# Experimental Investigation on Atomization and Dustfall Characteristics of Combined Nozzles for Shearer External Spray in Fully Mechanized Coal Mining Face

Authors: Qian Wang; Ruixin Song; Junpeng Wang; Cuicui Xu; Gang Zhou; Jiayuan Wang
Journal: Advances in Civil Engineering
 (2021)

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## Abstract

To explore the dustfall effect of combined nozzles used in a fully mechanized mining face, Phase Doppler Interferometry (PDI) system was used to test 6 types of high-efficiency atomizer under 8 MPa. A new nozzle group of nozzles 2#, 3#, and 5# is selected by atomization experiment. The atomization experiment and field application test of the preferred nozzle are performed. The experiment and field application results show that, with the spray pressure of 8 MPa, when the distance in the nozzle group is 200 mm and the angle change is 10 degrees, the atomization effect is the best. Under the optimal parameters, the average dustfall rates of the entire and respirable dust are 81.82% and 79.96%, respectively, which are 23.49% and 20.75% higher than those of the traditional shearer.


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## Body

## 1. Introduction

In recent years, the mining level of mechanization and automation of coal mines in China has been gradually improved [1–3], and the disasters caused by the production process of coal mines have become increasingly prominent [4–6]. In particular, the dust hazards in underground working areas have seriously threatened the safety of mine production and the physical and mental health of miners [7–9]. According to measurements, with no dust-proof measures, the dust production of a coal mining face is as high as 60% of the total dust of the mine [10–12]. When a shearer cuts coal, the instantaneous total dust concentration in some coal mines is higher than 4000–6000 mg/m3 [13, 14]. Even if dust-proof measures are taken, the working environment of most coal mining faces and the total dust and exhaust dust powder on the downwind side of the shearer remain very bad. The dust concentration instantaneously reaches 500 mg/m3 and 200 mg/m3 [15, 16]. Those dust concentrations far exceeded the upper limit stipulated in the national safety regulations. Cutting dust is the main dust produced in a coal mining face. Therefore, it is particularly important to effectively control the coal cutting dust production in the coal mining face [17, 18]. At present, most scholars at home and abroad mainly focus on two aspects: spray numerical simulations and spray dustfall experiments [19–22]. In the numerical simulation of sprays, Xu et al. used the VOF method to simulate the internal flow field of X-swirl pressure nozzle and found that the swirl angle and outlet diameter are two important parameters affecting the spray field velocity [23]. On the basis of VOF-LES method, Yu et al. introduced the adaptive mesh refinement (AMR) method to predict the breakup of liquid core in the process of primary atomization [24]. However, the aforementioned studies mostly focused on the internal flow field of the nozzle and did not involve the atomization distribution characteristics of the external spray field of the nozzle. In an experiment with spray and dust suppression, Charinpanitkul and Tanthapanichakoon studied the atomized droplet size distribution of different types of nozzles and their effects on the dustfall rate of different sizes of dust [25]. Based on the self-designed nozzle atomization test platform, Cheng Weimin and Zhou et al. performed atomized droplet size testing of several commonly used nozzles under different pressures in the coal mining face and studied the atomization effect of the high-pressure spray atomization droplet size [26]. Wang et al. used a self-designed spray dust reduction experimental system to study the quantitative relationship between the diameter and the atomization parameters of commonly used pressure nozzles [27, 28]. However, most of the above studies were aiming at single characteristics of the nozzles, which resulted in large randomness in the selection of nozzles.Although the above scholars have performed systematic research on two aspects, spray numerical simulations and spray dust reduction experiments, none of these analyses concerned the selection of different types of nozzle combinations, atomization parameters of multiple nozzles, and atomization and dust suppression effects of multiple nozzle groups. Therefore, based on the self-built spray and dust simulation experiment system, 6 types of nozzles of different types were selected for experimental study. The distribution law of droplet size distribution and the optimization of atomization parameters between different nozzles in fully mechanized coal face under different types of nozzle combinations are studied in detail and applied in fully mechanized coal mining face. This work provides a reference for improving the efficiency of shearer external spray and dust suppression.


## 2. Experimental System and Scheme

### 2.1. Experiment System

To clearly and accurately analyze the distribution characteristics of the external spray atomization characteristics of shearers in a fully mechanized working face, the macroscopic and microscopic characteristics of the droplet field were measured based on Phase Doppler Interferometry (PDI) test system and dustfall indoor simulation system. The experimental system is mainly composed of a phase Doppler laser interferometer (PDI200MD), a mine tunnel simulator, a K45-6 type mine energy-saving ventilator, a spray device, a SAG-410 dust aerosol generator, and a control and data acquisition device. The simulation roadway is mainly composed of two parts. They are the inlet diffuser with the shape of pyramid and the cuboid experimental section with the length, width, and height of 3.0 m, 3.0 m, and 2.5 m, respectively. The air inlet is a circle with a diameter of 1.4 m, and the air outlet is a rectangle with a section of 3.0 m × 2.5 m. Except that the air inlet and air outlet are not closed, the other 8 surfaces are sealed with 1 cm thick plexiglass. The layout of the test system is shown in Figure1.Figure 1
Doppler laser interferometric spray experiment system.


