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Dust removal effect of negatively-pressured spraying collector for advancing support in fully mechanized coal mining face: Numerical simulation and engineering application
Tunnelling and Underground Space Technology 95 (2020) 103149
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Dust removal effect of negatively-pressured spraying collector for advancing support in fully mechanized coal mining face: Numerical simulation and engineering application Gang Zhoua,b,c,1, Qingtao Zhanga,b, Biao Suna,b,1
⁎,1
T
, Yingying Hud,2, Danhong Gaoa,b,1, Shicong Wanga,b,1,
a
College of Mining and Safety 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 c Energy Flagship, Commonwealth Scientific and Industrial Research Organisation, P.O. Box 883, Kenmore, Brisbane, QLD 4069, Australia d Department of Chemical Engineering and Safety, Bin Zhou University, Binzhou, China b
ARTICLE INFO
ABSTRACT
Keywords: Fully mechanized coal mining environment Dust from advancing support Negatively-pressured spraying collector Numerical simulation Engineering application
Combining experimental measurements and field tests, a negatively-pressured spraying collector was developed for the dust source of advancing support in fully mechanized coal mining environments based on the results obtained from the numerical simulation of wind-dust diffusion law under the action of dust collector using discrete particle model of Computational Fluid Dynamics (CFD) in order to reduce dust from advancing support. Simulation results showed that advancing support produced a dust band with high concentrations of up to 1,000 mg/m3 near the sidewalk around 0–9 m away from the wind leeward side of the advancing support which gradually diffused to 0–36 m. After the use of negatively-pressured spraying collector, the concentration of dust band at the leeward side of the advancing support was decreased to below 300 mg/m3 and diffusion extent shrank to 0–5 m from the leeward side of shearer; the form of dust mass near sidewalk also changed to low concentration dust band. After laboratory tests, ultrasonic atomizing nozzle was selected as the final nozzle and negatively-pressured spraying collector equipped with this nozzle was applied under the same water and air pressures of 0.4 MPa which gave effective range of 5.03 m and atomization angle of 71°. The developed negatively-pressured spraying collector was applied to 73down23 fully mechanized coal mining environment in Nantun mine and it was found that it effectively decreased dust concentration from advancing support in the operation zone near sidewalk. Average total and respiration dust-decreasing rates were up to 81.5% and 79.1%, respectively, which confirmed excellent dust reduction efficiency of the proposed device.
1. Introduction Coal dust seriously threatens the health of miners (Chen et al., 2018). In underground coal mining, maximum dust concentration can reach 2,500–3,000 mg/m3 (Yu et al., 2018) in the parallel operations of multi-working procedures such as coal cutting, advancing support, etc. (Yuan et al., 2018). In fully mechanized coal mining environments, dust production mainly comes from drum cutting and advancing support (Zhou et al., 2018).
Since 1950s, many researchers (Faeth et al., 1995; Grundnig et al., 2006; Mckenna et al., 2009) have systematically studied the law of dust diffusion with wind. In 1960, Hodkinson (Hodkinson, 1960) investigated the distribution of airflow velocity and dust concentration under the action of wind through experimental tests in wind tunnel and obtained the relationships between dust concentration field, airflow velocity field and dust concentration variation curve along wind direction. Courtney et al. (Courtney, 1986) studied the settling law of respirable dust in roadways and developed the equations of dust
Corresponding author at: College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, China. E-mail address: [email protected] (Q. Zhang). Address: College of Mining and Safety Engineering, Shandong University of Science and Technology, 579 Qianwangang Road, Economic & Technical Development Zone, Qingdao, Shandong Province, Postcode: 266590, China. 2 Address: Department of Chemical Engineering and Safety, Bin Zhou University, 391 Huanghe 5th Road, Bincheng Zone, Binzhou, Shandong Province, Postcode: 256600, China. ⁎
1
https://doi.org/10.1016/j.tust.2019.103149 Received 26 March 2019; Received in revised form 17 September 2019; Accepted 14 October 2019 0886-7798/ © 2019 Elsevier Ltd. All rights reserved.
