The Influence of Wind Speed on Pneumatic Conveying Characteristics of Solid Feed in Horizontal Pipe by Simulation and Experiment

Abstract

Pneumatic conveying technology is an efficient, energy-saving and environmentally friendly means of solid feed conveying. In the process of pneumatic conveying, wind speed has a decisive influence on conveying characteristics. Here, computational fluid dynamics coupled with a discrete element method simulation and experiment were combined, and the conveying wind speed was used as the experimental variable to study the conveying characteristics of the conveying material in the tube, such as particle distribution state, solid phase mass concentration, coupling force on solid feed, average speed and pressure drop of solid feed in the pipe. The results show that when the conveying wind speed increases from 18 m/s to 20.6 m/s, the solid feed changes from sedimentary flow to suspended flow, the particle accumulation gradually decreases and the conveying efficiency is significantly improved. The particle slug greatly reduces the collision and friction between the internal particles and the pipe and reduces the crushing rate to a certain extent. When the conveying wind speed is about 23.2 m/s, there are almost no trapped particles in the pipeline, which can achieve rapid feed delivery, and conveying efficiency is greatly improved. Therefore, this paper provides a good theoretical basis for improving conveying efficiency and reducing crushing rate in the process of pneumatic conveying.

1. Introduction

China is a big pork consumer, accounting for about 60% of the world’s pork consumption. The transformation of the pig industry from traditional agricultural production to the development mode of science and technology, intelligence, scale and intensification [1,2] is the future development trend. In the pig industry, the transport of solid feed is the key to improving the efficiency of pig breeding, and the pneumatic conveying system is needed to transport solid feed quickly and efficiently.

Pneumatic conveying technology originated in the late 19th century, and Medhurst proposed the pneumatic conveying scheme of mail in 1810. In 1824, Vallance first built an experimental apparatus for pneumatic conveying. In 1891, the British Dockham successfully produced a negative pressure pneumatic conveying device, which was widely used in the United Kingdom, Germany and other food-importing countries. In 1924, Gasterstadt of Germany carried out pneumatic conveying experiments on wheat and published many theoretical and experimental research conclusions, and the pneumatic conveying theory tended to be perfect. Compared with foreign countries, China’s pneumatic conveying technology started late and began to be applied in China after the 1960s. With the rapid development of China’s economy, pneumatic conveying technology has been widely used in chemical, pharmaceutical and grain-processing fields [3].

Pneumatic conveying involves a device that drives the flow of dry granular materials by means of the flow of air or gas, so as to realize the transfer of materials from one location to another location. It can be widely used in various breeding enterprises, because of its simple structure, high transportation efficiency, long transportation distance and other advantages [4,5,6]. Tan Y et al. [7] applied pneumatic conveying technology to municipal solid waste transportation. Wypych [8] designed pneumatic conveying technology using inert carrier gas to safely handle highly explosive metal powders. Daolong Y et al. [9] applied pneumatic conveying technology in the field of pulverized coal transportation to avoid direct contact between pulverized coal and air. Ranjbari et al. adopted the coupled Eulerian–Lagrangian method. Able to capture the movement of solid particles and the development of liquid phase in three-phase (solid particles, liquid and gas) flow pipeline transportation [10], Sun et al. used a CFD-DEM coupled numerical simulation method to analyze the flow characteristics of hard shotcrete materials in the process of vertical pneumatic transportation [11].

Conveying wind speed is the key factor in pneumatic conveying, which determines a host of parameters of pneumatic conveying system. To investigate the impact of conveying wind speed on the pneumatic conveying system, based on the theory of gas–solid two-phase flow [12,13], this paper describes a suitable horizontal pneumatic conveying system for solid feed. The method combining computational fluid dynamics (CFD) and the discrete element method (DEM) [14,15] was used to numerically simulate the pneumatic conveying process under different conveying wind speed conditions. The analysis was conducted on the dynamic pressure, pressure drop and mass concentration of solid feed within the pipeline [16,17,18], and an experimental platform for pneumatic conveying in a horizontal pipeline was set up for experimental verification.