### 2.2. Selection of Experimental Nozzles

Based on the investigation of major coal mines and shearer nozzle manufacturers, 6 types of external spray nozzles for shearers with better dustfall effects [29, 30] were selected and numbered by the Roman number of the nozzle. The specific objects are shown in Figure 2. Nozzle 1# is a 1.8 mm linear hollow cone nozzle, nozzle 2# is a solid cone wide-angle nozzle with a diameter of 2.0 mm, and nozzle 3# is the cross opening with an aperture of 2.4 mm. The swirl centrifugal mixed solid conical with an orifice diameter of 3.0 mm is named nozzle 4#, mixed solid conical nozzle with 2.0 mm orifice is named nozzle #5, and solid conical fan nozzle with 2.8 mm orifice is named nozzle 6#.Figure 2
Nozzles used in the experiment.


### 2.3. Experimental Scheme

To accurately obtain the droplet size and droplet velocity distribution in the spray field, the droplet size distribution at different positions of the spray field was measured by using the Doppler laser interferometer PDI200MD spray dustfall simulation experiment system. According to the actual wind speed of fully mechanized face in Jianzhuang Coal Mine being 1 m/s, in order to make the experiment more accurate, the wind speed of the simulated tunnel will be kept at 1 m/s. To more clearly analyze the distribution of droplet characteristics, measuring points were set up in the simulated roadway, among which 9 measuring points were distributed on three different planes. The coordinates of the nine points in mm are A (500, 0), B (1500, 600), C (1500, 0), D (1500, −600), E (2500, 1200), F (2500, 600), G (2500, 0), H (2500, −600), and I (2500, −1200). The layout of the measuring points in the droplet field is shown in Figure3; experimental process is shown in Figure 4.Figure 3
Layout of the measuring points of the droplet field.Figure 4
Experimental diagram of nozzle atomization.


## 2.1. Experiment System

To clearly and accurately analyze the distribution characteristics of the external spray atomization characteristics of shearers in a fully mechanized working face, the macroscopic and microscopic characteristics of the droplet field were measured based on Phase Doppler Interferometry (PDI) test system and dustfall indoor simulation system. The experimental system is mainly composed of a phase Doppler laser interferometer (PDI200MD), a mine tunnel simulator, a K45-6 type mine energy-saving ventilator, a spray device, a SAG-410 dust aerosol generator, and a control and data acquisition device. The simulation roadway is mainly composed of two parts. They are the inlet diffuser with the shape of pyramid and the cuboid experimental section with the length, width, and height of 3.0 m, 3.0 m, and 2.5 m, respectively. The air inlet is a circle with a diameter of 1.4 m, and the air outlet is a rectangle with a section of 3.0 m × 2.5 m. Except that the air inlet and air outlet are not closed, the other 8 surfaces are sealed with 1 cm thick plexiglass. The layout of the test system is shown in Figure1.Figure 1
Doppler laser interferometric spray experiment system.


## 2.2. Selection of Experimental Nozzles

Based on the investigation of major coal mines and shearer nozzle manufacturers, 6 types of external spray nozzles for shearers with better dustfall effects [29, 30] were selected and numbered by the Roman number of the nozzle. The specific objects are shown in Figure 2. Nozzle 1# is a 1.8 mm linear hollow cone nozzle, nozzle 2# is a solid cone wide-angle nozzle with a diameter of 2.0 mm, and nozzle 3# is the cross opening with an aperture of 2.4 mm. The swirl centrifugal mixed solid conical with an orifice diameter of 3.0 mm is named nozzle 4#, mixed solid conical nozzle with 2.0 mm orifice is named nozzle #5, and solid conical fan nozzle with 2.8 mm orifice is named nozzle 6#.Figure 2
Nozzles used in the experiment.


## 2.3. Experimental Scheme

To accurately obtain the droplet size and droplet velocity distribution in the spray field, the droplet size distribution at different positions of the spray field was measured by using the Doppler laser interferometer PDI200MD spray dustfall simulation experiment system. According to the actual wind speed of fully mechanized face in Jianzhuang Coal Mine being 1 m/s, in order to make the experiment more accurate, the wind speed of the simulated tunnel will be kept at 1 m/s. To more clearly analyze the distribution of droplet characteristics, measuring points were set up in the simulated roadway, among which 9 measuring points were distributed on three different planes. The coordinates of the nine points in mm are A (500, 0), B (1500, 600), C (1500, 0), D (1500, −600), E (2500, 1200), F (2500, 600), G (2500, 0), H (2500, −600), and I (2500, −1200). The layout of the measuring points in the droplet field is shown in Figure3; experimental process is shown in Figure 4.Figure 3
Layout of the measuring points of the droplet field.Figure 4
Experimental diagram of nozzle atomization.


