The high-performance seed metering technology is used to improve seeding quality and achieve high-speed seeding^［1-3］. Through centralized seeding and pneumatic conveying method， the pneumatic centralized system can avoid the problems of difficulty in filling and clearing seeds at high speed； and has the advantages of reducing seed damage rate and increasing adaptability for crops of different particle sizes. Therefore， the pneumatic centralized seeding technology has been one of the most important technical methods for high-performance seeding^［4-5］.

In recent years， pneumatic centralized seeding technology has developed rapidly in China^［6-13］. For example， LEI et al.^［6］ carried out simulation analysis and experimental research on the structural parameters of the seed tube of the seed distributor. LIAO et al.^［7］ designed the dome-type seed distributor for the pneumatic centralized system and developed combined rape and wheat pneumatic centralized seeder. ZHANG et al.^［8］ developed the pneumatic centralized seeder of wheat and optimized the structural parameters of the distributor. DAI et al.^［11］ designed the pneumatic centralized seeder of rice and optimized the internal structure of the distributor. KUMAR et al. ^［14］ proposed three outlet types of the seed distributor； and completed the simulation and experimental comparison research.

CFD （Computational Fluid Dynamics） is widely used to design the pneumatic centralized system. By analyzing the motion parameters of the internal flow field， the researchers can obtain the flow distribution， to optimize the structural parameters of the pneumatic centralized system^［15-17］. However， CFD can only simulate the flow field， but not the movement of seed flow. With the development of computer simulation technology， CFD-DEM gas-solid coupling has been used to research the movement of the seed flow inside the pneumatic centralized system^［18-25］. FLUENT is used to simulate the flow field， and EDEM is used to build the seed model. The movement law of the seeds flowing inside the pneumatic centralized system can be simulated by the combination of that software. Compared with the traditional flow simulation， CFD-DEM can obtain simulation results closer to the actual seed metering， so the accuracy of the results is easier to verify.

The study is based on the seed distributor， which is the main component of the pneumatic centralized system. The CFD-DEM coupling simulation method is used for experimental research， and the simulation results are verified by benches and field experiments. The T-type， Y-type， and M-type seed distributors were designed to improve the seeding quality of wheat seeds. The seed distributor with the best outlet type was selected for further structural parameter optimization research. The study has theoretical value and practical significance.

The pneumatic centralized system is shown in Figure 1（a）， mainly including the fan， seed box， seed metering， air-seed mixer， seed tube， corrugated pipe， and seed distributor.

The seeds discharged from the seed box and the airflow generated by the fan are mixed to form seed flow in the air-seed mixer， which enters the seed distributor through the corrugated pipe； and exits from the flow outlet after being equally divided.

The seed distributor is the main component of the pneumatic centralized system， as shown in Figure 1（b）， it mainly includes a conical cover， inner tube， inlet， and several outlets. The seed flow enters the inner tube from the inlet， and under the guidance of the Y-type inner tube， the seed flow is equally divided and discharged from outlets. When the seed flow passes through the outlets， it will collide with the wall， which reduces the uniformity of seed metering. Therefore， the uniformity of seed metering can be improved by optimizing the outlet structure of the seed distributor.

The formation of seed flow depends on pneumatic conveying. Research shows that the damage rate of seeds increases at high speed， and the problem of accumulation occurs at low speed. Therefore， to reduce the damage rate， the minimum speed at which the seeds can be suspended is generally used as the pneumatic conveying speed of the seed flow. The suspension speed of wheat seeds is calculated by the Formula （1）^［26］.

where， v₀ is the suspension speed； K_s is the correction factor， the wheat seed is nearly spherical， K_s is 1.1； d_f is the equivalent particle size； ρ_f is the particle density； ρ_a is the air density； C is the coefficient of aerodynamic drag， which is 0.44 for wheat seeds； g is the gravitational acceleration. The measured results show that the particle density of wheat seeds is 1 335 kg/m³， and the equivalent particle size is 4.01×10^-3 m.