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the velocity field and pressure distribution of air in working face (Zhang et al., 2018). 3D steady-state incompressible N-S equation was used as the governing equation of airflow in coal face and k 2D model was used for turbulence model (Patankar and Joseph, 2001). Currently, these models are the most widely used engineering turbulence models in which only momentum transfer is considered and heat transfer is neglected (Qin et al., 2014). The specific forms of the equations as follows: Conservation of mass / continuity equation:
concentration distribution along centrally symmetrical roadways. Based on aerodynamic theory, Ni Guanhua (Ni, 2018, 2019) studied the causes and the influencing factors of the coal seam WBE are analyzed, and the critical micelle concentration (CMC), surface tension, viscosity and contact angle changes of the surfactants on the surface of coal. Liu Ronghua (Liu et al., 2000) studied the experimental characteristics of dust dispersion under the action of wind flow using similar simulation experiments. Wang Hetang (Wang et al., 2019b, 2019c) carried out an experimental comparison of the spray performance of typical waterbased dust reduction media, and carried out an investigation on the atomization characteristics of a solid-cone spray for dust reduction at low and medium pressures. Sun Biao (Sun et al., 2018, 2019) investigated the influences of turbulent airflow disturbance from coal cutting on pollution characteristics of coal dust, the airflow migration and the coal dust dispersion was numerically simulated, and the Venturi negative-pressure secondary dedust device was developed. Wang Pengfei (Wang et al., 2019e) studied the effects of structural parameters on the atomization characteristics and dust suppression performance of an air-assisted internal mixing atomizer. Thus, it can be seen that the current research on dust prevention and control in fully mechanized mining lacks the research on diffusion law and prevention technology of advancing support dust. Today, the main measure to decrease advancing support dust in fully mechanized coal mining environments is inter frame spray (Wang et al., 2018a). Cai Peng (Cai et al., 2019) determined the effect of air flowrate on pollutant dispersion pattern of coal dust particles at the fully mechanized mining face. However, due to high instantaneous mass concentration of advancing support dust, traditional spray dust control measures are not efficient (Wang et al., 2019d). For this reason, in this paper, we have proposed a method by high efficiency atomization to prevent and control dust production by advancing support and developed a negatively-pressured spraying collector to be installed on the top beam of hydraulic support for dust removal. In this method, on the one hand, water curtain formed by fog was used to seal dust on the sidewalk of support and the cable trench of working face. On the other hand, negative pressure was generated by spraying to suck foul air into collector for deep purification which could not only block the diffusion of dust to sidewalk, but also dedust and clean the air. This method addressed the disadvantage of traditional spraying in dealing with advancing support dust. In order to determine the effect of negatively-pressured spraying collector and guide its further development and application, this paper used FLUENT software based on discrete phase model (DPM) (Zhang and Li, 2015) to simulate and analyze the law of air-dust diffusion under the action of negatively-pressured spraying collector in the support of fully mechanized coal mining environments. Then selected the proposed nozzle to research a negatively-pressured spraying collector suitable for shifting process of fully mechanized coal mining environment by optimization of the atomization performance of nozzle. Field applications showed that the developed collector effectively prevented dust from spreading into the sidewalk of the support and sealed and collected the dust produced by advancing support. Cutting coal and moving support are the main sources of dust production in fully mechanized mining faces. Previous studies have mostly focused on cutting coal dust production and moving support dust production has been somehow neglected. However, moving support dust production greatly pollutes sidewalk and can easily cause occupational hazards to human body. As a spray device between hydraulic supports, the innovation of negatively-pressured spraying collector developed in this paper is that it can combine spray dust-fall and vacuum suction to effectively control the dust generated by moving support.
xi
( ui ) = 0
(1)
N-S equations was expressed as:
xi
( ui uj ) =
( µ + µi )
xi
uj xi
+
p xi
ui xj
(2)
Standard k equation is the equation of turbulent kinetic energy, also known as k-equation, was written as: xi xi
(µ + ) (µ + ) + µi
( ui k ) = xi
xi
(
k xi
k xi
+ Gk C1 k
Gk
C2
2
k
(3)
k2
µi = Cµ uj
k
µi
( ui ) =
Gk = µi
xi
uj xi
+
ui xj
)
(4)
where Gk is turbulent kinetic energy variation rate due to shear force variation (kg/(m·s3), k is the turbulent kinetic energy of fluid (m2/s2), ε is turbulent dissipation rate (m2/s3), μ is laminar viscosity coefficient (Pa·s), μi is turbulence viscosity coefficient (Pa·s), p is effective pressure of turbulence in (Pa); ρ is air density (kg/m3), xi,j is coordinates along x, y and z directions (m), ui,j is velocity along x, y and z directions (m/s), and the values of Cε1, Cε2, Cμ, σε and σk were assumed to be 108, 1.44, 1.92, 0.09, 1.3 and 1.0, respectively. 2.2. Mathematical model of discrete phase In Fluent software, the trajectories of dust particles in discrete phase are solved by differential equations of particle forces in integral pull coordinate system (Ren et al., 2014). The force balance equation of discrete phase particles in Cartesian coordinates was as follows (Jinag et al., 2013): (taking X-direction as an example)
du p dt
= FD (u
FD =
Re =
uP ) +
gx (
)
p p
18µ CD Re 2 p d p 24
d |up
+ Fx
(5) (6)
u|
µ
a a CD = a1 + 2 + 32 Re Re
(7) (8)
where u is fluid phase velocity (m/s), up is particle velocity (m/s), dp is particle diameter (m), ρp is particle density (kg/m3), Re is relative Reynolds number, CD is drag coefficient, gx is gravitational acceleration along x direction (m/s2), FD(u-up) is unit mass drag force of dust particles (N), and Fx is other forces acting long x direction (Ν). In a certain range of Reynolds numbers, for spherical particles, α1, α2 and α3 are constant. Because the particle size of discrete phase was small and mass concentration was sparse, the particle was mainly affected by the drag force of fluid, followed by gravity. Other forces such as apparent mass force, Brownian force and Saffman lift were relatively small and were neglected.