## 3. Performance of Single Nozzle in a Fully Mechanized Coal Face

### 3.1. Macroscopic Characteristics and Droplet Size Distribution

The investigation and study on the fully mechanized coal face in Jianzhuang Coal Mine showed that the nozzle pressure used at the site was 8 MPa. So, the experiment is carried out under 8 MPa. The results are shown in Table1 and Figure 5.(1)
According to the macroparameters of atomization characteristics of the nozzle, the atomizing angle of different nozzles is in the following order 5# > 2# > 6# > 4# > 1# > 3#. The range of different nozzles is 3# > 4# > 6# > 1# > 2# > 5#. Compared with the above macroscopic atomization parameter data, the range of atomization and the angle of atomization are two negative atomizing parameters. In addition, because of the difference in nozzle structure, the droplet kinetic energy is transformed into axial and radial kinetic energy at the nozzle, which results in the negative correlation difference between the atomization angle and the range.(2)
The average droplet size, volume probability, and cumulative volume distribution of droplets from different nozzles show that the droplet size distribution is consistent with the normal distribution under the spray pressure of 8 MPa and the droplet size dispersion of different nozzles is different. The standard deviation of the normal distribution of nozzle droplet size of 1–6# nozzle is 52.947, 35.186, 43.051, 58.542, 83.723, and 66.312 in order. The standard deviation of normal distribution of droplet size can measure the dispersion degree of droplet. The larger the standard deviation of the normal distribution, the more dispersed the droplet distribution. The smaller the standard deviation of the normal distribution, the more uniform the droplet distribution. The more uniform the droplet size distribution, the higher the dust reduction efficiency. It can be seen that the order of the dispersion degree of the droplet size distribution of the six nozzles is 5# > 6# > 4# > 1# > 3# > 2#.Table 1
Macroscopic characteristic parameters of different nozzles and mean value of the microdroplet size.
Nozzles numberMacroscopic characteristics parametersSpray droplet sizeAtomization angle (°)Effective range (m)D0.1/μmD0.5/μmD0.9/μmD32/μmD43/μm1#753.8727.4457.18117.3048.3268.232#883.6521.1348.3697.3139.4063.643#694.7330.0169.34133.2752.9480.594#734.3638.4887.46155.8167.9499.425#943.6018.9346.0686.2636.8154.886#804.1530.6171.60134.7563.4885.16Figure 5
Sketch of the droplet size, volume distribution, frequency distribution, and cumulative distribution of droplets for different nozzles.


### 3.2. Droplet Velocity Distribution

The velocity distribution of different measuring points is shown in Table2 and Figure 6.Table 2
Mean droplet velocity at different nozzles.
Velocity vectorNozzles numberDroplet velocity (m/s)ABCDEFGHIAxial velocity1#36.65225.99133.59427.29318.40619.92626.71622.88625.5752#32.77424.69328.83425.42616.39718.07123.35620.65922.0993#47.46832.69340.21535.05723.25625.65330.49926.65927.4714#46.22630.49938.52732.90320.40823.37329.14825.51226.5765#30.96221.31425.77622.92516.13419.06523.69520.29421.8156#41.58728.26538.20129.32419.17921.88427.25423.57325.127Radial velocity1#1.2652.4431.6352.6682.3232.1011.8402.3152.7032#1.7893.7432.5664.0293.7693.3933.0474.0724.3373#1.2182.3171.7552.6342.2361.9361.6571.9902.3034#1.5862.9802.1893.2452.8582.5602.4063.0363.8075#1.8703.3052.4193.7304.1553.4863.1164.2674.6756#1.7832.9462.2783.2203.6843.1712.8083.5424.292Figure 6
Droplet velocity curve of different nozzles. (a) Axial velocity. (b) Radial velocity.
(a)(b)Figure6 shows the following:(1)
The experimental data for the droplet velocity of different nozzles show that the droplet field of the nozzle shows different trends in the axial and radial directions: for the axial velocity, the axial velocity of the droplet gradually decreases with the increase in axial distance of the nozzle, and the fluctuation of the axial velocity of the droplet gradually decreases with the increase in radial distance; for the radial velocity, the axial distance increases. With the increase in radial distance, the radial velocity increases in varying degrees, but the radial velocity is smaller than the axial velocity of the droplets.(2)
The change rate of the droplet velocity in the axial direction of different nozzles is as follows: in the axial direction of A, C, and G, the average decrease rates of the axial and radial velocities of different nozzles are 36.15% and 17.63%, respectively. The axial and radial velocities have opposite change trends in both axial and radial directions. The velocity gradually decreases from the center to the surroundings, and the radial velocity gradually increases from the center to the surroundings. Taking E, F, G, H, and I with a distance of 3000 mm from the nozzle as an example, the average reduction rate of the axial velocity is 42.84%, and the average growth rate of the radial velocity is 32.37%. The above droplet velocity variation and droplet field macroscopic characteristic parameter show that the axial velocity plays a major role in the range of the fog field, and the radial velocity plays a key role in the coverage of the spray field.(3)
The change rates of the axial velocity and radial velocity show that the axial velocity plays a leading role in the effect of the radial velocity on the fog field, and the axial velocity of the droplet is basically maintained at 20∼30 m/s, which is conducive to capturing the respirable dust. The radial velocity plays a decisive role in the coverage of the spray field, while the droplet radial velocity basically remains at 1.5∼4.5 m/s. Intermediately, this velocity is the main factor that affects the size of the atomization angle.


