Design and Test of Air-Assisted Seed-Guiding Device of Precision Hill-Seeding Centralized Seed-Metering Device for Sesame

Abstract

Sesame seeds are flat and oval, with poor mobility, easily blocking a seed tube and reducing seeding quality. An air-assisted seed-guiding device was designed for a hill-seeding centralized seed-metering device for sesame. The core of the seed-guiding device is a distribution manifold that could restrict the trajectory of seeds and make seeds move in the same direction as airflow. Six-factor three-level orthogonal tests were carried out using CFD–DEM coupling simulation to study the influence of the structure and operation parameters of the seed-guiding device on airflow field, seed transport, and seeding performance. The simulation results derived optimal parameters: the depth of the circular section of the seed slide was 2.62 mm, the length of the expansion and contraction section was 188 mm and the length of the contraction section was 20 mm, the seed tube diameter was 19 mm, the airflow velocity was 6.3 m/s, and the rotation speed of the roller was 25 r/min. Under the optimal parameters, the positive pressure required for the seed-guiding device was 256.77 Pa, the time of seeds passing through the seed-guiding device was 0.77 ± 0.02 s, and the velocity of seeds when they came out of the seed tubes was 2.24 ± 0.30 m/s. The qualified rate was 88.33% (2 ± 1 seeds/hill), and the miss-seeding rate was 5.00% (0 seeds/hill). Bench test showed that the qualified rate was 86.80%, and the miss-seeding rate was 6.00%. The seeding performance of the bench test was consistent with the simulation results. Field tests showed that the average number of seedlings per hill was 1.32. The seed-guiding device could meet the requirements of precision hill-seeding for sesame. This study provides a reference for design of a seed-guiding device of a centralized seed-metering device for sesame.

1. Introduction

Sesame is an important specialty oil crop with a long history of cultivation. It is widely grown in Asia, Africa, and South America [1]. Sesame is rich in nutrients, containing oil of 34.4–63.2% content and protein of 17–32% content [2]. The existing seeding methods are mainly broadcast, drilling, single-seed seeding, and hill-seeding. Broadcast and drilling result in lower uniformity of seed distribution. Single-seed seeding and hill-seeding can make the seeds evenly distributed, and emergence and maturity time are consistent, which is essential to ensure crop yield and reduce harvest losses [3]. Sesame seeds are flat and ovoid, and single-seed seeding requires coating technology to improve the qualified index [4]. Hill-seeding does not require seed coating and can seed sesame seeds evenly and orderly into the soil according to the required seeds per hill and hill agronomic distance. Hill-seeding is the main form of mechanized precision seeding of sesame seeds [5,6]. Centralized seed-metering devices are suitable for multi-row seeding operations, with high operational efficiency and compact structure advantages. Centralized seed-metering is one of the trends in seeding technology [7]. Sesame seeds easily block seed tubes when seeding in multiple rows due to low sphericity and poor mobility, which increases the miss-seeding rate and seriously affects seeding quality. Developing a seed-guiding device and studying its structure and operation parameters on the movement of seeds is the key to solving the problem.

The main types of hill-seeding metering devices include vacuum-disk-type, vertical-rotor-type, and roller-type, etc., among which roller-type hill-seeding metering devices are most widely used. Groove structure, number of grooves, and seed guard device structure affect number of seeds per hill, hill distance, and position and time of seeds dropping from the roller, which, in turn, affect seeding performance [3,8]. The seed-guiding process impacts uniformity of seed distribution and seeding quality. Seed tube diameter, seed drop height, forward speed, and airflow pressure and speed are important influencing factors [9,10]. Collisions between seeds and seed tube openers and soil change speed and direction of seed movement, scattering seeds and reducing uniformity of seed placements [11,12]. Single-row precision seed-metering devices (single-seed seeding or hill-seeding) can reduce seed collision by lowering seed throwing height, setting the seed guide belt, adjusting the throwing direction of seed tubes, and adopting reasonable pressure and speed of airflow to improve guiding performance [10,13,14,15,16]. Sesame seeds have poor mobility, so airflow is needed in the guiding process of a centralized seed-metering device to avoid seeds blocking the seed tube. Common air-assisted centralized seed-metering devices use a dosing wheel to feed seeds and improve uniformity of seeds by using a Venturi device pressurized tube divider head and airflow [17,18,19,20]. Air-assisted centralized seed-metering devices can seed multiple rows and are more suitable for different seeds and high-speed seeding operations, but they are primarily for drill [21]. To sum up, the existing research mainly focuses on single-row precision seed-metering devices and air-assisted centralized seed-metering devices for drilling, and there is scant research on guiding devices of the hill-seeding centralized seed-metering device. The designed hill-seeding centralized seed-metering device for sesame adopts the roller to fill 2 ± 1 seeds each groove and air-assisted seed-guiding. Subsequently, optimal structural parameters of hole wheel type are determined [22]. How to design the structure of a seed-guiding device and adopt reasonable airflow parameters to guide seeds smoothly and ensure seed distribution uniformity needs to be studied.

In this paper, we aim to design an air-assisted seed-guiding device to solve the problem of choking sesame in seed tubes and improve hill-dropping uniformity. CFD–DEM coupling simulation was used to analyze the influence of the seed-guiding device’s critical structure and operation parameters on seed transport and seeding performance. We conduct bench and field tests to verify the accuracy of simulation results and the operating performance under optimal parameters. This work will contribute to designing an air-assisted seed-guiding device of a hill-seeding centralized seed-metering device for sesame and improving seeding quality.

2. Materials and Methods

2.1. Overall Structure and Working Principle

2.1.1. Overall Structure

The structure of the seeder is shown in Figure 1. Key components of the seeder are ditching devices, a rotary tillage device, ground wheel driving devices (driving encoder and fertilizer application device), fertilization devices, a frame, a hill-seeding centralized seed-metering device, seed tubes, double disc openers, etc. The hill-seeding centralized seed-metering device, seed tubes, and double disc openers make up the seeding system.

The detailed structures of the seeding system are shown in Figure 2, which mainly include a blower, distribution manifold, roller, seed box, shell, motor drive, control system, etc. The blower, distribution manifold, and seed tube constitute the seed-guiding device.