2. Computational Fluid Dynamics and Discrete Element Method

The CFD-DEM was first proposed by Tsuji et al. in 1992 for the study of slug flow pneumatic conveying [19] and subsequently proposed by many others and has now become an effective method for the study of particle–fluid systems [20]. In recent years, research has intensified in the development and application of full 3D CFD-DEM models and taking into account the influence of particle shape [21,22], which is essential for the authenticity of simulations.

FLUENT 18.0 software is a computational fluid dynamics (CFD) simulation software and, because of its powerful function and flexibility, it is widely used in aerospace, automotive, energy and other industries. EDEM 2020 software is primarily used for the simulation of particle dynamics, which uses discrete element methods (DEMs) to simulate materials of any shape and size. The coupling of FLUENT and EDEM is an interactive process, which can improve the reliability and accuracy of simulation. The two can transmit information to and receive information from each other while running independently [23,24,25]. FLUENT can receive the physical properties and motion information of particles in EDEM, and EDEM can exert force on particles under the fluency provided by FLUENT, and together they complete the simulation of gas–solid two-phase flow [26,27].

In the coupling solution, the solid feed is first created in EDEM and its parameters are set. The contact model between solid feeds is Hertz–Mindlin (no slip) with RVD rolling friction. The contact model between particles and pipes was selected as Hertz–Mindlin with Archard wear, the direction of gravity acceleration was set to the negative Z-axis, the size was set to 9.81 m/s² and the cell size was set to 4R.

For FLUENT, the three-dimensional double precision solver is selected. The mesh file was imported and checked. The pressure base and transient solution mode were set. The gravitational acceleration was set to the negative Z-axis and the size was 9.81 m/s². Then it is connected with EDEM, and the solution steps are as follows:

Models: Standard k-ε turbulence model is adopted;

Boundary Conditions: Select the speed inlet at the entrance and the pressure outlet at the exit;

Solution Methods: Use the SIMPLEC algorithm;

Solution Initialization: Select Standard Initialization and initialize from the entry;

Time Step Size (s): Set the time step to 8 × 10⁻⁴;

Number of Time Steps: 4500.

3. Mathematical Model

3.1. Particle Suspension Velocity

When the airflow speed reaches a certain level, the particles will be transported by the airflow, that is, the suspension speed [28]. There are many formulas for calculating the suspension speed [29,30,31] of particles in pneumatic conveying. The regional suspension velocity formula and its applicable particle size method are the most extensive, the specific formula is as follows:

(1)

Viscous resistance zone

$$u_b={\displaystyle \frac{d_b^2\left({\rho }_b-{\rho }_a\right)g}{18\mu }} \mathrm{a}\mathrm{n}\mathrm{d} d_b\le 1.225{\left[{\displaystyle \frac{{\mu }^2}{{\rho }_a\left({\rho }_b-{\rho }_a\right)}}\right]}^{{\displaystyle \frac{1}{3}}}$$

(1)

(2)

Transition region

$${ u}_b=1.195d_b{\left[{\displaystyle \frac{{\left({\rho }_b-{\rho }_a\right)}^2}{{\rho }_a\mu }}\right]}^{{\displaystyle \frac{1}{3}}} \mathrm{a}\mathrm{n}\mathrm{d} 2.2{\left[{\displaystyle \frac{{\mu }^2}{{\rho }_a\left({\rho }_{ab}-{\rho }_a\right)}}\right]}^{{\displaystyle \frac{1}{3}}}\le d_b\le 20.4{\left[{\displaystyle \frac{{\mu }^2}{{\rho }_a\left({\rho }_b-{\rho }_a\right)}}\right]}^{{\displaystyle \frac{1}{3}}}$$

(2)

(3)