## 3.1. Macroscopic Characteristics and Droplet Size Distribution

The investigation and study on the fully mechanized coal face in Jianzhuang Coal Mine showed that the nozzle pressure used at the site was 8 MPa. So, the experiment is carried out under 8 MPa. The results are shown in Table1 and Figure 5.(1)
According to the macroparameters of atomization characteristics of the nozzle, the atomizing angle of different nozzles is in the following order 5# > 2# > 6# > 4# > 1# > 3#. The range of different nozzles is 3# > 4# > 6# > 1# > 2# > 5#. Compared with the above macroscopic atomization parameter data, the range of atomization and the angle of atomization are two negative atomizing parameters. In addition, because of the difference in nozzle structure, the droplet kinetic energy is transformed into axial and radial kinetic energy at the nozzle, which results in the negative correlation difference between the atomization angle and the range.(2)
The average droplet size, volume probability, and cumulative volume distribution of droplets from different nozzles show that the droplet size distribution is consistent with the normal distribution under the spray pressure of 8 MPa and the droplet size dispersion of different nozzles is different. The standard deviation of the normal distribution of nozzle droplet size of 1–6# nozzle is 52.947, 35.186, 43.051, 58.542, 83.723, and 66.312 in order. The standard deviation of normal distribution of droplet size can measure the dispersion degree of droplet. The larger the standard deviation of the normal distribution, the more dispersed the droplet distribution. The smaller the standard deviation of the normal distribution, the more uniform the droplet distribution. The more uniform the droplet size distribution, the higher the dust reduction efficiency. It can be seen that the order of the dispersion degree of the droplet size distribution of the six nozzles is 5# > 6# > 4# > 1# > 3# > 2#.Table 1
Macroscopic characteristic parameters of different nozzles and mean value of the microdroplet size.
Nozzles numberMacroscopic characteristics parametersSpray droplet sizeAtomization angle (°)Effective range (m)D0.1/μmD0.5/μmD0.9/μmD32/μmD43/μm1#753.8727.4457.18117.3048.3268.232#883.6521.1348.3697.3139.4063.643#694.7330.0169.34133.2752.9480.594#734.3638.4887.46155.8167.9499.425#943.6018.9346.0686.2636.8154.886#804.1530.6171.60134.7563.4885.16Figure 5
Sketch of the droplet size, volume distribution, frequency distribution, and cumulative distribution of droplets for different nozzles.


## 3.2. Droplet Velocity Distribution

The velocity distribution of different measuring points is shown in Table2 and Figure 6.Table 2
Mean droplet velocity at different nozzles.
Velocity vectorNozzles numberDroplet velocity (m/s)ABCDEFGHIAxial velocity1#36.65225.99133.59427.29318.40619.92626.71622.88625.5752#32.77424.69328.83425.42616.39718.07123.35620.65922.0993#47.46832.69340.21535.05723.25625.65330.49926.65927.4714#46.22630.49938.52732.90320.40823.37329.14825.51226.5765#30.96221.31425.77622.92516.13419.06523.69520.29421.8156#41.58728.26538.20129.32419.17921.88427.25423.57325.127Radial velocity1#1.2652.4431.6352.6682.3232.1011.8402.3152.7032#1.7893.7432.5664.0293.7693.3933.0474.0724.3373#1.2182.3171.7552.6342.2361.9361.6571.9902.3034#1.5862.9802.1893.2452.8582.5602.4063.0363.8075#1.8703.3052.4193.7304.1553.4863.1164.2674.6756#1.7832.9462.2783.2203.6843.1712.8083.5424.292Figure 6
Droplet velocity curve of different nozzles. (a) Axial velocity. (b) Radial velocity.
(a)(b)Figure6 shows the following:(1)
The experimental data for the droplet velocity of different nozzles show that the droplet field of the nozzle shows different trends in the axial and radial directions: for the axial velocity, the axial velocity of the droplet gradually decreases with the increase in axial distance of the nozzle, and the fluctuation of the axial velocity of the droplet gradually decreases with the increase in radial distance; for the radial velocity, the axial distance increases. With the increase in radial distance, the radial velocity increases in varying degrees, but the radial velocity is smaller than the axial velocity of the droplets.(2)
The change rate of the droplet velocity in the axial direction of different nozzles is as follows: in the axial direction of A, C, and G, the average decrease rates of the axial and radial velocities of different nozzles are 36.15% and 17.63%, respectively. The axial and radial velocities have opposite change trends in both axial and radial directions. The velocity gradually decreases from the center to the surroundings, and the radial velocity gradually increases from the center to the surroundings. Taking E, F, G, H, and I with a distance of 3000 mm from the nozzle as an example, the average reduction rate of the axial velocity is 42.84%, and the average growth rate of the radial velocity is 32.37%. The above droplet velocity variation and droplet field macroscopic characteristic parameter show that the axial velocity plays a major role in the range of the fog field, and the radial velocity plays a key role in the coverage of the spray field.(3)
The change rates of the axial velocity and radial velocity show that the axial velocity plays a leading role in the effect of the radial velocity on the fog field, and the axial velocity of the droplet is basically maintained at 20∼30 m/s, which is conducive to capturing the respirable dust. The radial velocity plays a decisive role in the coverage of the spray field, while the droplet radial velocity basically remains at 1.5∼4.5 m/s. Intermediately, this velocity is the main factor that affects the size of the atomization angle.