2.1.2. Working Principle of the Seeding System

As shown in Figure 3, sesame seeds are flat-ovate. The sesame variety is Hangtianxinzhi T31-8, with a length of 3.17 ± 0.16 mm, a width of 1.88 ± 0.11 mm, a thickness of 0.90 ± 0.10 mm. The 1000-grain mass of sesame is 3.090 ± 0.019 g. According to Formula (1), the geometric average particle size is 1.75 ± 0.13 mm, and sphericity is 55.39%. Sesame seeds may block seed tubes for low sphericity and poor mobility and increase miss-seeding rate. Airflow is necessary to guide seeds smoothly when seeding multiple rows.

During the seeding operation, the motor drives the roller, and each groove fills 2 ± 1 seeds in the seed box. When the roller carries the seeds through a certain angle, the seeds will enter the seed slide at port tangentially due to gravity and centrifugal force. The seeds move orderly under the restriction of the seed slide, and seeds move in the same direction with the airflow velocity at the end of the seed slide. The seeds fall through the seed tube into the seed furrow formed by the double disc opener under airflow force, gravity, seed tube pressure, and friction. The air-assisted seed-guiding device consisting of a blower, distribution manifold, and seed tube is the key to avoiding seed clogging of the seed tubes in the process of multi-row and ensuring hill dropping uniformity.

2.2. Design of Air-Assisted Seed-Guiding Device

Structure parameters of the distribution manifold and seed tubes and the airflow velocity are essential factors in determining the seed movement characteristics and distribution uniformity in the seed-guiding process. Conduct theoretical analysis of the air-assisted seed-guiding device to determine the range of structural and operation parameters for CFD–DEM coupling simulation.

2.2.1. Structural Design of the Distribution Manifold

Figure 4 shows the structure of the distribution manifold, which is the core component of the seed-guiding device. The distribution manifold is mainly composed of a seed slide and airflow channel. The seed slide is the transition section between the roller and the seed tube. An arc section concentric with the roller and a straight section tangent to the arc comprise the seed slide, whose tangent direction is consistent with the seed movement trajectory. Airflow moves in the same direction as seeds, which can reduce the disturbance of seeds and the probability of collision between seeds and the air-assisted seed-guiding device. It is essential to improve hill dropping uniformity.

Figure 4 shows the main structural parameters of the seed slide are the depth of the circular section of the seed slide h_d1, the depth of the straight section of the seed slide h_d2, and the width of the seed slide w_d. The width of the seed slide is designed to be as wide as the churning groove of the roller to avoid seed jamming. The depth of the circular section of the seed slide is designed according to the depth of the groove and the size of sesame seeds, and the lower limit should ensure that the seeds standing vertically in the groovecan pass smoothly. When the depth of the circular section of the seed slide is too large, it separates seeds from the grooveearlier. It is adverse for the hill dropping uniformity, so the upper limit is set equal to the maximum length of the seed. In the straight section of the seed slide, seeds have separated from the groove. The depth of the lower limit should achieve stable seed-guiding, and the upper limit is mainly limited by the seed tube diameter. The depth of straight section of the seed slide was taken 1.1 times the maximum length of seeds [23]. From the structural parameters of the roller, the width and depth of the seed guard slide satisfy calculation Equation (1) [22]:

$$\left\{\begin{array}{l}{w}_d=2.1(\overline{a}+3{\mathsf{\sigma }}_a)\\ 0.5(\overline{a}+3{\mathsf{\sigma }}_a)-\frac{d_s\Psi }{4}\le h_{d1}\le \overline{a}+3{\mathsf{\sigma }}_a\\ h_{d2}=1.1(\overline{a}+3{\mathsf{\sigma }}_a)\\ d_s={(\overline{a}\overline{b}\overline{c})}^{\frac{1}{3}}\end{array}\right.$$

(1)

where w_d is the width of the seed slide, mm; h_d1 is the depth of the circular section of the seed slide, mm; h_d2 is the depth of the straight section of the seed slide, mm; $\overline{a}$ is the mean seed length, mm; $\overline{b}$ is the mean seed width, mm; $\overline{c}$ is the mean seed thickness, mm; σ_a is the standard deviation of seed length mm; d_s is the seed equivalent diameter, mm; Ψ is the seed sphericity, %.

Substituting the sesame seed material parameters into Equation (1) yields width of the seed slide is 7.67 mm, depth of the circular section of the seed slide is 1.58–3.65 mm, and depth of the straight section of the seed slide is 4.02 mm. The angle β₁ corresponding to the starting point of the seed slide should be no less than the maximum value of the initial seed throwing angle. The previous study yielded a seed throwing angle range of 28.5°–52.2° within the design speed range of 10–40 r/min. β₁ should be greater than the top seed throwing angle to ensure that all seeds from the groove fall into the seed slide. At the same time, due to the overall size of the seed-metering device, take β₁ for 60°. The suitable value of angle β₂, which is between the end of the circular section of the seed slide and the center of the roller and the vertical line, is 10°15° [24]. Take β₂ at 10° to allow seeds more time to detach from the groove.

From the conservation of mass and Bernoulli’s equation, the smaller the cross-section area, the greater the flow rate and the smaller the static pressure [25]. Setting the seed slide outlet at the end of the contraction section can format negative airflow in the seed slide, avoiding the seeds being blown upward along the seed slide by the airflow and promoting the seeds to fall into the seed slide smoothly. The center of the exit surface of the seed slide coincides with the exit axis of the airflow channel so that the direction of seed movement is in the same direction as the airflow.

The airflow channel structure is shown in Figure 4c and consists of blower mounting section L₁, expansion section L₂, contraction section L₃, and cylindrical section L₄. Figure 3 and Figure 4 show that the distribution manifold and the airflow channel length take the maximum value when the end of the distribution manifold is at the bottom of the seed-metering device shell. When the size of the distribution manifold decreases, the angle between the seed slide’s straight section and the cylindrical section’s axis increases. The partial velocity of the seed along the vertical cylindrical axis increases, and the probability of the seed colliding with the side wall of the cylindrical section after leaving the seed slide increases. The length of the expansion section increases, the strength of the connection between the distribution manifold and seed tubes decreases, and the upper limit of the tapering section length is 20 mm. When the length of the contraction section is 0 mm, the cross-sectional area of the flow field at the intersection of the cylindrical section and the expansion section suddenly becomes smaller. That will aggravate the turbulent flow. Take the lower limit of the contraction section length as half of the upper limit value. The inlet diameter of the contraction section is equal to the height of the end plane of the expansion section. From Figure 4, we can see that the airflow channel structure size satisfies:

$$\left\{\begin{array}{l}192\le L_1+L_2+L_3+L_4\le 220\\ 10\le L_3\le 20\\ W_1+2L_2\tan \frac{{\theta }_1}{2}=W_2\\ H_2-2L_3\tan \frac{{\theta }_2}{2}=d_d-2\delta \\ H_1-2L_2\tan \frac{{\theta }_3}{2}=H_2\end{array}\right.$$