2. Mathematical model of airflow-dust movement 2.1. Mathematical model of airflow The mathematical model of airflow was mainly used to determine 2
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3. Geometric model construction and numerical simulation parameter setting
arranged vertical to coal wall, the distance between adjacent nozzles was 0.5 m, and the nozzles were injected from roof to floor with the angle of 45°. Therefore, the air-water curtain formed by the nozzle injection generated a full-face closure between coal wall and hydraulic support in the coal mining space of fully mechanized coal face. On the pillars of hydraulic supports, three nozzles were arranged horizontal to coal wall, the distance between adjacent nozzles was 0.5 m, and the nozzles were injected from roof to coal wall with the angle of 45°. The air-water curtain formed by nozzle injection effectively captured and sealed small particle dusts which diffused from mining space to the front of support and sidewalk. The inlet and outlet of negatively-pressured spraying collector are shown in the enlarged part of Fig. 3. The design value of the air volume of collector was 3.5 m/s, average air velocities of inlet and outlet were 11.66 and 10.27 m/s, respectively.
3.1. Establishment of the geometrical model and mesh generation In 73down23 fully mechanized coal face in Nantun Coal Mine (located in Jining City, Shandong Province, China), air distribution capacity of working face was 2000 m3/min, the height and width of the intake airflow roadway were 3.5 and 4.2 m, respectively, cross-section area was 14.7 m2, and wind speed of intake airflow roadway was 2.27 m/s. The geometric model of numerical simulation was established by SolidWorks software and grids were divided by the integrated computer engineering and manufacturing code for computational fluid dynamics (ICEMCFD). The geometric model was composed of working space, shearer, hydraulic support and cable trough. Fully mechanized mining face was U-shaped and the sizes of intake and return airways were 10.0 m × 4.2 m × 3.5 m, with middle mining space of 90.0 m × 7.0 m × 3.5 m. Shearer mining height and cutting depth were 3.5 and 0.8 m, respectively. While cutting coal, coal cutter moved behind the advancing support and delayed the 3 operations of the rear drum of the shearer. Fig. 1 shows the diagram of equipment layout and air flow. The geometric model was divided into five parts, i.e. intake airway, advancing support area, coal-mining area, non-mining area and return airway. Mesh size (maximum size) was set at 0.8 for intake and returning airways, 0.3 for advancing support area, 0.25 for coal mining area, and 0.2 for non-mining area. The minimum size was the default value, which was generally assumed to be 1.55e-05 m to get unstructured mesh of geometric model, making numerical simulation results more accurate. Fig. 2 shows the distribution of finite element mesh after optimization where A is intake airway, B is coal mining area, C is return airway and D is shearer. Grid independence test was performed to eliminate the influence of mesh generation on the analysis results obtained from dimension control test of finite element model meshes. The density of mesh generation directly depended on the results of finite element analysis. Due to sparse meshing, the size of the whole model element was large and the number of elements was small resulting in inaccurate calculations. Mesh division was too dense, calculation accuracy was acceptable, but computer configuration requirements were very high, calculation time was too long, and it was not suitable for large-scale batch finite element analysis. Therefore, controlling the size and quantity of the grid was very important. Independence tests revealed that the simulation scheme described in Table 1 had the highest efficiency.
3.3. Numerical simulation parameters and boundary condition settings A steady state simulation with 1000 iterations was performed in this study with both reporting and profile update intervals of 1. The reflection of dust particles after contact with coal wall, that is, the particles rebound after meeting with coal wall and return to the fluid domain for migration. The partitioned grid files were input into FLUENT software and the boundary conditions of the developed geometric model and dust source parameters were set according to Table 2. 4. Analysis of numerical simulation results of airflow-dust migration law Analysis section was selected based on the height of working face and the direction of coal wall-hydraulic support. In this paper, the heights of 0.5, 1.5, and 3.0 m from the floor and the longitudinal section of pavement were taken to analyze the results obtained from wind-dust field simulation. The negatively-pressured spraying collector is not applied in Figs. 4–10(a) and applied in Figs. 4–10(b). 4.1. Numerical simulation of air flow law As shown in Fig. 4, when air entered from airflow roadway, velocity distribution as high in middle section and small on both sides. A small low speed area appeared on both sides of the corner when the air flow entered the advancing support zone from the corner. Because of the influence of hydraulic support, air flow was divided into two strands around both sides. The air flow on one side of the working face was larger which flew to the shearer and its speed was decreased because wind was blocked by the hydraulic support. The other flew along the side of the gob. In the non-mining area, the airflow in the working face mainly flew along hydraulic support-coal wall and that on the side of gob mainly came from wind leakage along support gap. After the application of dust collector, the velocity of airflow was increased and a wider high-speed wind belt appeared in the middle and rear of non-
3.2. Geometric modeling and mesh generation of negatively-pressured spraying collector The developed collector was applied on the top beam of hydraulic support as shown in Fig. 3. On the side of top beam, four nozzles were
Fig. 1. Geometric model of the fully-mechanized mining face. 3
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Fig. 2. Meshing model of the fully-mechanized mining face. Table 1 Geometric model meshing characteristics for optimum simulation conditions. Conditions
Number of grids
Number of faces
Number of nodes
Negatively-pressured spraying collector not used Negatively-pressured spraying collector used
469,220 482,248
1,026,426 1,059,069
121,586 127,152
Fig. 3. Layout of the developed negatively-pressured spraying collector. Table 2 Settings of boundary conditions and parameters of the dust source. Item
Name
Parameter
Item
Name
Parameter
Boundary condition
Inlet boundary type Inlet velocity /(m/s)
VELOCITY_INLET 2.27
Main parameters of dust source
Discrete phase model Turbulent diffusion ratio /(m2/s3) Outlet boundary type
ON 0.48 Outflow
Particle size distribution of dust Distribution index Mean diameter/(m) Max diameter /(m) Min diameter /(m) Dust output /(kg/s)
Rosin-Rammler 1.13 6.7e-06 28.5e-06 1.6e-06 0.012
mining area which promoted dust removal efficiency. As shown in Fig. 5, Whether or not a dust collector is used, the cross section of the sidewalk decreases and the wind speed increases under the influence of the shearer in the advancing support zone and coal mining zone; in the non-mining zone, the cross section of the sidewalk is larger than the advancing support zone, and the hindrance of the upright column and other equipment leads to the wind speed decreasing gradually. The difference is that after the application of dust collector, there were three wind masses with higher airflow velocities in the sidewalk area of 18 m, 40–45 m, 66 m on the leeward side of advancing support. This resulted in dust dilution and discharge from workface in the shortest time possible.