Pressure difference resistance zone

$$u_b=5.45\sqrt{{\displaystyle \frac{d_b\left({\rho }_b-{\rho }_a\right)}{{\rho }_a}}} \mathrm{a}\mathrm{n}\mathrm{d} 20.4{\left[{\displaystyle \frac{{\mu }^2}{{\rho }_a\left({\rho }_b-{\rho }_a\right)}}\right]}^{{\displaystyle \frac{1}{3}}}\le d_b\le 1100{\left[{\displaystyle \frac{{\mu }^2}{{\rho }_a\left({\rho }_b-{\rho }_a\right)}}\right]}^{{\displaystyle \frac{1}{3}}}$$

(3)

$u_b$ is the particle suspension speed, ${\rho }_a$ and ${\rho }_b$ are air and particle density, respectively, $d_b$ is particle size, $\mu$ is aerodynamic viscosity (1.81 × 10⁻⁵ Pa·s). The corresponding particle size ranges are respectively the viscous resistance regions: $d_b$ ≤ 0.065, transition region: 0.065 < $d_b$ ≤ 0.60, pressure difference resistance zone: 0.60 < $d_b$ ≤ 32.56, unit: mm.

The solid feed used in this paper is located in the pressure difference resistance area and, accordingly, the suspension speed can be obtained by taking 10.3 m/s in the corresponding formula.

3.2. Governing Equation of Gas–Solid Phases

The Navier–Stokes equations are fundamental equations that describe the motion of fluids and are used in the CFD section. For incompressible fluids, the equation is expressed as:

$${\displaystyle \frac{\delta u}{\delta t}}+u\cdot \nabla u=-{\displaystyle \frac{1}{\rho }}\nabla p+\nu {\nabla }^{2}u+f$$

(4)

$u$ is the fluid velocity, m/s, $p$ is pressure, Pa, $p$ is the fluid density, m/s, $\nu$ is kinematic viscosity, m²/s, $f$ is for volume force (e.g., gravity), N.

The motion of each particle satisfies Newton’s second law [32], and the equation is expressed as:

$$m{\displaystyle \frac{dv}{dt}}=F_{total}$$

(5)

m is the particle mass, v is the particle velocity, m/s, $F_{total}$ is the total force on the particle (including gravity, collisions between particles and fluid drag on the particle), N.

3.3. The Force Between Gas and Particles

The drag force of the fluid on the particle can be calculated according to Newton’s law of resistance [33], expressed as:

$$F_{drag}=C_D{\displaystyle \frac{\pi d_p^2{\rho }_f\left|u_f-u_p\right|}{2}}(u_f-u_p)$$

(6)

$C_D$ is the drag coefficient, $d_p$ is the particle diameter, ${\rho }_f$ is the fluid density, $u_f$ and $u_p$ are the velocities of the fluid and particles, respectively.

4. Simulation of Pneumatic Conveying

In order to elucidate the flow characteristics of gas–solid two-phase flow in the pneumatic conveying system studied in this paper, the coupled CFD-DEM was used to simulate the conveying pipeline.

4.1. Model Construction and Grid Division

The solid feed used in the experiment was cylindrical with four different particle sizes. The particle tool was used in EDEM to model the solid feed, and the modeling results are shown in Figure 1a. In this model, the radius of the spheres is set to 1 mm, and different numbers of spheres are selected to be stacked according to the particle size, similar in shape and size to real solid feed.

The horizontal conveying pipeline was modeled using Solidworks 2018 and meshed in ICEM CFD 18.0 software. The hexahedral non-structural meshing method was adopted. The wall of the pipeline was refined to varying degrees, FLUENT was selected to verify the grid independence and the standard turbulence model was set. The inlet velocity was 15.5 m/s, the outlet pressure outlet was selected, the time step was 0.1 s, the number of operation steps was 600 and each step was iterated 20 times. The results show that the simulation time hardly changes with the increase in the number of meshes when wall encryption is selected and the number of meshes is 403,200. Figure 1b shows the result of the mesh division for a pipeline with a diameter of 84 mm and the length is 10,000 mm, where a-a represents the cross-section of the mesh and b-b is the sampling section.

4.2. Setting of Pipeline Parameters and Basic Working Conditions

In order to better observe the state of solid feed in the pipeline during the experiment, acrylic plate with strong insulation, excellent transparency and high recovery rate was selected as the pipeline material. The density, Poisson’s ratio and shear modulus of the conveying pipeline are shown in

Table 1. Conveying pipeline parameters.