(2)

where: L₁ is the length of the blower mounting section, 12 mm; L₂ is the length of the expansion section, mm; L₃ is the length of the contraction section, mm; L₄ is the length of the cylindrical section 20 mm; W₁ is the inlet width of the distribution manifold, mm; θ₁ is the left and right side angle of the expansion section, (°); W₂ is the width of the exit plane of the expansion section 176 mm; H₂ is the height of the exit plane of the expansion section, 28 mm; θ₂ is the top and bottom side angle of the expansion section (°); d_d is the seed tube diameter, mm; δ is the shell thickness of distribution manifold, 2 mm; H₁ is the height of the distribution manifold, mm; θ₃ is the cone angle of contraction section, (°).

If the size of the blower outlet, that is, the airflow channel inlet, is known, Equations (1) and (2) show the depth of the circular section of seed slide h_d1, the expansion and contraction section length (L₂ + L₃), the contraction section length L₃, and the seed guide tube diameter d_d, which can determine the airflow channel and seed slide structure. The expansion and contraction section length is 160–188 mm.

2.2.2. Analysis of the Airflow Parameters of the Distribution Manifold

According to Figure 4 and Figure 5, the airflow channel’s local resistance loss mainly includes loss of the expansion section and the contraction section and the sudden expansion loss at the exit of the seed slide [26]. The local resistance of the airflow channel meets:

$$\left\{\begin{array}{l}{p}_j=p_{j1}+p_{j2}+p_{j3}\\ p_{j1}=\frac{{\xi }_1{\rho }_a{v}_{a1}^2}{2}\\ p_{j2}=\frac{{\xi }_2{\rho }_a{v}_{a2}^2}{2}\\ p_{j3}=\frac{{\rho }_a{(\frac{S_3}{S_2}-1)}^2{v}_{a3}^2}{2}\\ S_1=W_1(H_1-2L_1\tan \frac{{\theta }_3}{2})\\ S_2=N\left[\frac{\mathsf{\pi }{H}_2^2}{4}-(h_{d2}+2\delta )(w_d+2\delta )\right]\\ S_3=N\left[\frac{\mathsf{\pi }{\left(d_d-2\delta \right)}^2}{4}-(h_{d2}+2\delta )(w_d+2\delta )\right]\\ S_1{v}_{a1}=S_2{v}_{a2}=S_3{v}_{a3}\end{array}\right.$$

(3)

where: p_j is the total local pressure loss in the airflow channel, Pa; p_j1 is the local loss of the expansion section, Pa; ξ₁ is the local loss of the expansion section coefficient; v_a1 is the inlet airflow velocity of the expansion section, m/s; p_j2 is the local loss of the contraction section, Pa; ξ₂ is the local loss of the contraction section coefficient; v_a2 is the inlet airflow velocity of the contraction section, m/s; p_j3 is the sudden expansion loss at the exit of the seed slide, Pa; ρ_a is the density of airflow, 1.205 kg/m³; S₁ is the inlet cross-sectional area of expansion section, m²; S₂ is the inlet cross-sectional area of contraction section, m²; S₃ is the outlet cross-sectional area of contraction section, m².

Equation (3) shows that the local resistance loss at the airflow channel is mainly determined by the inlet and outlet airflow velocities and resistance coefficients of each section of the airflow channel. When the airflow required for the seed-guiding process is fixed, the air velocity of each section is determined by its cross-sectional area. The resistance coefficient is mainly affected by airflow velocity and structure shape. Sudden changes in airflow direction or cross-section area aggravate airflow disturbance will increase local pressure loss [26]. Designing the structure of the airflow channel is designed as a symmetrical, inclined gradient structure with the same inlet and outlet flow direction to reduce the vortex intensity pressure loss and energy consumption and improve the uniformity of seed distribution in the seed-guiding process. Due to the irregular shape of the airflow channel and the mutual interference of turbulent flow in each section, it is challenging to calculate the local loss coefficient accurately. It is proposed to determine the influence of crucial structure parameters (the depth of the circular section of the seed slide h_d1 contraction section length (L₂ + L₃), the contraction section length L₃ the seed guide tube diameter d_d) on the airflow field, and seeds motion with the help of CFD–DEM coupling simulation analysis.

2.2.3. Structural Design of the Seed Tube

The seed tube’s diameter, installation size, and airflow velocity impact the force and movement of seeds in the seed tube. Air-seeders mostly use 25, 30 mm diameter seed tubes to avoid blocking the seed tube when seeding wheat, barley, and other large grain seeds. Research has shown that a 20 mm seed tube can achieve a stable seed guide for small size seeds, and the suitable diameter of seed tubes can reduce the energy consumption of guiding seeds and improve the seeding quality [21,27]. Select PVC steel wire hoses with diameters of 16, 19, and 22 mm for the study due to the size of sesame seeds, the width limitation of the seed slide, and the seed tube diameter.

The angle between the seed tube and the ground impacts the force and movement of the seeds. As shown in Figure 1 and Figure 2, the installation size of the seed-metering device on the seeder is determined by the combination of planting agronomy, the structure of the whole machine, and the recommended installation distance of the double disc opener [24].

As shown in Figure 5, simplify the seed tube into three straight sections. The radius of the transition arc between straight sections is 100 mm, the total height is 525 mm, the height of the middle section is 280 mm, and the height of the end section is 185 mm.

Seeds can move smoothly at the start and end of the seed guide because the angle of repose of seeds is less than the angle between the seed guide and the ground. The angle between the middle section of the seed tube and the ground determines whether the seeds block the seed tube. Calculate the angle between the middle section of the seed tube and the ground as:

$$\varphi =\arcsin \frac{L_h}{\sqrt{L_{\mathrm{h}}^2+L_{\mathrm{w}}^2+L_{\mathrm{l}}^2}}$$

(4)

where: Ø is the angle between the middle section of the seed tube and the ground (°); L_h is the height of middle section of the seed tube, m; L_w is the width of seed guide tube (vertically the forward direction of the tractor), 0.135–0.675 m; L_l is the length of seed guide tube (along the forward direction of tractors), 0.15 m.