4.2. Numerical simulation of dust diffusion law In this paper, y = 0.5, 1.5, and 3 m sections as well as the longitudinal section of pavement along height direction were selected according to the airflow velocity distribution map to carry out more detailed dust concentration analyses. In Figs. 6–8, it was seen that dust concentration distribution was very complicated but had certain regularity. As shown in Fig. 6, before the application of dust collector, dust mainly deposited on the floor near the bottom of the shearer. In nonmining area, only a small amount of dust was distributed along the cable trough and coal wall. After the application of dust collector, the 4
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Fig. 4. Comparison of air velocity distribution in workface (y = 1.5 m).
Fig. 5. Comparison of airflow velocity distribution in pavement.
high concentrations of dust mass near the leeward side of shearer was almost disappeared, but low concentration dust of 150 mg/m3 was distributed in a certain area in the second half of non-mining area and return airway. In all, after the application of dust collector, dust concentration near the floor was decreased and the average concentration of dust was decreased from 316.5 to 96.0 mg/m3. As can be seen from Fig. 7, when collector was not applied, the main
area of dust accumulation at the height of respiratory zone was 0–16 m on the leeward side of advancing support which was diffused along coal wall, support and sidewalk to the side of gob, with concentrations of as high as 1000 mg/m3, which seriously threatened the health of workers. High concentration dust mass mainly gathered in front drum, driver and hydraulic support. Dust distribution was lower in the second half of non-mining area. After the application of dust collector, dust 5
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Fig. 6. Comparison of dust concentration distribution in workface (y = 0.5 m).
Fig. 7. Comparison of dust concentration distribution in workface (y = 1.5 m).
concentration was greatly decreased with maximum concentration of only 300 mg/m3 and diffusion range was reduced to 0–4.5 m on the leeward side of shearer. The dust form near the sidewalk was also changed from a large area of high concentration agglomeration to a low concentration band. In non-mining area, only low-concentration dust of no more than 150 mg/m3 was spread along coal wall. Under the action of negatively-pressured spraying collector, the average dust concentration of the whole working face was decreased from 198.83 to 61.74 mg/m3. From Fig. 8, it can be seen that dust near the roof was mainly concentrated at 0–22.5 m on the leeward side of advancing support. High concentrations of dust was mainly spread along support column and coal wall, which caused dust concentration on the top of sidewalk to increase to as high as 1500 mg/m3. After the application of dust collector, the range of high-concentration dust cluster near advancing support was decreased and only a small amount of low-concentration dust was settled near the floor such that the amount of dust 22.5 m
away on the leeward side was very small. As shown in Fig. 9, dust particles generated by advancing support were diffused with wind, with relatively smooth trajectories. Under pressure, a large number of droplets were sprayed from nozzles and negative pressure entrainment effect was created. Due to the presence dust removal cover which formed a positive pressure field in ejection cover accelerating the diffusion of droplets and increasing dust removal efficiency ultimately resulting the formation of a negative pressure field in throat tube. Therefore it can effectively inhale the dust generated by moving support from negative pressure suction port. In another word, after using dust collector, the running speeds of dust particles were obviously increased and trajectory variation became larger; consequently, the settling speed of larger particles was decreased. As shown in Fig. 10, the dust generated by advancing support moved along the wind direction and gradually settled under the action of wind current with continuous increase of speed. Under the shearer and space between shearer and cable trough, a large number of low6
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Fig. 8. Comparison of dust concentration distribution in workface (y = 3.0 m).