Density (kg/m3)	1190
Poisson ratio	0.32
Shear modulus (Pa)	1.07 × 109

.

The basic conditions of pneumatic conveying solid feed are established: the standard k-ε turbulence model is adopted, the conveying wind speed is 15.5 m/s, the particle mass flow rate is 0.288 kg/s, the conveying pipe diameter is 84 mm and the sampling section is 5 m away from the blanking port. First, the reference working condition is simulated, subsequently the control variable method is employed to simulate the pipe conditions under different wind speeds, and its influence on the pneumatic conveying system is analyzed.

4.3. Influence of Different Wind Speeds on Conveying Characteristics

The conveying wind speed is the power source of the whole pneumatic conveying system, which directly affects the flow characteristics of the gas–solid phases. According to the calculation, the suspension speed of solid feed is 10.3 m/s. In actual transportation, due to the collision and friction between particles and particles, particles and pipelines, as well as the uneven flow of air in the pipeline, the conveying wind speed generally used is 1.5 to 2.5 times [34] the particle suspension speed. Considering the adjustability of the experimental device, the transport wind speed adopted in this study is 1.5, 1.75, 2.0, 2.25, 2.5 times the particle suspension speed, that is, 15.5 m/s, 18.0 m/s, 20.6 m/s, 23.2 m/s and 25.8 m/s. The maximum mass flow rate of solid feed was 0.288 kg/s. The five different wind speed conditions were simulated and the experiment was carried out to study the influence of the change in conveying wind speed on the conveying characteristics.

4.3.1. Effect of Conveying Wind Speed on Distribution State of Solid Feed in Tube

After stable transportation, the motion state and distribution of solid feed are observed, as well as the distribution state of solid feed in the pipe within the sampling section (4.5–5.5 m from the entrance of the pipeline) under different transportation wind speeds, as shown in Figure 2. Different colors represent different particle size ranges of solid feed. Solid feed according to particle size from small to large is colored in black, red, green and blue. The simulation results show that, under the same conditions, with the increase in the conveying wind speed, the speed difference between the upper and lower end of the particles at the bottom of the pipeline increases, and the particles will be subjected to upward force according to the Bernoulli principle. At the same time, the particle rotation causes the Magnus effect lift caused by the superposition of the circulation around the particles and the air flow, and the amount of accumulated feed at the bottom of the pipeline decreases, the suspension amount increases and the flow pattern of the particle group transitions from sedimentation to suspension flow.

When the conveying wind speed is 1.5 times the suspension speed of solid feed (V = 15.5 m/s), the solid feed accumulates at the bottom of the pipeline, resulting in a dense deposit flow. Under this condition, most of the solid feed is transported in the front of the pipeline, there is more friction and collision between the wall and the bottom of the pipeline and the wear is greater. In the actual conveying process, with the increase in the conveying wind speed, the accumulation of solid feed at the bottom of the pipe gradually decreased, the suspension amount in the pipe increased and the particles suspended in the upper part of the pipe were still small. When the conveying wind speed is 2 times the suspension velocity (V = 20.6 m/s) or higher, the quantity of particles deposited at the bottom of the pipeline is markedly decreased, the interspace of particle groups is increased, the friction collision between particles and the pipe wall is reduced, the wear of the pipe wall is reduced, the transport time was shortened from 8.25 s to 6.83 s and the conveying efficiency is greatly improved.

The analysis of the simulation results shows that the sediment flow of solid feed is formed because it is not only affected by the drag force provided by the wind but also affected by its own gravity during transportation. When the conveying wind speed is low, the gravity of the solid feed itself dominates and, consequently, particles accumulate at the bottom of the pipe and are transported along the pipe, forming a sedimentary flow at the bottom of the pipe. With the increase in conveying wind speed, according to the Bernoulli principle and Magnus effect, the lift force on solid feed gradually increases, and the influence on the lift force of small particle size solid feed gradually becomes greater than the influence of gravity. Accordingly, the suspended solid feed increases with the increase in conveying wind speed. In the conveying process, in order to achieve suspension transportation of solid feed, the actual conveying wind speed should be greater than 2 times the suspension speed.