Equation (4) shows that the angle of the middle section of the seed tube takes the value range of 22.0°–54.2°. The angle of repose of the sesame variety Hangtianxinzhi T31-8 is 26.1°. Because the lower limit angle of the seed tube’s middle section is smaller than the repose angle of sesame, sesame seeds easily block the seed tube due to the lack of mobility and using airflow to achieve a stable seed-guide.

2.2.4. Analysis of the Airflow Parameters of the Seed Tube

Suspension velocity is one of the crucial parameters to determine airflow velocity. Its expression is determined by the value of drag coefficient zoning and is influenced by material characteristics parameters, such as seed size and density in

Table 1. Mechanical parameters of materials for simulation.

Items	Sesame	ABS	PVC Steel Wire Hose	Rubber
Density/(kg·m−3)	930	1060	1500	1350
Poisson’s ratio	0.48	0.39	0.40	0.47
Shear modulus/Pa	2.5 × 106	8.96 × 108	1.5 × 109	2.9 × 109
Coefficient of restitution (between with sesame)	0.278	0.36	0.42	0.001
Coefficient of static friction (between with sesame)	0.57	0.49	0.92	1
Coefficient of rolling friction (between with sesame)	0.01	0.03	0.01	1

Note: to avoid seeds bouncing, the mechanical parameters of contact between sesame and rubber are not true values.

. The critical value of drag coefficient zoning and theoretical suspension velocity satisfy the calculation equation [28]:

$$\left\{\begin{array}{l}k=\frac{d_s}{1000{(\frac{{\mu }^2}{{\rho }_a({\rho }_s-{\rho }_a)})}^{\frac{1}{3}}}\\ v_x=\frac{5.451}{\sqrt{K_x}}\sqrt{\frac{d_s({\rho }_s-{\rho }_a)}{1000{\rho }_a}}\end{array}\right.$$

(5)

where: k is the critical value of drag coefficient zoning; ρ_s is the seed density, kg/m³; μ is the aerodynamic viscosity, 18.1 × 10⁻⁶ Pa·s; v_x is the seed theoretical suspension velocity, g/s; v_x is the seed theoretical suspension velocity, m/s; K_x is the shape correction factor.

Substituting material characteristics parameters of sesame into Equation (5) yields a critical value of drag coefficient zoning k of 26.36. Compared to the critical values of the drag coefficient zoning, the sesame seed suspension velocity drag coefficient is in the differential pressure resistance zone or Newton zone. Take the correction factor of the sesame seed shape as 1.1 [29] and obtain the theoretical suspension velocity as 6.036 m/s. The suspension speed was determined to be 3.23–6.31 m/s with the help of PS-20 test bench for material suspension velocity and airflow velocity and pressure tester (ZC1000-1F with L˗shaped Pitot tube, Shanghai Yi’Ou Instrument Equipment Co., Ltd., Shanghai, China). The results verified the accuracy of the theoretical calculation. The experimental results show that the seeds are suspended velocity at a wide range due to the low sphericity and variation in seed particle size. The theoretical suspension velocity based on the seed equivalent diameter is within the scope of the measured suspension velocity, is close to the upper limit of the interval, and can be used to calculate the theoretical airflow velocity.

Suitable airflow velocity can ensure stable seed delivery and reduce power consumption and seed breakage rate [27]. The upper limit of airflow velocity in the seed tube should meet the regular suspension transport of seeds because sesame is loose, dry material and there is no vertical transport section in the transport process. The upper limit of wind speed is 1.5 times the suspension speed [30]. Airflow velocity and blower flow rate satisfy the calculation equation:

$$\left\{\begin{array}{l}{v}_{a\max }=1.5v_x\\ V_a=\frac{9N\pi d_d^2{v}_{a\max }}{{10}^4}\end{array}\right.$$

(6)

where: v_amax is the upper limit of airflow velocity in the seed tube, m/s; V_a is the blower flow rate, m³/h.

The lower limit of the stable airflow velocity of the seed tube corresponds to the state of force when the seeds are moving along the wall of the seed tube at a uniform velocity, and the seed force is as shown in Figure 6, which satisfies the calculation equation:

$$\left\{\begin{array}{l}{F}_R+G\sin {\varphi }_{\min }-fG\cos {\varphi }_{\min }=0\\ F_R=\frac{kC\overline{b}\overline{c}}{{10}^6}{\rho }_a\frac{{(v_{a\min }-v_s)}^2}{2}\\ G=\frac{k\mathsf{\pi }{d}_s^3{\rho }_{\mathrm{s}}g}{6\times {10}^9}\\ v_s=\frac{n\mathsf{\pi }{d}_x}{60}\\ R_{e\min }=\frac{v_{a\min }{d}_d{\rho }_a}{1000\mu }\end{array}\right.$$

(7)

where: F_R is the airflow coupling force on seeds, N; G is the gravity on seeds, N; ø_min is the angle between the middle section of the outermost seed tube and the ground 22.0°. k is the number of seeds in a single groove; f is the coefficient of static friction between sesame seeds and the seed tube, 0.92; C is the bypass resistance coefficient; v_amin is the lower limit of the stable airflow velocity of the seed tube, m/s; v_s is the seed velocity, m/s; g is the gravitational acceleration, 9.8 m/s²; n is the rotation speed of roller, r/min; d_x is the roller diameter 0.12 m; R_emin is the Reynolds number minimum.

Equation (7) shows that the airflow coupling force decreases when the relative velocity of seeds and airflow decreases, so take seed velocity as the linear velocity at 40 r/min of the roller. To simplify the calculation, ignore the influence of the distribution manifold and the starting section of the seed tube on the seed velocity. The resistance coefficient C is 0.44 [31], and the lower limit of the stable airflow velocity of the seed tube v_amin is calculated as 5.477 m/s. Considering the impact of gas leakage, airflow velocity should be reserved 15% margin [31]. The lower limit of the stable airflow velocity is 6.30 m/s. When the seed tube diameter is 16 mm, it takes a Reynolds number minimum of 6710.72.

