and then increased,and there was a negative relation between the particle velocity and pressure.When the horizontal pipe was a stratified flow,the unit pressure drop,particle velocity and particle concentration distribution in the pipe appeared no significant change.The power spectrum analysis of different flow patterns showed that there was an obvious dominant frequency in the power spectrum of slug flow,the particle concentration fluctuated greatly and the flow was unstable;in dune flow,the dominant frequency became less obvious and the fluctuation of particle concentration weakened,but it was still in unstable flow;in stratified flow,the dominant frequency disappeared and the particle flow was relatively smooth and stable.The probability density distribution of solid concentration was related to flow pattern,and different flow patterns had different peak and shape of probability density function.According to the current transportation conditions,the characteristic parameters of solid concentration were used to quantitatively identify the flow pattern in the pipe.There was a power function relationship between Ar number and Re number in the flow pattern transition region,and the quantitative expression of the transition gas velocity between different flow patterns was obtained.The dense phase pneumatic conveying in the bend was investigated.The flow characteristics of solid phase and its variation under different transport conditions were revealed while powder flowed through the 90 ~o bend under high pressure.The results show that as the superficial gas velocity increased,the solid concentration in the horizontal pipe decreased at a higher rate than those in the bend and vertical sections.Based on the calculation conditions adopted in the base case,the solid concentration in the horizontal section was almost the same as that in the vertical section for U_g≥10 m/s,indicating that the stable and consistent conveying regime was achieved.The fluctuation intensity of solid concentration in the bend was higher compared to those in the straight sections.Similar profiles of the average particle velocity along the bend section(from horizontal to vertical)were obtained for different superficial gas velocities with the average particle velocity consistently reaching the trough in the middle of the bend section.As the bend radius increased,the particle flow in the horizontal section experienced a transition from the slug flow pattern to the stratified flow pattern.A bend radius of 3.0D was shown to be sufficient to achieve the stable particle conveying process.As the bend radius became larger,the trough point shifted to the location close to the bend entrance(i.e.,from 75~o to 30~o for R=1D and 5D,respectively).The role of bend radius on the particle velocity entering the bend was subtle,yet was significant on the particle velocity exiting the bend.Distinctly different particle flow patterns changing from stratified flow pattern to slug flow pattern were observed in the horizontal section by altering the particle size.The solid concentration in all sections generally increased as the particle size increased.The average particle velocity increased as the particle size decreased.Also,the velocity variation profile through the bend section was consistent for different particle sizes with all profiles reaching the trough in the middle of the bend.The storage and discharge of powder is an essential part in pneumatic conveying system.It is important to understand the discharge flow characteristics of hopper.Through the study of flow pattern,discharge flow rate and mixing characteristics of different materials in conical hopper during discharge process of different structure hoppers,it is shown that the discharge outlet size has a great influence on the discharge flow rate,and it has a non-linear relationship.Under the same discharge outlet diameter,the discharge flow rate decreased with the increase of the half angle of hoppers when the half angle of hoppers was less than 45~o,and that the discharge flow rate did not change much when the half angle of hoppers was increased beyond this value.In addition,in order to describe the mixing characteristics of different layered powder materials in the unloading process of hoppers,the mixing degree was effectively described by the variation coefficient of powder mass fraction.On this basis,the mixing rules of different structure hoppers were obtained.The results show that increasing the half angle of the hopper was helpful for the mixing of materials in different layers during unloading process,and reducing the outlet diameter of the hopper will reduce the mixing index(i.e.enhance the mixing),but the decrement was small.