Fig. 9. Comparison of dust concentration particle distribution in workface.
speed dust particles were accumulated. In non-mining area, most of dust particles settled on the floor and the remaining particles were spread in the whole roadway section. After the application of dust collector, the number of dust particles in coal mining area was obviously decreased and low-speed dust particles which were collected near the shearer were disappeared. Only limited dust particles with higher speeds migrated with the wind.
better controlled dust production by advancing support. Therefore, in this paper, an efficient nebulizer and negative-pressure entrainment dust collector capable of effectively dealing with high concentration dust mass has been developed to control dust production by advancing support. As a spray device between hydraulic supports, negatively-pressured spraying collector was capable of combining spray dust-fall and vacuum suction to effectively control the dust generated by advancing support. Therefore, the efficiency of the method highly relied on its atomization angle, effective range and droplet size.
5. Development and performance measurement of negativelypressured spraying collector
5.1. Technical principle of negatively-pressured spraying collector
Through numerical simulations, it was found that dust concentration in coal mining area, near the roof of non-mining area and the respiratory zone were higher due to the advancing support in fully-mechanized coal mining environments, but the developed negative-pressure collector
As shown in Fig. 11, negatively-pressured spraying collector produced a high speed air-water jet from nozzle and a negative pressure in the tapered section to inhale dust-laden air from suction nozzle. The 7
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Fig. 10. Comparison of dust velocity particle distribution in workface.
inlet on the back part of the collector was used to suck dirty air flow in the sidewalk area of advancing support to purify. 5.2. Comparison of the atomization performance of nozzle Based on the experimental platform of Doppler laser interference spray and dust reduction (as shown in Fig. 12), the nozzle of negatively-pressured spraying collector was compared through atomization characteristic parameter experiments. To fully investigate the structural characteristics and atomization efficiencies of different types of nozzles (Dehkordi et al., 2017), two types of nozzles were selected as alternatives for experiments. The types and diameters of the selected nozzles are shown in Fig. 13. Taking into account the requirements of negatively-pressured spraying collector regarding particle size distribution and range of nozzle to fully investigate the structural characteristics and atomization efficiencies of different types of geomantic nozzles (Wang et al., 2018b), ultrasonic atomizing nozzle and air blast atomizer were selected for the experiments. Because droplet size can easily be controlled in ultrasonic atomizing nozzles, it can produce smaller droplets which result in dust reduction. Air blast atomizer is a two-phase flow nozzle (Prostanski and Dariusz, 2013) and has the advantages of stable atomization quality and non-blocking characteristic.
Fig. 11. Schematic diagram of the principle and structure of negatively-pressured spraying collector.
developed negatively-pressured spraying collector was installed on the top beam of hydraulic support and was arranged along the length and width directions of advancing support. Dust was collected by spraying (Zhou et al., 2014; Wang et al., 2019a) and negative pressure suction
Fig. 12. Laser Doppler spray dust suppression experimental device. 8
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Fig. 13. Practicality and structural parameters of alternative nozzles.
Fig. 14. Effective range contrast of nozzles.
Ultrasonic atomizing nozzle is an electronically controlled nozzle. Its greatest advantage is that it can obtain excellent atomization quality at very low liquid transfer rates and generate small and uniform droplets. Air blast atomizer breaks up liquids using air kinetic energy and transforms them into droplets. The main characteristic of this type of nozzle is that it can mix air and water to produce small droplets with relatively low pressures. Experimental equipment was consisted of a three-phase coupling test platform (Zhou et al., 2017b) of air flow-droplet-dust independently developed by Shandong University of Science and Technology. Total platform length was 15.5 m. The main components of the platform were PDI-200MD laser phase doppler interferometer (Manufacturer: Artium Technologies Inc., Droplet size range capability:
0.5 μm to 2000 μm or greater, Measures in droplet number densities: 100,000/ml) and ASA signal analysis system, Topas SAG-410 dust aerosol generator (Manufacturer: Topas Gmbh, Dresden, Germany, Frequency: 67–63 Hz), transparent simulation roadway, frequency conversion ventilation device, and Aero Trak APC 9303 (Manufacturer: TSI Incorporated, Size range: 0.3–25 μm, Flow rate: 2.83 L/min). Main parameters were as follows: droplet size measurement range of 0.3–7000 μm, total droplet volume measurement range of 100–106/ cm3, velocity measurement range of −150 ~ 300 m/s, dust concentration detection range of 0–10000 mg/m3, and spray pressure of 0.1–10 MPa. The equipment could perform different types of experimental analyses under coupling conditions of spray pressure, air supply pressure, airflow velocity and dust concentration. 9
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Fig. 15. Atomization angle contrast of nozzles.