The air–material flow rate ratio in the seed tube determines the choice of the theoretical calculation model and CFD–DEM coupling model of the transport process. The air–material flow rate ratio in the seed guide tube is determined by the seed feed rate, airflow velocity, and seed tube diameter. The seed feed rate and air–material flow rate ratio satisfy the calculation equation:

$$\left\{\begin{array}{l}{q}_s=\frac{m_0{v}_m{B}_m{Q}_f}{360Nu}\\ \eta =\frac{4000q_s}{\mathsf{\pi }{d}_d^2{\rho }_a{v}_{a\min }}\end{array}\right.$$

(8)

where: q_s is the seed feed rate of each seed tube, g/s; m₀ is the 1000-grain mass, g; v_m is the operation speed of planter, 5 km/h; B_m is the planter width, 1.8 m; Q_f is the planting density, 2.25 × 10⁵ plants/hm²; η is the air–material flow rate ratio in the seed tube; u is the sesame seedling germination rate, 0.38.

The maximum seed feed rate in the seed tube is calculated to be 0.0762 g/s. When the airflow velocity is 6.30 m/s and the seed tube diameter is 16 mm, the maximum air–material flow rate ratio in the seed tube is 0.05, and the air–material flow is dilute phase flow [32]. The effect of sesame on the airflow field can be ignored.

The seed tube is dilute phase flow, and the bending radius of the seed guide tube is large, ignoring the local pressure loss caused by two-phase flow and bending of the seed tube. The pressure loss along the seed tube mainly includes pressure loss from friction acceleration and suspension lifting. Because the layout of the seed tube form is inclined downward, it can be obtained from the blower full pressure calculation equation [29]:

$$\left\{\begin{array}{l}p=p_j+p_f+p_a+p_x\\ p_f=(0.2\eta +\sqrt{\frac{30}{v_{a\max }}}){\lambda }_a\frac{1000L}{d_d}{\rho }_a\frac{v_{a\max }^2}{2}\\ p_a=(1+\eta \frac{v_s^2}{v_{a\max }^2}){\rho }_a\frac{v_{a\max }^2}{2}\\ p_x=\frac{{\rho }_{a}g\eta L(v_x-v_s\sin {\varphi }_{\min })}{v_s}\end{array}\right.$$

(9)

where: p is the blower full pressure, Pa; p_f is the pressure loss from friction Pa; p_a is the pressure loss from acceleration Pa; p_x is the pressure loss from suspension lifting Pa; λ_a is the frictional resistance coefficient; L is the average length of seed tube, 0.9 m.

The value of the Reynolds number influences the expression of the frictional resistance coefficient. When the seed tube diameter is 16 mm and the airflow velocity is 9.1 m/s, the frictional resistance coefficient is the largest, and its Reynolds number is 9693.26, which is in Blasius turbulent smooth zone and satisfies the empirical Equation (10) [26]:

$${\lambda }_a=\frac{0.3164}{R{e}^{0.25}}$$

(10)

Calculated from Equation (10), the frictional resistance coefficient is 0.032. Substituting the relevant parameters into Equation (9) yields ∆P_f, ∆P_a, ∆P_x of 163.38, 49.89, and 12.56 Pa, respectively, and the total pressure drop caused by the seed tube is 225.84 Pa. The initial choice of Speer 008-B100-93D blower, the blower performance curve is shown in Figure 7, the outlet width is 98.8 mm, and the height is 45 mm.

The hydraulic diameter and turbulence intensity are the basic parameters of CFD–DEM coupling simulation, and inflation thickness has a significant impact on airflow velocity and airflow resistance, and the relevant parameters satisfy the calculation equation [33,34]:

$$\left\{\begin{array}{l}{d}_{w1}=\frac{2W_1{H}_1}{W_1+H_1}\\ I=16{(R_{\mathrm{e}})}^{-1/8}\\ {\delta }_a=\frac{34.2d_{w1}}{R_e{}^{0.875}}\end{array}\right.$$

(11)

where: d_w1 is the hydraulic diameter of the airflow channel inlet section, mm; I is the turbulent intensity, %; δ_a is the inflation thickness, mm.

Equation (11) shows that the hydraulic diameter turbulence intensity and inflation thickness of the airflow channel inlet section are 61.84 mm 4.67–5.29% 0.179–0.255 mm, respectively. The turbulence intensity of the seed tube is 4.89–5.32%.

From the above analysis, the suitable seed tube diameter was determined to be between 16–22 mm. The study concluded that sesame seeds must be delivered by airflow due to poor mobility. The critical airflow velocity, pressure loss, hydraulic diameter, and turbulence intensity were determined. These parameters provide a theoretical basis for CFD–DEM coupling simulation.

2.3. Test Design

2.3.1. Establishment of CFD–DEM Coupling Simulation Model

The material of the distribution manifold, seed tube, and conveyer belt in the air-assisted guiding device are acrylonitrile-butadiene-styrene copolymer (ABS), transparent polyvinyl chloride (PVC) steel wire hose, and rubber plate, respectively. The density, Poisson’s ratio, and shear modulus of these materials refer to Refs. [35,36]. Three-axis dimensions of sesame are displayed in Section 2.1.2. Sesame seed density was measured using the density bottle method [30]. The coefficient of restitution shear modulus and Poisson’s ratio of sesame seeds were determined according to the method described in Refs. [37,38]. The main testing instruments include a 100 mL volume density bottle, TMS-PRO texture analyzer from FTC (Sterling, VA, USA), Phantom v1840 high-speed camera (Wayne, NJ, USA), etc. The experimental procedures are shown in Figure 8.

The simulation parameters are shown in Table 1, and the simulation model is shown in Figure 9.

2.3.2. Experiment Methods and Appraisal Indices of CFD–DEM Coupling Simulation

Equations (1) and (2) show that depth of the circular section of seed slide h_d, expansion and contraction section length (L₂ + L₃), contraction section length L₃, and seed tube diameter d_d are the key structure parameters of the airflow channel and seed slide. The seed-filling process and seed-guiding process are continuous processes. Meanwhile, the rotational speed of the roller significantly influences the number of seeds in a single groove, the seed throwing angle, and the velocity of seeds. Airflow velocity determines coupling force on the seed. These parameters are important factors affecting seed movement and hill dropping uniformity. Therefore, depth of the circular section of the seed slide (A), expansion and contraction section length (B), contraction section length (C), seed tube diameter (D), airflow velocity (E), and rotational speed of the roller (F) were selected as the influencing factors. The appraisal indices of hill-seeding are the qualified rate, miss-seeding rate, and replay rate. Ensuring a high qualified rate is a primary goal; the miss-seeding rate and replay rate are secondary objectives. The sum of qualified rate, miss-seeding rate, and replay rate is 100%. When qualified rate and miss-seeding rate are fixed, the replay rate is specific, so qualified rate (Y₁) and miss-seeding rate (Y₂) are selected as the response indicators for analysis. Three levels were set for each factor. The L₂₇ (3¹³) orthogonal test was selected. The experimental scheme and results are shown in

Table 2. Test scheme and results.