4.3.2. Particle Mass Distribution in Conveying Pipes at Different Wind Speeds

In order to investigate the distribution state of solid feed along the pipeline direction in pneumatic conveying, the pipeline section 5 m away from the feeding port was taken as the sampling section. The simulation results are as follows: Figure 3 is the cloud map of the mass distribution of solid particles at different wind speeds, the solid particles are mainly concentrated at the bottom of the pipeline, other conditions remain unchanged and, with the increase in the conveying wind speed, the particles at the bottom of the pipeline gradually spread out from the accumulation state to both sides. When the conveying wind speed increased to 20.6 m/s, “suspended” particles began to appear, and the number of “suspended” particles increased with the increase in the wind speed, indicating that the transportation efficiency of solid particles in the conveying pipeline was gradually improved. In addition, the maximum concentrations of particle mass distribution were 234.71, 107.15, 63.22, 43.57 and 38.64 kg/m³. This shows that the greater the conveying wind speed, the more dispersed the solid particles in the pipeline, and the friction and collision between the particles are reduced and the transportation efficiency is improved.

4.3.3. Influence of Conveying Wind Speed on Average Speed and Pressure Drop of Solid Feed

In the dynamic balance stage, the speed of conveying materials has a great impact on the working time and efficiency of the entire system, and the pressure drop as the power source of conveying not only to ensures the continuous and stable movement of solid feed but also overcomes the various resistance generated by collision and friction during the conveying process. Therefore, in this section, the average speed and pressure drop of particle groups under different conveying wind speeds in the conveying pipeline are studied. Figure 4a shows the relationship between the average speed of solid feed and time at different conveying wind speeds. When conveying tends to be stable, the average speed of solid feed is 13.8 m/s, 17.4 m/s, 18.9 m/s, 21.8 m/s and 24.9 m/s. It can be seen from the figure that, at the beginning of transportation, the average speed of solid feed increases linearly and rapidly to the maximum, then decreases and tends to be stable. It was also observed that the higher the conveying wind speed, the faster the average solid feed speed increased at the beginning of conveying.

Figure 4b shows the changes in pressure drop in the pipe at different conveying wind speeds after particle movement in the sampling section reaches dynamic equilibrium. As can be seen from the figure, the pressure drops corresponding to the five wind speeds are 72.4 Pa, 95.2 Pa, 147.6 Pa, 217.5 Pa and 309.1 Pa, respectively, and the pressure drop increases are 22.8 Pa, 52.4 Pa, 69.9 Pa and 91.6 Pa, respectively. As the conveying wind speed increases, the change in pressure drop becomes more pronounced.

4.3.4. Influence of Conveying Wind Speed on Particle Coupling Force

Figure 5 shows the coupling force experienced by particles at different wind speeds, illustrating the variation of the coupling force over time under five different velocity conditions. The coupling force is the force exerted by the fluid on the particles, including drag force, lift force and gravity, among others. At the initial stage of the simulation, the coupling force increases rapidly as the particles are accelerated and begin to interact with the gas. Subsequently, the coupling force gradually stabilizes, reflecting the dynamic equilibrium state of the particles within the air.

As the conveying wind speed increases, the peak value of the coupling force also correspondingly rises, indicating that higher wind speeds result in greater forces acting on the particles, thereby affecting their suspension and transportation. When the wind speed reaches 25.8 m/s, the peak of the coupling force is the highest, suggesting that, at high wind speeds, the efficiency of particle transportation is greater, but this may also lead to increased wear and energy consumption of the system.

5. Experimental Analysis

5.1. Experimental Media

In this paper, compressed air is used as the conveying gas, and solid feed is used as the conveying medium, as shown in Figure 6. The feed divided into four groups according to particle size, which are 3–5 mm, 5–7 mm, 7–9 mm and 9–11 mm, respectively.

Table 2. Related parameters of solid feed.

