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

College of Safety and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China State Key Laboratory of Mining Disaster Prevention and Control Co-Founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China Energy Heating Limited Company of Qingdao West Coast Utility Group, Qingdao 266400, China


Introduction
In recent years, the mining level of mechanization and automation of coal mines in China has been gradually improved [1][2][3], and the disasters caused by the production process of coal mines have become increasingly prominent [4][5][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][8][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][11][12]. When a shearer cuts coal, the instantaneous total dust concentration in some coal mines is higher than 4000-6000 mg/m 3 [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. e dust concentration instantaneously reaches 500 mg/m 3 and 200 mg/m 3 [15,16].
ose 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. erefore, 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][20][21][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. erefore, based on the self-built spray and dust simulation experiment system, 6 types of nozzles of different types were selected for experimental study. e 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. is work provides a reference for improving the efficiency of shearer external spray and dust suppression.

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. e 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. e simulation roadway is mainly composed of two parts. ey 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. e 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. e layout of the test system is shown in Figure 1.

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. e 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. e 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#.

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. e 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). e layout of the measuring points in the droplet field is shown in Figure 3; experimental process is shown in Figure 4.

Macroscopic Characteristics and Droplet Size
Distribution. e 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. e results are shown in Table 1 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 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.   e larger the standard deviation of the normal distribution, the more dispersed the droplet distribution.
e smaller the standard deviation of the normal distribution, the more uniform the droplet distribution. e 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

Droplet Velocity Distribution.
e velocity distribution of different measuring points is shown in Table 2 and Figure 6. Figure 6 shows the following: (1) e 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) e 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. e axial and radial velocities have opposite change trends in both axial and radial directions. e 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%. e 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) e 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. e 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.   Advances in Civil Engineering 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. e height of the measuring point is the height of the respiratory zone. e dust is sampled and dried. Each measurement point was measured three times; the average value was measured. e 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 Table 3; the histogram of dust particle size distribution is shown in Figure 7. e experimental results in Figure 7 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. e 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. e 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.

Nozzle Optimization for Shearer External
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.

Distribution of Atomization Parameters of the Combined Nozzles.
e atomization characteristics of the selected nozzles were obtained at different distances and spray angles (shown in Table 4 and Figure 8). e 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 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 angle α. When α � 10 degrees, the droplet size is minimal. 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 in d. 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. e 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.

Field Application
e 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. e 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. e 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. e field application test chart of the shearer external spray device is shown in Figure 9. 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.
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. e height of the measuring point is the height of the respiratory zone. e dust is sampled and dried. Each measurement point was measured three times; the average value was measured. e results are shown in Table 5. 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      Advances in Civil Engineering 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.

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) e 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. e 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) e experiment of atomization parameters of the combined nozzles shows that, with the increase in distance d 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. e 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.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding this work.