No.		Depth of the Circular Section of Seed Slide/mm	Length of Expansion Contraction Section/mm	Length of Contraction Section/mm	Diameter of the Seed Tube/mm	Airflow Velocity/ (m/s)	Rotation Speed of the Roller/(r/min)	Qualified Rate/%	Miss- Seeding Rate/%
1		1.58	160	10	16	6.3	10	68.00	22.67
2		1.58	160	10	16	7.7	25	61.33	27.67
3		1.58	160	10	16	9.1	40	49.33	47.33
4		1.58	174	15	19	6.3	10	71.33	16.00
5		1.58	174	15	19	7.7	25	77.00	13.00
6		1.58	174	15	19	9.1	40	74.00	20.67
7		1.58	188	20	22	6.3	10	72.33	14.67
8		1.58	188	20	22	7.7	25	77.33	15.00
9		1.58	188	20	22	9.1	40	82.00	16.00
10		2.62	160	15	22	6.3	25	83.67	7.67
11		2.62	160	15	22	7.7	40	80.00	16.67
12		2.62	160	15	22	9.1	10	66.67	16.00
13		2.62	174	20	16	6.3	25	81.33	13.33
14		2.62	174	20	16	7.7	40	79.67	19.33
15		2.62	174	20	16	9.1	10	65.33	25.67
16		2.62	188	10	19	6.3	25	87.33	5.33
17		2.62	188	10	19	7.7	40	80.33	15.33
18		2.62	188	10	19	9.1	10	77.33	8.33
19		3.65	160	20	19	6.3	40	81.00	16.33
20		3.65	160	20	19	7.7	10	68.33	16.33
21		3.65	160	20	19	9.1	25	77.00	14.00
22		3.65	174	10	22	6.3	40	80.00	18.00
23		3.65	174	10	22	7.7	10	72.67	13.00
24		3.65	174	10	22	9.1	25	77.67	12.33
25		3.65	188	15	16	6.3	40	80.00	17.33
26		3.65	188	15	16	7.7	10	69.00	17.67
27		3.65	188	15	16	9.1	25	71.00	21.67
Qualified rate	k1	70.30	70.59	72.67	69.44	78.33	70.11
	k2	77.96	75.44	74.74	77.07	73.96	77.07
	k3	75.19	77.41	76.04	76.93	71.15	76.26
	R	7.67	6.81	3.37	7.63	7.19	6.96
Miss-seeding rate	k1	21.44	20.52	18.89	23.63	14.59	16.70
	k2	14.19	16.81	16.30	13.93	17.11	14.44
	k3	16.30	14.59	16.74	14.37	20.22	20.78
	R	7.26	5.93	2.59	9.70	5.63	6.33

.

The software used for gas–solid coupling simulation is EDEM2018 and ANSYS Fluent (version 19.2). The coupling interface is the DPM model. The calculation model in Fluent is Standard k-ε. The simulation step in Fluent is 1 × 10⁻⁴ s, improving the quality of the fluid domain mesh with the help of the Proximity and Curvature advanced dimension function, setting the boundary layer of the seed tube, and locally meshing the inlet and outlet surfaces of the flow field. The simulation step in EDEM is 5 × 10⁻⁶ s. The number of sesame seeds in the seed box is 80,000. The rotation speeds of the roller are 10 r/min, 25 r/min, and 40 r/min, respectively. The length of the conveyer belt is 12 m. Equation (11) calculates the speed of the conveyor belt. The distance between hills with sesame is 185 mm. The number of grooves of each roller is 17. When the rotation speed of the seed-metering is 10r/min, 25 r/min, and 40 r/min, respectively, the moving speed of the conveyer belt is set as 0.52 m/s, 1.31 m/s, and 2.10 m/s, respectively. The simulation time is 24 s, 9 s, and 6 s, respectively. Based on AutoCAD software (2017), the number of sesames on each hill and the distance between hills with sesame on the conveyer belt can be obtained. Six rows of data for a total of 300 hills were counted for each experiment. Referring to NY/T1143-2006 “Technical specifications of quality evaluation for drills”, the qualified rate of seed numbers per hill and miss-seeding rate can be obtained.

$$v_b=\frac{n{N}_w\Delta s}{\mathrm{60,000}}$$

(12)

where: v_b is the moving speed of conveyer belt, Δs is the distance between hills, and N_w is the number of grooves of each roller.

2.3.3. Bench Test Program

The centralized seed-metering device is installed on the JPZS-16 digital triaxial vibration test bench (Heilongjiang Academy of Agricultural Machinery Sciences, Harbin, China) for seeding quality. The width of the conveyer belt is 1.2 m. The rowing space of the sesame is 300 mm. The installation position of seed tube is consistent with the theoretical calculation and simulation model. The seed tube was divided into the left and right groups, and the tests of each group were repeated 5 times. In the test, 250 hills are counted in each row. 2 ± 1 seeds per hill are qualified, 0 seeds per hill are miss-seeding, and more than 3 seeds per hill is replay-seeding. The speed of the conveyer belt is set to 1.31 m/s according to the simulation experiment. The airflow velocity is set to 6.3 m/s according to theoretical calculation.

The hill-seeding seed-metering device should first meet the requirements of seed number per hill. In addition, hill distance and the scattering distance ratio are performance indicators to evaluate hill-seeding performance. The test and calculation methods refer to Refs. [3,5]. Each group of test statistics has 6 rows and each row statistic 10 hills; the test was repeated 5 times, totaling 300 hills.

2.3.4. Field Experiment Program

A sesame seeding experiment was conducted at Huazhong Agricultural University on 10 June 2022 to verify the field seeding quality of the centralized seed-metering device. The sesame variety used in the experiment was Hangtian Xingzhi T31-8. The power machine in the experiment was a LeiWo 1004 tractor (Weichai Lovol lntelligent Agricultural Technology Co., Ltd., Qingdao, China). The speed of the tractor during the experiment was 4.37 km/h. The distance between hills with sesame and rowing space is consistent with the CFD–DEM coupling simulation and the bench test.