Density (kg/m3)	Poisson Ratio	Shear Modulus (Pa)	Collision Recovery Coefficient	Coefficient of Sliding Friction	Coefficient of Rolling Friction
800	0.4	3.93 × 107	0.53	0.41	0.08

shows the relevant parameters of the solid feed used in the experiment.

5.2. Experimental Equipment

As is shown in Figure 7, the pneumatic conveying system mainly consists of three inverters of SKI600 series from Sako Hangzhou, China and a 2PB810H27 7.5KW model high-pressure fan from Suzhou FENRZ, feeding funnel, acrylic transparent pipe, pipe rack, CHWVN-WD4100YA28 model sensor from Zhenweil Hangzhou, China, HTSUA1333GC-T high-speed camera and collection device.

The inverter can achieve an output frequency of 0–50 Hz, and the corresponding fan conveying wind speed can be adjusted from 0–29.3 m/s according to the experimental requirements at any time to adjust the wind speed of the fan through the inverter to control the fan for high-precision control but also reduce energy consumption to ensure the stable operation of the motor. The external power supply of the CHWVN-WD4100YA28 sensor is 24 V. The CHWVN-WD4100YA28 sensor can measure the wind speed, pressure, temperature and air volume through a 220 V to 24 V transformer. In order to ensure the stable pressure of the conveying air in all parts of the conveying pipe and the smooth discharge of the air from the pneumatic conveying system, the outlet of the collection device is designed to be significantly larger than the inlet, and the end of the conveying pipe is controlled to have a certain appropriate distance from the outlet.

5.3. Experimental Process

The corresponding frequencies of 15.5 m/s, 18.0 m/s, 20.6 m/s, 23.2 m/s and 25.8 m/s are 28.9 Hz, 34.6 Hz, 38.7 Hz, 43.5 Hz and 48.7 Hz. The experimental equipment was organized, it was checked whether the air tightness of the transportation pipeline was perfect and whether the air outlet can be successfully discharged and the power supply was switched on after the check is completed.

The solid feed was poured into the feeding funnel, the frequency converter was adjusted to 28.92 Hz, the run was begun, the feeding funnel valve was opened when the wind speed at the inlet of the transport pipeline was 15.5 m/s, the mass flow rate was controlled at 0.288 kg/s and the movement of the solid feed in the sampling pipeline was observed and shot through high-speed cameras. Using a second sensor to measure wind speed, pressure and other data inside the sampling pipe, the solid feed was transported from the conveying pipe to the collection device, where it was recovered and reused.

5.4. Experimental Results and Analysis

5.4.1. Distribution of Solid Feed in Pipes

When the conveying wind speed is less than 18 m/s, as shown in Figure 8a, due to the low wind speed, there are more collisions between particles, and the friction resistance between particles and the pipe wall is relatively high. It can be obviously observed that most solid feed is transported forward along the lower pipe wall. With the continuous feeding of the hopper and the increase in the conveying distance, the front solid feed has insufficient power, and the speed gradually decreases until eventually it stops and builds up. As shown in Figure 8b, the transported solid feed collided with the accumulation, most of which stayed at the bottom of the pipeline and became part of the accumulation, and the rest continued to be transported along the pipeline after passing over the accumulation, resulting in low transportation efficiency and much residue in the pipeline. The relationship between accumulation position and conveying wind speed is shown in

Table 3. Accumulation location of solid feed.

Conveying wind speed (m/s)	15.5	16.9	17.5	20.6	25.8
Motor frequency (Hz)	28.9	32.3	34.1	38.7	48.7
Distance from the accumulation to the entrance (m)	3.4	6.1	None	None	None

. In order to avoid accumulation, the maximum feeding amount corresponding to different conveying wind speeds is measured, as shown in

Table 4. Maximum feeding amount corresponding to different conveying wind speeds.

Conveying wind speed (m/s)	15.5	16.9	17.5	20.6	25.8
Mass flow rate (kg/s)	0.128	0.239	0.288	0.288	0.288

. During the experiment, only the feeding amount corresponding to a 15.5 m/s conveying wind speed needed to be controlled.