was set at coordinate origin, the direction of injection was along x axis, and the direction perpendicular to x axis was considered as y axis. Nine measuring points were selected in the experiment as shown in Fig. 16. The coordinates of these seven points along with their names were A (1, 0), B (2, 0.8), C (2, 0), D (2, −0.8), E (3, 1.6), F (3, 0.8), G (3, 0), H (3, −0.8), and I (3, −1.6), and the atomization performance of the two nozzles were measured. When both water and air pressures were 0.4 MPa, the range and atomization angle of the nozzle were moderate and droplet size and velocity distribution were measured (Anubhav et al., 2015). Fig. 17 shows the contrast diagrams of particle size distribution at different measuring points of nozzles and Table 3 compares average particle sizes ¯ 50 , D ¯ 90 and D ¯ 100 represent the ¯ 10 , D produced by nozzles where D characteristic diameters of droplets; that is, droplets with lower diameters accounted for 10, 50, 90 and 100% of total droplets. The change trend of droplet size distribution was similar for two nozzles. The droplet sizes of nozzle No. 1 were smaller than those of nozzle No. 2. For example, the average median diameter of nozzle No. 1 was 41.1 μm while corresponding value for nozzle No. 2 was 48.4 μm. Therefore, the atomization efficiency of nozzle No. 1 was higher in which could result in better dust suppression. The radial and axial velocities of the two nozzles are shown in Fig. 18. The radial average velocities of nozzles No. 1 and No. 2 were 1.781 and 1.611 m/s, and their axial average velocities were 24.721 and 18.605 m/s, respectively.
Fig. 16. Location of spray points.
5.2.1. Measuring macroscopic atomization characteristics of the nozzle Macroscopic atomization parameters of nozzles are important indicators of the atomization efficiency of nozzles (Zhou et al., 2017a). In this paper, effective range and atomization angle of nozzles No. 1 and No. 2 under air pressures of 0.3–0.6 MPa and water pressures of 0.3–0.6 MPa have been measured, as show in Fig. 14 and Fig. 15, respectively.
5.3. Performance test of negatively-pressured spraying collector
5.2.2. Measuring microscopic atomization characteristics of the nozzle Particle size and velocity of sprayed droplets of wind-water two phase nozzle were measured at micro level by Doppler laser interferometer (Ren et al., 2011). The nozzle was installed in the experimental device of mine roadway, as shown in Fig. 12. This device was consisted of four stages, namely air inlet stage, diffusion stage, experimental stage and contraction stage. The experimental device was cuboid with dimensions 6.0 m × 3.0 m × 3.0 m. The position of nozzle
For the acquisition of macro-parameters of the nozzles, the measurement of the effective ranges of nozzles was relatively simple such that once the atomization field of nozzles was stabilized, it could be measured directly by a tape ruler. Atomization angle was obtained by photographic method and then the negativity of the recorded images were processed by Photoshop software to obtain more accurate atomization angles.
10
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Fig. 17. Comparison of droplet size distribution produced by nozzles at different measuring points.
Fig. 18. Comparison of axial and radial velocities of nozzles at different measuring points.
The experimental results showed that the macro and micro atomization parameters of nozzle No. 1 were better than those of nozzle No. 2. Therefore, nozzle No. 1 was considered to be more effective in preventing dust from spreading to sidewalk along length and width directions of hydraulic support. When both water and air pressures were 0.4 MPa, the effective range and atomization angle of negatively-pressured sucker collector on nozzles were 5.03 m and 71°, respectively. The average wind speed of ejector hood outlet and average air suction volume were 12.53 m/s and 3.76 m3/min, respectively.
measured several times in different time periods. While doing this, shearer was not open and the only dust source was advancing support. Therefore, the collector effect of the dust collector for dust of advancing support is obtained. A total of 6 dust sampling points were selected with the specific distribution described below: #1, #2, #3, #4, #5, #6 sampling points located at 10, 15, 20, 25, 30 and 35 m behind advancing support, respectively. When collecting dust from advancing support in fully-mechanized mining face using filter membrane method, the collecting time of each dust sample was not less than 5 min. Dust filter membrane had to be packaged, numbered and recorded clearly at the end of each sample collection cycle. The results of engineering application showed that, after the application of collector in hydraulic support, the average dust fall rate of total dust and respiratory dust were 81.5 and 79.1%, respectively, as shown in Figs. 20 and 21. It was seen that negatively-pressured
6. Engineering applications The developed negatively-pressured spraying collector was applied to 73down23 fully-mechanized mining face on Nantun coal mine (Shandong province), as shown in Fig. 19. Dust concentrations before and after turning the collector on were
11
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Fig. 19. Engineering application of the dust collector.
Fig. 20. Comparison of total and respirable dust concentrations before and after using negatively-pressured suction collector.
spraying collector had played a positive inhibitory role in removal link and its dust reduction effect was remarkable.
(2) By using the developed spraying collector, high concentration dust cluster generated near the floor was transformed into a low concentration band, dust diffusion range in respiratory zone was decreased, dust was mostly dispersed in front of mining area and nonmining area was in low concentration state, and the range of high concentration dust cluster near the roof was decreased. (3) Ultrasonic atomizing nozzle with diameter of 2.0 mm had large atomizing angle, long range and good droplet effect under water and air pressures of 0.4 MPa, therefore, it was considered as suitable for negatively-pressured spraying collector. After the use of the developed collector in fully mechanized mining face, dust concentration in the wind side and pavement area on the leeward side of the advancing support was obviously reduced. The total dust and exhaled dust fall rates were 81.5 and 79.1%, respectively, and dust removal efficiency was excellent.
7. Conclusion According to the numerical simulation results and atomization experiment of nozzle, a negatively-pressured spraying collector for the dust produced in advancing support was developed and applied in field. The following conclusions were drawn: (1) After the use of negatively-pressured spraying collector, wind speed in the non-mining area of fully mechanized mining face was increased, especially at 20, 30, 40 and 75 m distances on the leeward side of advancing support, where there are large air masses or wind belts, which makes the dust expedite the dilution and discharge, reduce its concentration and narrow its diffusion range.
