Study On Multiphase Flow Field Characteristics In Pneumatic Conveying Of Bulk Grain

Pneumatic conveying has the advantages of simple structure,good sealing,good cleanliness and flexible conveying,which can well solve the problems of leakage,mildew and pollution in the traditional conveying process of bulk grain.In the process of pneumatic conveying bulk grain,the flow characteristics and movement mechanism of particles are very complicated due to the gas-solid two-phase flow.Studying the flow characteristics is the key to improve the conveying efficiency and stability of bulk grain particles.It is difficult to obtain the specific flow characteristics simply by using the experimental method.Therefore,this paper takes wheat particles as the object,adopts the method of combining experiment and numerical simulation to study the pneumatic conveying of wheat,provides theoretical basis and practical guidance for the application of pneumatic conveying of wheat,and expands the application field of pneumatic conveying technology.Firstly,based on the Euler-Euler two-fluid model,particle kinetics,Gidaspow drag force model and Johnson&Jackson wall model,the force of bulk grain particles in the pipeline was analyzed,and a mathematical model of gas-solid two-phase flow in wheat pneumatic conveying was established.The flow characteristics and pressure drop in straight pipe and curved pipe are predicted by the simulation.The pressure drop error is 14% and13% respectively compared with the test,which verifies the reliability of the model.Secondly,Fluent was used to numerically simulate the pneumatic conveying process of wheat in horizontal pipeline,and the influences of conveying speed,conveying concentration and other factors on the flow characteristics in the pipeline were analyzed.The results show that the particles accumulate at the bottom of the pipe due to gravity,the gas-solid velocity is fast on the upper part of the pipe,and the wall shear force is at the lower wall of the pipe.With the increase of gas-solid two-phase velocity,the accumulation height at the bottom of the pipeline decreases.With the increase of transport concentration,the gas-solid two-phase velocity decreases and the accumulation height at the bottom of the pipeline increases.Particles will not break when they collide with the wall in horizontal pipeline.The numerical simulation of pneumatic conveying process of wheat with curved pipe was carried out to analyze the influence of conveying speed,conveying concentration,bending diameter ratio and other factors on the flow characteristics in horizontal and vertical curved pipe.The results show that when particles flow through the bend,the velocity of gas-solid phase at the wall of the bend is large.Under the action of centrifugal force,particles accumulate on the outer wall of the bend.The shear force at 30°～45° of the outer wall of the bend is large,and the inner and outer walls wear seriously in this range,and wheat particles may be broken.The accumulation height at the bend decreases with the increase of gas-solid velocity.With the increase of conveying concentration,the gas-solid two-phase velocity decreases,the accumulation height at the bend increases,and the shear force decreases.When the conveying concentration is above 0.3,the shear force does not change.With the increase of bent-diameter ratio,the gas-solid two-phase velocity increases,the accumulation height at the bend decreases,and the shear force decreases.When the bent-diameter ratio is above 4,wheat particles will not be broken at all.Finally,the pneumatic conveying platform of bulk grain was built independently,and the conveying experiments of horizontal pipeline and horizontal curved pipe were carried out at different conveying speeds and concentrations,and the pressure drop characteristics of pipeline were analyzed.The results show that the pipe pressure drop increases with the increase of conveying speed and conveying concentration,and the pressure drop of curved pipe is greater than that of horizontal pipe under the same experimental conditions.Based on the Barth-attached pressure drop theory,the empirical formula of wheat particle pressure drop coefficient was obtained by regression fitting with dimensional analysis method.The error of wheat phase pressure drop coefficient calculated according to the empirical formula and the test value was within 15%.