According to Formula （1）， the suspension speed of wheat seeds is 10.63 m/s. About the empirical coefficient of pneumatic conveying speed， when the flow field is relatively complex， the pneumatic conveying speed is 2.6-5.0 times the suspension speed. Therefore， the pneumatic conveying speed of wheat seeds v_a in the simulation and experiment is 28 m/s.

According to the relevant technical requirements of wheat and other major food crops in Shaanxi Province， the main technical parameters of the wheat pneumatic centralized seeder are determined as follows： the maximum working width of the seeder is 2 m， and the number of sowing rows is 6. According to the surface conditions of different planting areas of wheat， the control range of the working speed of the seeder is determined to be 4-12 km/h， and the sowing amount of dryland wheat is 130-140 kg/hm². The seed metering device adopts the outer groove wheel seed metering device， the speed range is 20-60 r/min， the number of grooves is 10， and the adjustable length is 110 mm.

Based on the traditional Y-type seed distributor， the movement and force of seed particles passing through the inclined tube of the outlet were analyzed， as shown in Figure 2.

Under the combined function of gravity （W）， drag force （F_R）， and viscous force （T_f）， it is assumed that the seed flow passes through the distributor outlet at the speed v_s without collision and rotation. The axial mechanical equilibrium equation of seed flow is established， as shown in Formula （2）^［27］.

where， G_s is the mass flow； ΔL is the displacement of seed flow movement； t is the time of seed flow movement； θ is the angle between the inclined tube and horizontal plane. The force can be calculated by the Formula （3）.

where， A is the frontal area of seed particles； μ is dynamic viscosity； r and D are the radius and diameter of the outlet pipe； W_f is the quality of seed particles transported per unit time.

The wheat seeds used in the experiment were “XINONG 585”. Based on the CFD-DEM coupling simulation method， EDEM 2021.2， and ANSYS Fluent 2021R1 software were used to carry out the coupling simulation of wheat seed flow and conveying airflow in the distributor. According to the parameter calibration method in Reference ［28］ and the technical requirements of wheat sowing， the CFD-DEM simulation parameters are determined as shown in Table 1.

According to the measured physical parameters， the manual filling of the 5-ball model is established through the EDEM software， as shown in Figure 3.

The seed distributor simulation model is established through the CFD software， as shown in Figure 3 and Figure 4. The outlet of the Y-type seed distributor is upward， and the angle of outlet a is 120°， the outlet of the T-type seed distributor is horizontal， the outlet of the M-type seed distributor is downward， and the angle of outlet b is 120°. The inlet diameter and outlet size of the three types of seed distributors are d=60 mm and L₁=300 mm respectively.

The gas-solid coupling simulation is carried out in ANSYS software. The N-S calculation model is used to solve the motion of the seed flow， the inlet velocity of wheat seeds is 28 m/s， and the seed production rate is 2 000 per second. The Realizable k-ε model is used to describe the airflow movement.

Evaluating the quality of seed metering involves using the coefficient of variation of seeding uniformity in accordance with the standards of seeder operation quality as defined in GB/T 9478—2005^［29］. By setting the monitor in the EDEM software， the number of seeds discharged at each outlet can be obtained to calculate the number of seeds， as shown in Figure 5.

The coefficient of variation of seeding uniformity is calculated as shown in Formula （4）.

where， x_i is the number of seeds in outlet i； x is the average amount of seeds； S is the standard deviation of the seeding uniformity； V is the coefficient of variation of seeding uniformity， %； n is the number of outlets （n=6 in this study）.

The bench experiment was used in the laboratory of Northwest A&F University to verify the simulation results， as shown in Figure 6. The M-type seed distributor is manufactured by 3D printing technology. Through the self-made experiment bench， the seed rate of each outlet is measured， and the coefficient of variation of seeding uniformity is calculated by the Formula （4）. Repeat the above process three times and take the average value as the result. In the bench experiment， the working parameters are consistent with the simulation experiment. The flow speed is measured by the outlet of the fan， and measured with a Pitot tube.