Because of the small size of the sesame, it is difficult to count the number of sesame seeds after the sesames fall into the soil. Referring to NY/T2709-2015 “Operat quality for rape seeders” and NY/T1143-2006 “Technical specifications of quality evaluation for drills”, we used number of emergence plants to evaluate seeding quality. Five ridges were randomly selected 2 weeks after seeding. The length of the ridges is 2 m.

3. Results and Discussion

3.1. Influence of Structure and Operation Parameters on Seeding Quality

Table 2 shows that the primary and secondary order and optimal level of influence on qualified rate of seed numbers per hill are A₂D₂E₁F₂B₃C₃, and the primary and secondary order and favorable factors of influence on miss-seeding rate are D₂A₂F₂B₃E₁C₂. According to range analysis, the optimal level of other factors is the same except for the length of cone section C. This means that depth of the circular section of the seed slide, length of expansion contraction section, diameter of seed tube, airflow velocity, and rotation speed of the roller are 2.62 mm, 188 mm, 19 mm, 6.3 m/s, and 25 r/min, respectively. The influence of length of contraction section on qualified rate and miss-seeding rate is a secondary factor, and the optimal level is 20 mm and 15 mm.

The experiment results were analyzed by variance analysis to quantify the significance of each factor. The variance analysis result is shown in

Table 3. Variance analysis of simulation results.

Variance Source	Qualified Rate				Miss-Seeding Rate
	Sum of Squares	Degree of Freedom	F Value	p Value	Sum of Squares	Degree of Freedom	F Value	p Value
Model	0.14	12	5.51	0.002 **	0.13	12	6.00	0.001 **
A	0.027	2	6.48	0.010 *	0.025	2	6.87	0.008 **
B	0.022	2	5.29	0.019 *	0.016	2	4.41	0.033 *
C	0.005	2	1.24	0.319	0.003	2	0.95	0.411
D	0.034	2	8.18	0.004 **	0.054	2	14.79	0.000 **
E	0.024	2	5.64	0.016 *	0.014	2	3.92	0.045 *
F	0.026	2	6.23	0.012 *	0.019	2	5.07	0.022 *
Residual	0.029	14			0.026	14
Cor Total	0.17	26			0.16	26

Note: ** shows highly significant (p < 0.01); * shows significant (p < 0.05).

. Table 3 shows that the regression models of the qualified rate and miss-seeding rate are highly significant. Diameter of seed tube (D) has a highly significant impact on qualified rate (p < 0.01). Depth of circular section of seed slide (A), length of expansion contraction section (B), airflow velocity (E), and rotation speed of roller (F) have a significant impact on qualified rate (p < 0.05). Depth of circular section of seed slide (A) and airflow velocity (E) have a highly significant impact on qualified rate (p < 0.01). Length of expansion contraction section (B), airflow velocity (E), and rotation speed of the roller (F) have a significant impact on qualified rate (p < 0.05). Other factors have no significant influence on the experiment results. The optimization objectives are maximum qualified rate and minimum miss-seeding rate, and the optimization design is carried out using Design-Expert 8.0 software. The optimization results show that the length of contraction section should be 20 mm, and the value of other factors is consistent with the range analysis. A verification simulation experiment was performed to verify the optimization results. The verification simulation experiment results show that the qualification, miss-seeding, and replay rate of the air-assisted guiding device are 88.33%, 6.67%, and 5.00%, respectively. An optimized distribution manifold was manufactured to conduct bench and field experiments.

3.2. Analysis of Airflow Field Distribution and Sesame Movement Characteristics

Differences in pressure and flow characteristics of airflow field lead to differences in coupling force, velocity of movement, and distribution uniformity of sesames. Test 3 has the lowest qualified rate. Test 3 and the optimal results are selected for analysis. The reasons for the difference between the qualified rate of seed numbers per hill and miss-seeding rate in the two tests were identified by analyzing the distribution characteristics of airflow field and sesame movement characteristics.

The distribution of airflow velocity and pressure in the flow field at the distribution manifold and initial segment of the seed tube is shown in Figure 10. In test 3, the depth of the circular section of the seed slide, the length of the expansion contraction section, the diameter of the seed tube, and the length of the contraction section are minimum values. At this time, the cross-sectional area of the seed slide is the minimum value. Meanwhile, the airflow velocity is the maximum value, the coupling force increases, and static pressure decreases. The positive pressure, negative pressure, and airflow velocity at the distribution manifold are 4243.97 Pa, −2977.74 Pa, and 102.64 m/s, respectively. The maximum positive pressure and airflow velocity of the optimal result are only 6.05% and 24.06% of test 3. According to the theoretical calculation performance curve of the blower’s air pressure in Section 2.2, when the diameter of the seed tube is 16 mm, the selected blower cannot meet the air volume and air pressure required for the air-assisted seed-guiding process. When the diameter of the seed tube is 19 mm, the maximum air volume of the blower is 63.65 m³/h, and the maximum air volume is greater than the theoretical air volume calculated by Equation (6). The blower selected for the air-assisted guiding device meets the design requirements.

The agronomic requirement for the air-assisted guiding device designed in this paper is 2 ± 1 seeds/groove. Seeds were randomly selected for filling one, two, and three seeds/groove, respectively, to analyze sesame’s force and velocity characteristics in the grooves. Each case was set three times and eighteen sesames were analyzed. The force and velocity distribution characteristics in the analysis were statistical. The force and speed analysis curve of sesames was named. The first number is the groove number. The second number is the number of sesames in the grooves. The third number is the sesame serial number in the groove. As shown in Figure 11, the eighth groove contains three sesame seeds with seed numbers 831, 832, and 833.

The coupling force, velocity, and collision pressure distribution characteristics from the inlet of the seed slide to the outlet of the seed tube are shown in Figure 12.

The coupling force, velocity of sesame, compressive force, time of guiding process, and velocity of sesame at the end of the seed tube are shown in

Table 4. Statistical force and velocity of seeds in guiding process.

Test Indicators	Coupling Force/(×10−5 N)	Velocity of Sesame/(m/s)	Compressive Force/(×10−5 N)	Time of Guiding Process/s	Velocity of Sesame at the End of Seed Tube/(m/s)
Test 3	8.13 ± 13.18	1.31 ± 1.16	2.92 ± 8.29	0.57 ± 0.13	2.39 ± 0.55
Optimal result	0.79 ± 1.39	1.10 ± 0.81	1.35 ± 2.00	0.77 ± 0.02	2.24 ± 0.30

.