## 4. Atomization Characteristics of Combined Nozzles

### 4.1. Nozzle Optimization for Shearer External Spray

The Winner 99 microparticle image analyzer was used to measure the dust particles produced during the upwind cutting in the area of shearer of No. 3 upper 901 fully mechanized mining face in Jianzhuang Coal Mine. The distribution characteristics of dust particles in the area of a fully mechanized mining face and the downwind side of the shearer were obtained. According to the characteristics of dust production in the shearer area of the fully mechanized mining face, five dust sampling points were set up: front drum of the shearer, driver of the shearer, back drum of the shearer, downwind side of the shearer at 10 m, and downwind side of the shearer at 20 m; the number of measuring points is 1, 2, 3, 4, and 5, respectively. When the flow field in the working space is relatively stable, the AKFC-92A mine dust sampler is placed at the measuring points for sampling. The height of the measuring point is the height of the respiratory zone. The dust is sampled and dried. Each measurement point was measured three times; the average value was measured. The volume frequency and dust concentration of the dust particle size distribution at different locations and distribution zones in the production area of the fully mechanized mining face are shown in Table3; the histogram of dust particle size distribution is shown in Figure 7.Table 3
Particle size distribution, volume frequency, and dust concentration at different positions of the fully mechanized coal mining face.
Measurement pointFrequency of dust particle size rangeDust concentration (mg/m3)0∼2.5μm2.5∼4μm4∼10μm10∼20μm≥20μmTotal dust concentrationRespirable dust concentration1#8.6319.2025.7828.9217.47674.2309.52#12.5426.0929.0121.1611.20627.0263.83#7.3013.9727.4634.1217.15826.1344.74#26.3331.6822.1316.263.60431.3205.25#29.5335.0919.5312.473.38385.9171.6Figure 7
Frequency distribution of the dust particle size in the fully mechanized coal mining face.The experimental results in Figure7 show that the particle size distribution of dust in different production areas of the fully mechanized mining face varies and that the particle size distribution is basically 0–20 μm. The particle size of dust in each dust-measuring area has the following order: rear drum of the shearer > front drum of the shearer > coal mining machine. The average particle size of dust was calculated from the data of different measuring points (11.26 μm, 8.85 μm, 13.03 μm, 3.19 μm, and 2.70 μm). Hence, the large particles of dust are settled by gravity in the dust migration process with the air flow, the smaller particles are further moved with the air flow, and the proportion of exhaled dust in the total dust gradually increases.According to the distribution law of dust particle size in different dust-producing areas of a fully mechanized mining face and according to previous studies, the best relationship between the droplet size and the dust particle size captured is that the droplet size is 8–10 times of the dust particle size [31, 32], which is conducive to dustfall. According to the droplet size and droplet velocity of different nozzles, three kinds of nozzles meeting the droplet size distribution requirements were selected from six kinds of nozzles. 5# nozzles are suitable for short distance, large atomizing angle, and smaller droplet size as the inner nozzle for shearer external spray. 3# nozzle is also suitable for long distance, small atomizing angle, large droplet size, and large droplet velocity as the outside spray nozzles for shearers. 2# nozzle is suitable for medium distance, large atomizing angle, droplet size, and droplet velocity as the center nozzle.