The field experiment was carried out on the farm of Northwest A&F University to verify the simulation results， as shown in Figure 7.

LUTONG-55 tractor drags the pneumatic centralized wheat seeder， and the forward speed is 10 km/h in the field experiment. The soil of the field is typical columbine soil in Shaanxi， the water content measured by the oven drying method is 17%， the average compaction of tillage layer soil （0-10 cm） is 10 kPa. The rotary tillage is prepared before seeding， to level the field.

The sampling is obtained by setting the inoculation bag at the end of the seed tube in the field experiment. The seed rate in each inoculation bag is measured， and the coefficient of variation of seeding uniformity is calculated by the Formula （4）. Repeat the above process three times and take the average value as the result.

The seed particles and flow field velocity distribution of Y-type， T-type， and M-type seed distributors are shown in Figure 8. The horizontal and vertical sections of the outlets are established respectively. The results show that the flow field velocity at the outlets of the M-type seed distributor is higher than the Y-type and T-type， and the M-type seed distributor has lower pressure loss than the other two types. The three types of seed distribution have seed particle accumulation， and the M-type seed distributor has little accumulation of seeds and has better uniformity than the other two types. The pressure loss of the seed flow through the M-type seed distributor is less than the other types， therefore， the seed flow can quickly pass through the seed distributor， and avoid the accumulation of the seeds.

The coefficient of variation of seeding uniformity is calculated by the Formula （4）. The effect of different types of seed distributors on the coefficient of variation of seeding uniformity is shown in Table 2.

The simulation result shows that compared with the T-type and Y-type seed distributors， the coefficient of variation of seeding uniformity of the M-type seed distributor is reduced by 7.41% and 3.72% respectively. The seed accumulation in Y-type and T-type seed distributors cases the difference of seed rate in each outlet， and the coefficient of variation of seeding uniformity increased.

As shown in Figure 9， the structure parameters of the M-type distributor include the inclination of outlet pipe θ₁， the cone angle of top cover θ₂， inlet diameter D_r， fillet radius R， and pneumatic conveying speed V_c.

As shown in Table 3， the single-factor control variable test method was designed， and the median value of each parameter level was taken as the national parameter， the test results are shown in Figure 10.

Flow field uniformity coefficient （λ） based on both area-weighted and mass-weighted average velocity as an evaluation index of flow field uniformity of seed distributor.As shown in Figure 11， the λ values of the outlet’s section and longitudinal section are under different factor levels.

The single factor test results show that the inclination angle of outlet pipe θ₁ has a significant effect on the flow field uniformity of both the outlet’s section and longitudinal section， and with the increase of the inclination angle from 50° to 70°， the λ value increases to a maximum and then decrease.

When the inclination angle is 60°， the λ value of the outlet’s section reaches a maximum of 0.954. When the inclination angle is 65°， the λ the value of longitudinal section reaches a maximum of 0.997. The reason may be that when θ₁ is relatively small， the flow line at the connection between the inner tube and outlet is stable， but it is sharp at the end of the outlet.

The cone angle of top cover θ₂ has a significant effect on the flow field uniformity of both the outlet’s section and longitudinal section， and with the increase of the cone angle from 100° to 180°， The λ value keeps decreasing. When the cone angle is 120°， the λ value of both the outlet’s section and longitudinal section reach the maximum， respectively 0.982 and 0.991. This is consistent with the research results of the optimal parameters of LEI et al.^［6］ in the seed distributor cone angle.

The fillet radius R has the same trend as the inclination angle θ₁， with the increase of the inlet diameter from 62 mm to 78 mm， the λ value increases to a maximum and then decreases. When the inlet diameter is 74 mm， the λ value of both the outlet’s section and longitudinal section reach the maximum， respectively 0.972 and 0.975.

The inlet diameter D_rand pneumatic conveying speed V_c have no significant effect on the flow field uniformity of both the outlet’s section and longitudinal section. With the increase of R and V_c， the λ value remains unchanged， respectively 0.982 and 0.984 for the outlet’s section， and 0.970 and 0.969 for the longitudinal section.