Figure 12b,d,f shows the movement of sesames in the air-assisted guiding device. The seeds enter the air-assisted guiding device from the upper side of the seed slide at 0s. The seeds rotate with the roller in the groove at 0–0.18 s, and seeds are subjected to compression forces from the roller. At this time, the seeds are not subjected to coupling force. The seeds left the groove and entered the seed slide at 0.18–0.26 s, and the average seed feeding time was 0.22 ± 0.03 s. Because the depth of the circular section of the seed slide and the rotation speed of the roller are better, the consistency of seed feeding time is higher, and the compression force on the seed when it leaves the type hill is 0 N. At 0.26–0.40 s, the seeds move in the seed slide, and the velocity of sesame gradually increases due to the coupling force of airflow field and gravity. Seed movement occurs to the end of the seed slide at 0.40–0.50 s. At 0.50–0.80 s, the seeds move in the seed tube and the coupling force on the seeds is larger due to higher velocity of airflow at the end of the seed slide. The seed is accelerated by the coupling force and gravity during the seed-guiding process. Meanwhile, the compressive force on the seed increases and the velocity decreases when it collides with the air-assisted guiding device.

Figure 12 and Table 4 show that the average mean and standard deviation of coupling force on the seeds in the optimal result is slight. The mean values and standard deviations were 9.72% and 10.55% of the values in test 3. Smaller values and fluctuations in coupling force make the acceleration of the seeds smaller and smoother, and the intensity of the collision between seed and air-assisted guiding device was reduced. Smaller average mean and standard deviation of collision generated by compressive force and smaller seed acceleration due to collision results in smoother seed transport during the guiding process. A smaller standard deviation of guiding time and velocity at the end of the seed tube improves uniformity of seed distribution and qualified rate of seed numbers per hill.

The higher airflow velocity (theoretical value in the tube) and smaller cross-sectional area resulted in higher airflow velocity inside the air-assisted guiding device, and the average value and standard deviation of the coupling force on the seeds were larger.

Figure 12c shows that the No. 621 seed acquires a high velocity due to airflow acceleration. The seeds collide with the air-assisted guiding device and fly upward from the entrance of the seed slide. In addition, the processes of seed filling and guiding are serial structures. Effective seed filling is a necessary condition to ensure seeding quality. The rotation speed in the optimal result and test 3 are 25 r/min and 40 r/min, and the qualified rates of seed filling are 95.33% and 89.67%. The reduction in the qualified rate of seed filling and the seeds flying out from the upper side of the seed slide during the guiding process increased the miss-seeding rate to 47.33% in test 3.

In summary, coupling force is an essential factor affecting the seed movement characteristics of the air-assisted guiding process. Reasonable structure of an air-assisted guiding device, suitable airflow velocity, and rotating speed of rollers can improve seed distribution uniformity and qualified rate of seed numbers per hill and reduce the miss-seeding rate during the guiding process.

3.3. Bench Test

A bench test of seeding quality was carried out to verify the accuracy of the simulation results. First, the relationship between the blower controller’s indication and the seed tube’s airflow velocity was calibrated to ensure accuracy of airflow velocity during the test process. The equipment used for calibration is ZCF1000-1F Anemometer, which Shanghai Yi’ou Instrument Equipment Co., Ltd. (Shanghai, China) makes. The experimental procedure and calibration results are shown in Figure 13. Figure 13 shows that the indication of the blower controller is approximately proportional to the airflow velocity.

The bench test process and seeding quality are shown in Figure 14. The test showed that the qualified rate of seed numbers per hill was 86.80%, and the miss-seeding rate was 6.00%, the hill distance was 182 mm, and the scattering distance ratio was 23.6%. Further, [3] shows that, the smaller the scattering distance ratio is, the better the hill-seeding performance, and the scattering distance ratio can meet the requirement of precision hill-seeding when it is lower than 30%.

3.4. Field Experiment

The experiment procedure and crop growth are shown in Figure 15.

The average number of sesame is 1.32 plants per hill, the qualified rate is 83.45% (2 ± 1 plants per hill), the miss-seeding is 8.51% (0 plants per hill), the hill distance is 196 mm, and the scattering distance ratio is 28.9%. The result of the field experiment shows that the performance centralized seed-metering device meets the requirements of sesame seeding.

The longer lengths of seed tubes are due to the overall structural limitation of the planter. Longer seed tube lengths increase collision probability in the seed-guiding process, and the scattering distance ratio of the designed centralized seed-metering device is slightly higher compared to single-row precision seed-metering devices [5]. Under better structural and operating parameters, the seed-guiding device avoids seed clogging of bent seed tubes and improves seed distribution uniformity. The centralized seed-metering device can meet the performance requirements for hill-seeding with a scattering distance ratio of not more than 30% [3]. Further, refs. [3,9] show that seed tube type and installation height affect hill-seeding performance. Additional research is needed to investigate the influence law of the installation position of the centralized seed-metering device, types of seed tubes, and layout of seed tubes on hill-seeding performance.

4. Conclusions

(1)

A precision hill-seeding centralized seed-metering device for sesame was designed. The design and experimental study focused on an air-assisted guiding device. The core of the air-assisted guiding device is a distribution manifold with tangential seed protection and seed guide function. Based on theoretical analysis, the parameter range of the distribution manifold was determined. The depth of seed slide, length of expansion contraction section, and length of contraction section are 1.58 mm–3.65 mm, 160 mm–188 mm, and 10 mm–20 mm, respectively. Moreover, the suitable airflow velocity range to achieve stable transport is 6.3 m/s–9.1 m/s.

(2)

CFD–DEM coupling simulation obtained an optimal parameter combination for the air-assisted guiding device. When the depth of seed slide, diameter of seed tube, airflow velocity, rotation speed of the roller, length of expansion contraction section, and length of contraction section are 2.62 mm, 19 mm, 6.3 m/s, 25 r/min, 188 mm, and 20 mm, respectively, the seeding quality of the air-assisted guiding device is better. At this time, the qualified rate of seed numbers per hill and miss-seed rate are 88.33% and 5.00%, respectively. Bench test verifies the accuracy of simulation results.

(3)

The field experiment results showed that the qualified rate is 83.45% (2 ± 1 plants per hill) and the miss-seeding rate is 8.51% (0 plants per hill). The air-assisted guiding device of the precision hill-seeding centralized seed-metering device meets the requirements of sesame seeding.