### 4.2. Distribution of Atomization Parameters of the Combined Nozzles

The atomization characteristics of the selected nozzles were obtained at different distances and spray angles (shown in Table4 and Figure 8). The angle of the nozzle spray changed from inside to outside. When the angle changed to 5 degrees, the angle of the nozzle was −5 degrees, 0 degrees, and 5 degrees from inside to outside, and the distance between the nozzle and the nozzle group is expressed in terms of d and α.Table 4
Mean values of the atomization characteristic parameters of spray group under different spacings and angle changes.
d(mm)α (°)Spray droplet sizeDroplet velocityD0.1/μmD0.5/μmD0.9/μmD32/μmD43/μmVx/m/sVy/m/s150037.0290.35161.2878.4397.1425.491.34536.9089.65158.7977.8694.2224.851.411034.2785.13149.64.73.2588.7323.561.461535.4687.02154.5175.3995.5123.131.55200035.9785.20154.1675.8187.6327.691.45533.2481.73143.7373.2585.8126.851.501031.1579.34137.5769.7382.0825.401.581534.5283.58145.3172.3689.4524.721.66250041.8597.21171.6085.7396.9129.161.53539.4693.47169.0483.3698.4827.721.591035.0186.90157.1379.6090.6526.341.631536.3391.52165.7982.0993.1425.511.69Figure 8
Changes in the droplet size at different spacings and angles of the nozzle groups. (a) Curve of the change in droplet size with nozzle group angle. (b) Curve of the change in droplet size with nozzle group distance.
(a)(b)Table4 and Figure 8 show the following:(1)
When the distance between the nozzles is fixed, the droplet size of the nozzle droplet field first decreases and subsequently increases with the increase in angleα. When α = 10 degrees, the droplet size is minimal. For example, when d = 150 mm, the droplet size parameters of the nozzle decreased from (37.02 μm, 90.35 μm, 161.28 μm, 78.34 μm, and 97.14 μm) at α = 0 degree to (36.90 μm, 89.65 μm, 158.79 μm, 77.86 μm, and 394.22 μm) at α = 10 degree; then, it increased to (35.46 μm, 87.02 μm, 154.51 μm, 75.39 μm, and 95.51 μm) at α = 15 degree. The results show that, with the increase in α, the overall distribution of droplet size is more uniform, but when α increases to a certain limit, the droplet is not easy to break up, and the distribution of droplet size is more dispersive, which is not conducive to dust capture.(2)
When the inclination of the nozzle group is constant, the droplet size of the nozzle droplet field first decreases and subsequently increases with the increase ind. When d = 200 mm, the droplet size is minimal. For example, when α = 10 degrees, the droplet size decreased from (34.27 μm, 85.13 μm, 149.64 μm, 73.25 μm, and 88.73 μm) at d = 150 mm to (31.15 μm, 79.34 μm, 137.57 μm, 69.73 μm, and 82.08 μm) at d = 200 mm; then, it increased to (35.01 μm, 86.90 μm, 157.13 μm, 79.60 μm, and 90.65 μm) at d = 250 mm. A larger distance between the nozzles corresponds to a weaker effect between the droplet fields. However, when the distance between the nozzles is short, the effect between the droplet fields is greater, which is not conducive to the breakup of droplets. When the distance increases to a better equilibrium value, a smaller droplet size is conducive to the breakup of the droplets.(3)
When the distance between the nozzles and the angle of the nozzles increase, the probability of droplet collision and fragmentation increases, the droplet size decreases as a whole, and the velocity distribution is uniform. However, when the droplet collision and fragmentation increase to a certain extent, the probability of reunion increases, and the droplet size distribution becomes more dispersed, which is not conducive to dust capture. The above data show that when the distance between the nozzles is 200 mm and the angle is 10 degrees, the droplet size and velocity distribution of the droplet field are the best, and the condition is more suitable to capture dust particles.


## 4.1. Nozzle Optimization for Shearer External Spray

The Winner 99 microparticle image analyzer was used to measure the dust particles produced during the upwind cutting in the area of shearer of No. 3 upper 901 fully mechanized mining face in Jianzhuang Coal Mine. The distribution characteristics of dust particles in the area of a fully mechanized mining face and the downwind side of the shearer were obtained. According to the characteristics of dust production in the shearer area of the fully mechanized mining face, five dust sampling points were set up: front drum of the shearer, driver of the shearer, back drum of the shearer, downwind side of the shearer at 10 m, and downwind side of the shearer at 20 m; the number of measuring points is 1, 2, 3, 4, and 5, respectively. When the flow field in the working space is relatively stable, the AKFC-92A mine dust sampler is placed at the measuring points for sampling. The height of the measuring point is the height of the respiratory zone. The dust is sampled and dried. Each measurement point was measured three times; the average value was measured. The volume frequency and dust concentration of the dust particle size distribution at different locations and distribution zones in the production area of the fully mechanized mining face are shown in Table3; the histogram of dust particle size distribution is shown in Figure 7.Table 3
Particle size distribution, volume frequency, and dust concentration at different positions of the fully mechanized coal mining face.
Measurement pointFrequency of dust particle size rangeDust concentration (mg/m3)0∼2.5μm2.5∼4μm4∼10μm10∼20μm≥20μmTotal dust concentrationRespirable dust concentration1#8.6319.2025.7828.9217.47674.2309.52#12.5426.0929.0121.1611.20627.0263.83#7.3013.9727.4634.1217.15826.1344.74#26.3331.6822.1316.263.60431.3205.25#29.5335.0919.5312.473.38385.9171.6Figure 7
Frequency distribution of the dust particle size in the fully mechanized coal mining face.The experimental results in Figure7 show that the particle size distribution of dust in different production areas of the fully mechanized mining face varies and that the particle size distribution is basically 0–20 μm. The particle size of dust in each dust-measuring area has the following order: rear drum of the shearer > front drum of the shearer > coal mining machine. The average particle size of dust was calculated from the data of different measuring points (11.26 μm, 8.85 μm, 13.03 μm, 3.19 μm, and 2.70 μm). Hence, the large particles of dust are settled by gravity in the dust migration process with the air flow, the smaller particles are further moved with the air flow, and the proportion of exhaled dust in the total dust gradually increases.According to the distribution law of dust particle size in different dust-producing areas of a fully mechanized mining face and according to previous studies, the best relationship between the droplet size and the dust particle size captured is that the droplet size is 8–10 times of the dust particle size [31, 32], which is conducive to dustfall. According to the droplet size and droplet velocity of different nozzles, three kinds of nozzles meeting the droplet size distribution requirements were selected from six kinds of nozzles. 5# nozzles are suitable for short distance, large atomizing angle, and smaller droplet size as the inner nozzle for shearer external spray. 3# nozzle is also suitable for long distance, small atomizing angle, large droplet size, and large droplet velocity as the outside spray nozzles for shearers. 2# nozzle is suitable for medium distance, large atomizing angle, droplet size, and droplet velocity as the center nozzle.