In summary， the structural parameters of the M-type distributor affect the flow field uniformity， of which the inclination of outlet pipe θ₁， the cone angle of top cover θ₂， and the fillet radius R have a significant effect， and the inlet diameter D_r， the pneumatic conveying speed V_c has no significant effect.

Based on the results of the single-factor test of different structure parameters of the M-type distributor， the multi-factor quadratic rotation orthogonal experiment based on CFD-EDM coupling is used to obtain the optimal structure parameter combination of the seed distributor. The factor level table is shown in Table 4. The coefficient of variation of seeding uniformity is the experiment index. The inlet diameter D_r and pneumatic conveying speed V_c is 60 mm and 28 m/s respectively.

As shown in Figure 12， the experimental results show that all the main factors have a highly significant effect on the coefficient of variation of seeding uniformity （P≤0.01）. In addition， the interaction of the inlet diameter and the cone angle has a highly significant effect too.The results of response surface analysis show that with the increase of outlet pipe inclination angle， the cone angle of the top cover and the fillet radius increase， and the coefficient of variation of seeding uniformity decreases first and then increases. The reason is that when the outlet pipe inclination angle is large or small， the internal flow field of the seed distributor will produce an eddy current， which will lead to uneven seed discharge.

The volume of the flow field inside the seed distributor is affected by the fillet radius. When the space is large， the airflow will form more turbulence and vortex. When the space is small， the collision of seed particles will increase， and both the vortex and collision will lead to uneven seed discharge.

Parameter optimization analysis of the regression equation was finished by using Design Expert software， the results showed that the optimal combination of structural parameters of the seed distributor was： the cone angle of the top cover was 117.03°， the inclination angle of the outlet pipe was 59.50°， the internal fillet radius was 69.46 mm， and the minimum coefficient of variation of seeding uniformity was 6.05%.

The bench experiment and field experiment were used to verify the simulation results. Based on the optimal combination of the structure parameter of the M-type seed distributor， the simulation experiment is carried out. The verification result of the M-type seed distributor is shown in Table 5.

The relative deviation of the M-type seed distributor between the simulation result and verification result of bench and field is 1.00% and 1.55% respectively. The coefficient of variation of seeding uniformity in the bench experiment and field experiment is 7.03% and 7.58% respectively. The data of the three measurement samples from the field experiment show that the upper and lower deviation is much higher than that in the bench experiment.

The verification results show that the coefficient of variation of seeding uniformity in the field experiment is higher than that in the bench experiment， and there is no difference in the results of the bench experiment and the simulation experiment. This is because field operations are greatly affected by soil conditions and the machinery， for example， the performance of the seed metering device and the field vibration of the seeder will affect the experiment results. Under the same operation parameters， the average seeding rate in the field experiment is higher than that in the bench experiment， which shows that the field vibration can promote seeding of the pneumatic centralized system and the effect is significant.

1） Compared with the T-type and Y-type seed distributors， the M-type seed distributor can significantly improve the seeding quality of the wheat， and the simulation test shows that the coefficient of variation of seeding uniformity is reduced by 7.41% and 3.72% respectively.

2） The structural parameters of the M-type distributor affect the flow field uniformity， of which the inclination of outlet pipe θ₁， the cone angle of top cover θ₂， the fillet radius R have a significant effect， and the inlet diameter D_r， pneumatic conveying speed V_c has no significant effect， and multi-factor orthogonal experiment results show that the optimal combination of structural parameters is θ₂=117.03°， θ₁=59.50°， R=69.46 mm， the minimum coefficient of variation of seeding uniformity obtained through simulation experiment is 6.05%.

3） The coefficient of variation of seeding uniformity in the bench experiment and field experiment is 7.03% and 7.58% respectively. The absolute errors of the M-type seed distributor between the simulation result and verification result of bench and field are 1.00% and 1.55% respectively.