12
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Fig. 21. Comparison of simulated and measured dust concentrations. Table 3 Comparison of mean droplet size of nozzle. Average particle size (μm)
No. 1 nozzle
No. 2 nozzle
¯ 10 D ¯ 50 D ¯ 90 D ¯ 100 D
17.4 41.1 66.4 76.7
21.0 48.4 77.3 91.6
airway. United States Bureau of Mines. 19041. Dehkordi, P.B., Colombo, L.P.M., Guilizzoni, M., Sotagia, G., 2017. CFD simulation with experimental validation of oil-water core-annular flows through Venturi and Nozzle flow meters. J. Petrol. Sci. Eng. 149, 540–552. Faeth, M.G., Hsiang, L.P., Wu, P.K., 1995. Structure and breakup properties of sprays. Int. J. Multiph. Flow 21, 99–127. Grundnig, Peter Werner, Wilhelm, Höflinger, Gerd, Mauschitz, Zechang, Liu, Guiqin, Zhang, Zhiqiang, Wang, 2006. Influence of air humidity on the suppression of fugitive dust by using a water-spraying system. China Particuology 4 (5), 229–233. Hodkinson, J.R., 1960. The relation between ventilation airspeed and respirable airbornedust concentration in coal mines. Colliery Eng. 37, 236–239. Jiang, Zhong’an, Lan, Jiang, Jushi, Chen, 2013. Numerical simulation of dust concentration distribution regularities of down-the-hole drilling in open-pit mine. J. Shenzhen Univ. Sci. Eng. 30 (3), 313–318. Liu, Ronghua, Haiqiao, Wang, Shiliang, Shi, Heqing, Liu, 2000. The simulating test study on using air curtain to frevent excavating driver's area from dust. J. Hunan Univ. Sci. Technol. 15 (3), 17–21. Mckenna, N., Cheryl, B., Sanderson, S., 2009. Wind tunnel simulation of environmental controls on fugitive dust emissions from mine tailings. Atmos. Environ. 43 (3), 520–529. Ni, Guanhua, Zhao, Li, Hongchao, Xie, 2018. The mechanism and relief method of the coal seam water blocking effect (WBE) based on the surfactants. Powder Technol. 323, 60–68. Ni, Guanhua, Hongchao, Xie, Shang, Li, Qian, Sun, Dongmei, Huang, Yanying, Cheng, Ning, Wang, 2019. The effect of anionic surfactant (SDS) on pore-fracture evolution of acidified coal and its significance for coalbed methane extraction. Adv. Powder Technol. 30, 940–951. Patankar, N.A., Joseph, D.D., 2001. Modeling and numerical simulation of particulate flows by the Eulerian – Lagrangian approach. Int. J. Multiph. Flow 27 (10), 1659–1684. Pengfei, Wang, Yongjun, Li, Ronghua, Liu, Yijie, Shi, 2019d. Effect of forced-to-exhaust ratio of air volume on dust control of wall-attached swirling ventilation for mechanized excavation face. Tunn. Undergr. Space Technol. 90, 194–207. Prostanski, Dariusz, 2013. Uing of air-water spraying systems for improving dust control in mines. J. Sustainable Min. 12 (2), 29–34. Qin, Yueping, Zhenjun, Jiang, Miaomiao, Zhang, 2014. Comparison of simulation and measurement on dust migration regularity in fully mechanized workface. J. Liaoning Tech. Univ. 33 (3), 289–293. Ren, Tingxiang, Brian, Plush, Naj, Aziz, 2011. Dust controls and monitoring practices on australian longwalls. Procedia Eng. 26, 1417–1429. Ren, Tingxiang, Zhongwei, Wang, Graeme, Cooper, 2014. CFD modelling of ventilation and dust flow behaviour above an underground bin and the design of an innovative dust mitigation system. Tunn. Undergr. Space Technol. 41, 241–254. Sun, Biao, Cheng, Weimin, Wang, Jiayuan, Wang, Hao, 2018. Effects of turbulent airflow from coal cutting on pollution characteristics of coal dust in fully-mechanized mining face: A case study. J. Cleaner Prod. 201, 308–324. Sun, Biao, Cheng, Weimin, Wang, Jiayuan, Wang, Hao, Ma, Youying, 2019. Development of Venturi negative-pressure secondary dedust device and application of local spray closure technique. Adv. Powder Technol. 30 (1), 42–54. Wang, Hetang, Du, Yunhe, Wei, Xiaobin, He, Xinxin, 2019b. An experimental comparison of the spray performance of typical water-based dust reduction media. Powder Technol. 345, 580–588.