## 4.2. Distribution of Atomization Parameters of the Combined Nozzles

The atomization characteristics of the selected nozzles were obtained at different distances and spray angles (shown in Table4 and Figure 8). The angle of the nozzle spray changed from inside to outside. When the angle changed to 5 degrees, the angle of the nozzle was −5 degrees, 0 degrees, and 5 degrees from inside to outside, and the distance between the nozzle and the nozzle group is expressed in terms of d and α.Table 4
Mean values of the atomization characteristic parameters of spray group under different spacings and angle changes.
d(mm)α (°)Spray droplet sizeDroplet velocityD0.1/μmD0.5/μmD0.9/μmD32/μmD43/μmVx/m/sVy/m/s150037.0290.35161.2878.4397.1425.491.34536.9089.65158.7977.8694.2224.851.411034.2785.13149.64.73.2588.7323.561.461535.4687.02154.5175.3995.5123.131.55200035.9785.20154.1675.8187.6327.691.45533.2481.73143.7373.2585.8126.851.501031.1579.34137.5769.7382.0825.401.581534.5283.58145.3172.3689.4524.721.66250041.8597.21171.6085.7396.9129.161.53539.4693.47169.0483.3698.4827.721.591035.0186.90157.1379.6090.6526.341.631536.3391.52165.7982.0993.1425.511.69Figure 8
Changes in the droplet size at different spacings and angles of the nozzle groups. (a) Curve of the change in droplet size with nozzle group angle. (b) Curve of the change in droplet size with nozzle group distance.
(a)(b)Table4 and Figure 8 show the following:(1)
When the distance between the nozzles is fixed, the droplet size of the nozzle droplet field first decreases and subsequently increases with the increase in angleα. When α = 10 degrees, the droplet size is minimal. For example, when d = 150 mm, the droplet size parameters of the nozzle decreased from (37.02 μm, 90.35 μm, 161.28 μm, 78.34 μm, and 97.14 μm) at α = 0 degree to (36.90 μm, 89.65 μm, 158.79 μm, 77.86 μm, and 394.22 μm) at α = 10 degree; then, it increased to (35.46 μm, 87.02 μm, 154.51 μm, 75.39 μm, and 95.51 μm) at α = 15 degree. The results show that, with the increase in α, the overall distribution of droplet size is more uniform, but when α increases to a certain limit, the droplet is not easy to break up, and the distribution of droplet size is more dispersive, which is not conducive to dust capture.(2)
When the inclination of the nozzle group is constant, the droplet size of the nozzle droplet field first decreases and subsequently increases with the increase ind. When d = 200 mm, the droplet size is minimal. For example, when α = 10 degrees, the droplet size decreased from (34.27 μm, 85.13 μm, 149.64 μm, 73.25 μm, and 88.73 μm) at d = 150 mm to (31.15 μm, 79.34 μm, 137.57 μm, 69.73 μm, and 82.08 μm) at d = 200 mm; then, it increased to (35.01 μm, 86.90 μm, 157.13 μm, 79.60 μm, and 90.65 μm) at d = 250 mm. A larger distance between the nozzles corresponds to a weaker effect between the droplet fields. However, when the distance between the nozzles is short, the effect between the droplet fields is greater, which is not conducive to the breakup of droplets. When the distance increases to a better equilibrium value, a smaller droplet size is conducive to the breakup of the droplets.(3)
When the distance between the nozzles and the angle of the nozzles increase, the probability of droplet collision and fragmentation increases, the droplet size decreases as a whole, and the velocity distribution is uniform. However, when the droplet collision and fragmentation increase to a certain extent, the probability of reunion increases, and the droplet size distribution becomes more dispersed, which is not conducive to dust capture. The above data show that when the distance between the nozzles is 200 mm and the angle is 10 degrees, the droplet size and velocity distribution of the droplet field are the best, and the condition is more suitable to capture dust particles.