When the conveying wind speed reaches 20.6 m/s and above, the solid feed transportation speed is accelerated, the deposited feed in the conveying pipeline is reduced and the suspended amount is increased. The solid feed almost becomes a suspended flow, and there is no residue in the transportation pipeline, which can be completed well.

5.4.2. Particle Slug in Pipe

Through the observation experiment, the solid feed has a certain movement law. During the transportation process, a particle group will be formed in the pipeline, the particle group will move forward as a unit and there is a certain distance between each particle group, which is called a slug. As the transport process progresses, the forward slug gradually slows down, the rear slug continues to advance and collide with it, the two slugs can merge into a stable slug and the front part of the particles gains kinetic energy to move forward. When the wind speed is too low, the particles formed cannot overcome the friction resistance with the pipe wall, and as there is not enough power to transport forward, they will accumulate here, resulting in pipeline blockage. With the gradual increase in wind speed, the slug speed increases, and the accumulation is gradually reduced.

The movement of slug formation at five speeds is shown in Figure 9. It can be observed that, during the conveying process, the rear particles collide with the front slug. Some particles continue to move forward together with the slug, while the other part passes along the upper profile of the slug, and these particles also reach the middle and upper part of the pipeline. Since the speed of the rear particles is faster than that of the slug, when the two contact, the particles at the front end of the slug gain kinetic energy and accelerate the forward transport. When the slug moves, it will take away the particles in the front settling layer and leave a new settling layer, and the particles in the slug will continue to renew over time. The particle density of the slug is relatively high, and during the transportation process, the particles inside the slug are “surrounded”, which is greatly reduced by the impact of other particles and the friction between the pipe wall, reducing the wear of the particles. Therefore, the slug also has a certain degree of protection of particle integrity and reduction of particle breakage.

5.4.3. Measurement of Average Velocity of Suspended Solid Feed

In order to calculate the speed of suspended particles, a high-speed camera is used to shoot the motion state of the solid feed in the sampling pipe section, so as to calculate the speed here. As shown in Figure 10, the length of the sampling pipeline captured by the high-speed camera is 0.3 m, and its FPS parameter is set to 12 frames per second. In any time period, five suspended particles are randomly selected to measure their movement time and distance, and their numbers are 1–5. The speed of the five particles is calculated and the average value is taken, as shown in

Table 5. Distance and speed of suspended particles.

Particle numbering	1	2	3	4	5
Movement distance (m)	0.291	0.253	0.272	0.259	0.261
Movement time (s)	0.032	0.032	0.032	0.032	0.032
Speed (m/s)	9.09	7.91	8.50	8.09	8.12
Average speed (m/s)	8.34

. The average speed is 8.34 m/s. According to the above method, ten different time periods were selected in the transport process, and the average value was calculated. The solid feed speed corresponding to the five transport wind speeds was divided into 8.57 m/s, 10.29 m/s, 12.28 m/s, 14.66 m/s and 17.36 m/s. According to the data, the particle velocities were 55.3%, 57.1%, 59.6%, 63.2% and 67.3% of the conveying wind speed, respectively. With the increase in conveying wind speed, the solid feed velocity in a suspension state increased, and its increment gradually increased.

6. Conclusions

In this paper, the pneumatic conveying system is designed and modeled by FLUENT and EDEM software, a simulation was carried out by the coupled CFD-DEM and the experimental platform of a pneumatic conveying pipeline was built according to the model. It was observed that with the increase in conveying wind speed, the solid feed gradually changed from “deposition flow” to “suspension flow”, the particle accumulation gradually decreased, the friction with the pipe wall decreased, the average speed increased and the coupling force of particles increased correspondingly. A particle slug is formed in the pipeline during transportation. The particles outside the slug collide and friction with other particles or pipe walls protects the particles inside the slug and effectively reduces the particle breakage rate. When the conveying wind speed is about 2.25 times the particle suspension speed, the solid feed can quickly pass through the conveying pipe, there is almost no residue in the pipe and the conveying efficiency is improved.