| At present,the main conveying methods of grain particles are bucket conveying,belt conveying and spiral conveying,etc.However,due to the complex and diverse conveying environment conditions and supporting equipment structure,the above conveying methods cannot effectively meet the needs of operating occasions.Foreign pneumatic conveying technology developed in the 80 s from metallurgy and chemical industry has been widely used for conveying bulk materials such as rice,wheat and corn because of its simple structure,flexible configuration and convenient operation.However,in the conveying process,when the particles flow through the bend,under the action of inertia and centrifugal force,the particles gather on the outer wall of the bend,resulting in uneven local mixing of gas and solid phases,causing vibration,impact and erosion of the bend,which affects the service life of the bend.Therefore,this paper intends to construct three different curvature radius bends and their corresponding vertical pipes for pneumatic conveying of crop pellets to investigate the influence characteristics of different curvature radius pipes on the movement of gas-solid phases in the pneumatic conveying process.First,the equivalent diameter,sphericity distribution,density and collision recovery coefficient of three crop particles,corn,wheat and soybean,were measured.Subsequently,the gas-phase velocity distribution,particle-phase velocity distribution,particleparticle/particle-wall collision number and particle collision energy loss in the pipe were analyzed by coupled CFD-DEM calculations for R/D=3.75,5 and 6.25 bends and their corresponding vertical pipes.It was found that the number of particle collisions was higher in the bends with small curvature radius,but the number of collisions gradually decreased as the curvature radius of the bends increased.The energy loss when conveying particles with large curvature radius bends is less and more favorable to the flow of particles in the pipe.Then,the optimal conveying air velocity(MPD velocity),system pressure drop distribution,and power dissipation coefficient of R/D=3.75,R/D=5,and R/D=6.25 bends for conveying wheat,corn,and soybean pellets at different mass flow rates were investigated,and the artificial neural network technique was used to predict the pressure drop of the pneumatic conveying system.It was found that the pressure drop of the system when using the bend with R/D=6.25 for conveying pellets was lower than the remaining two bends,and the maximum reduction of MPD velocity was 3.18% and 5.27% for R/D=5 and R/D=6.25,respectively,compared with R/D=3.75,and the energy loss of the system could be effectively reduced when using a large curvature radius bend for conveying pellets.The pressure drop model established by artificial neural network can predict the pressure drop value of the system more accurately,and the error is within ±1.5%.Finally,the velocity of particles was measured at MPD velocity using a high-speed particle image velocimetry(PIV).The motion characteristics of particles in different curvature radius bends and their corresponding vertical tubes are analyzed in terms of the intensity,power spectrum and time-frequency characteristics of the particle pulsation velocity,and the continuous wavelet transform and one-dimensional discrete orthogonal wavelet decomposition are used to reveal the motion characteristics of the particles.The results show that the particle is dominated by large scale motion in the vertical tube,while the small-scale motion increases with the increase of conveying height.Based on the power spectrum analysis of the particle pulsation velocity,it can be determined that the particle pulsation velocity in the vertical tube corresponding to the large curvature radius bend produces larger power spectrum peaks in the low frequency range,which is related to the reduction of the pressure drop. |
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