## 5. Field Application

The present study conducts field testing for the external spray atomization dust reduction application test at the shearer outside of the 905 fully mechanized mining face at Jianzhuang Coal Mining Co., Ltd., Shandong Energy ZaoZhuang Mining Group. The fully mechanized mining face has a working face height of 3200 mm; the working face wind speed is 1.05 m/s; it uses a G500/1130-WD AC traction shearer to cut coal; the shearer provides a spray pressure of 8 MPa; the working face of the original shearer spray and interrack spray and other dust-proof measures are available. In the field test, the experimental 2#, 3#, and 5# nozzle groups were selected as the external spraying group of the shearer. The spray unit was designed and developed, and the dust concentration was measured by the atomizing dust suppression effect of the nozzle group at 10 degrees of different nozzle spacings. The field application test chart of the shearer external spray device is shown in Figure9. Because the shearer external spray is mainly aiming at the dust production during coal cutting process, 1, 2, 3, 4, and 5 measuring points were located on the downwind side of the shearer’s front cylinder at 5 m, 10 m, 20 m, 30 m, and 40 m, respectively.Figure 9
Field application diagram of the shearer external spray unit.When the flow field in the working space is relatively stable, the AKFC-92A mine dust sampler is placed at the measuring points for sampling. The height of the measuring point is the height of the respiratory zone. The dust is sampled and dried. Each measurement point was measured three times; the average value was measured. The results are shown in Table5. Table 5 shows that when the traditional coal shearer is sprayed outside, only the coal and dust sources are cut off, and the average dust and dusting rates at different measuring points of the fully mechanized coal mining face are 62.48% and 62.21%, respectively. When the new type of shearer outside the spray unit is opened, the total dust and dust concentration of the 5 measuring points first decrease and subsequently increase with the increase in distance between the external spraying units of the shearer. The dust concentration at 5 m of the downwind side of the front drum of the shearer decreased from 227.5 mg/m3 and 115.4 mg/m3 at the distance d = 150 mm to 148.5 mg/m3 and 70.3 mg/m3 at the distance d = 200 mm; then, it increased to 179.2 mg/m3 and 84.9 mg/m3 at d = 250 mm. After calculation, the average values of total dust and dustfall changed from 70.17% to 69.72% at d = 150 mm; they were 85.82% and 82.96% when the new spray group was applied in the field, and the dustfall rate decreased to 76.03% and 76.87% when d = 250 mm was applied at d = 200 mm. Thus, when the angle of the external spray unit is 10 degrees, the dustfall effect is the best when the distance between the nozzles is 200 mm, and the total dust and dust removal rate of the traditional coal shearer atomizer group increase by 23.34% and 20.75%, respectively.Table 5
Average dust concentration at different measuring points at the coal dust source.
Measurement pointNondust-proof measuresOpen traditional sprayOpening a new type of external spray unit (distance between nozzles)/mm150200250Total dust concentration (mg/m3)Respirable dust concentration (mg/m3)Total dust concentration (mg/m3)Respirable dust concentration (mg/m3)Total dust concentration (mg/m3)Respirable dust concentration (mg/m3)Total dust concentration (mg/m3)Respirable dust concentration (mg/m3)Total dust concentration (mg/m3)Respirable dust concentration (mg/m3)1#774.0376.1305.2143.7227.5115.4148.570.3179.284.92#993.4482.8382.9186.4278.3136.7171.680.2194.597.33#435.6221.9223.6104.1140.767.195.240.7109.447.84#262.1118.7132.764.782.241.560.424.068.930.65#217.5103.494.346.564.830.247.322.152.025.7


## 6. Conclusion

(1)
According to the analysis of the macroatomization characteristic parameters of different nozzles, there is a negative correlation between the spray angle and the range of the spray nozzle when the spray pressure is constant. Moreover, the droplet size distribution conforms to the normal distribution. A more concentrated dispersion corresponds to a more uniform droplet size distribution, which can improve the efficiency of the dust suppression.(2)
The droplet velocity of different atomization parameters of different nozzles shows that axial velocity plays a major role in capturing dust and is the main factor that affects the range of the spray field. The radial velocity has little effect on dust capture, but the radial velocity determines the coverage of the spray field, and the radial velocity is the main factor that affects the atomization angle.(3)
The experiment of atomization parameters of the combined nozzles shows that, with the increase in distanced and angle a between the nozzles, the probability of droplets colliding and breaking up in the overlapping area between the nozzles increases. When the distance between the nozzles is 200 mm and the angle of nozzle group changes to 10 degrees, the distributions of droplet size and velocity are the best, and it is more suitable to capture dust particles. According to the field application test of the shearer external spraying unit on the fully mechanized face, when the interval between the outside shearers is 200 mm, the dust removal efficiency of the fully mechanized mining face is the best. The field application effect is basically identical to the experimental results, which has theoretical guiding significance for the optimization design of the external spray unit of a fully mechanized mining face.



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*Source: 9441355-2021-12-15.xml*