Declaration of Competing Interest The authors declare that there are no conflicts of interest regarding the publication of this paper. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant no. 51774198, 51474139, 51904032, 51904171), the National Key Research and Development Program of China (Grant no. 2017YFC0805202), the Outstanding Youth Fund Project of Provincial Universities in Shandong Province (Grant no. ZR2017JL026), the Qingdao City Science and Technology Project (Grant no. 16-6-2-52-nsh), the Qingdao Postdoctoral Applied Research Project (Grant no. 2015194). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2019.103149. References Anubhav, Sinha R., Surya, Prakash, Mohan, A.M., Ravikrishna, R.V., 2015. Airblast spray in crossflow–Structure, trajectory and droplet sizing. Int. J. Multiph. Flow 72, 97–111. Cai, Peng, Wen, Nie, Dawei, Chen, Zhiqiang, Yang Shibo Liu, 2019. Effect of air flowrate on pollutant dispersion pattern of coal dust particles at fully mechanized mining face based on numerical simulation. Fuel 239 (2019), 623–635. Chen, Lianjun, Guoming, Liu, Weimin, Cheng, Zhaoxia, Liu, 2018. Development of cement dust suppression technology during shotcrete in mine of China-A review. J. Loss Prev. Process Ind. 55, 232–242. Courtney, W.G., Cheng, L., Divers, E.F., 1986. Deposition of respirable coal dust in an
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G. Zhou, et al. Wang, Jinhua, Kang, Hongpu, Liu, Jianzhong, Chen, Peipei, Fan, Zhizhong, Yuan, Weiming, Liu, Yupeng, 2018a. Layout strategic research of green coal resource development in China. J. China Univ. Mining Technol. 47 (1), 15–20. Wang, Hao, Nie, Wen, Cheng, Weimin, Liu, Qiang, Hu, Jin, 2018b. Effects of air volume ratio parameters on air curtain dust suppression in a rock tunnel’s fully-mechanized working face. Adv. Powder Technol. 29, 230–244. Wang, Pengfei, Shi, Yijie, Zhang, Lianyang, Li, Yongjun, 2019e. Effect of structural parameters on atomization characteristics and dust reduction performance of internal-mixing air-assisted atomizer nozzle. Process Saf. Environ. Prot. 128, 316–328. Wang, Hetang, Wu, Jinlong, Du, Yunhe, Wang, Deming, 2019c. Investigation on the atomization characteristics of a solid-cone spray for dust reduction at low and medium pressures. Adv. Powder Technol. 30 (5), 903–910. Wang, Jiayuan, Zhou, Gang, Wei, Xing, Wang, Shicong, 2019a. Experimental characterization of multi-nozzle atomization interference for dust reduction between hydraulic supports at a fully mechanized coal mining face. Environ. Sci. Pollut. Res. 26 (10), 10023–10036. Yu, Haiming, Cheng, Weimin, Peng, Huitian, Xie, Yao, 2018. An investigation of the nozzle’s atomization dust suppression rules in a fully-mechanized excavation face based on the airflow-droplet-dust three-phase coupling model. Adv. Powder Technol. 29, 941–956. Yuan, Liang, Zhang, Nong, Kan, Jiaguang, Wang, Yang, 2018. The concept, model and reserve forecast of green coal resources in China. J. China Univ. Mining & Technol. 47
(1), 1–8. Zhang, Tao, Li, Hongwen, 2015. Simulation optimization of DPM on gas-solid two-phase flow in complex pipeline flow field. J. Tianjin Univ. (Sci. Technol.) 48 (1), 39–48. Zhang, Qi, Zhou, Gang, Qian, Xinming, Yuan, Mengqi, Sun, Yanlong, Wang, Dan, 2018. Diffuse pollution characteristics of respirable dust in fully-mechanized mining face under various velocities based on CFD investigation. J. Cleaner Prod. 184, 239–250. Zhou, Gang, Nie, Wen, Cheng, Weimin, Wang, Hao, 2014. Influence regulations analysis of high-pressure atomization dust-settling to dust particle’s microscopic parameters in fully mechanized caving coal face. J. China Coal Soc. 39 (10), 2053–2059. Zhou, Gang, Zhang, Qi, Bai, Ruonan, Fan, Tao, Wang, Gang, 2017b. The diffusion behavior law of respirable dust at fully mechanized caving face in coal mine: CFD numerical simulation and engineering application. Process Saf. Environ. Prot. 106, 117–128. Zhou, Gang, Xu, Mao, Qiu, Han, Nie, Wen, Cheng, Weimin, Chen, Cong, 2017a. Experimental investigation about the influence of airflow on droplet sizes of mechanical nozzles for coal mining face. Tehnički Vjesnik-Tehnical Gazette 24 (6), 1713–1721. Zhou, Gang, Feng, Bo, Yin, Wenjing, Wang, Jiayuan, 2018. Numerical simulations on airflow-dust diffusion rules with the use of coal cutter dust removal fans and related engineering applications in a fully-mechanized coal mining face. Powder Technol. 339, 354–367.
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Dust removal effect of negatively-pressured spraying collector for advancing support in fully mechanized coal mining face: Numerical simulation and engineering application Gang Zhoua,b,c,1, Qingtao Zhanga,b, Biao Suna,b,1
⁎,1
T
, Yingying Hud,2, Danhong Gaoa,b,1, Shicong Wanga,b,1,
a
College of Mining and Safety 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 c Energy Flagship, Commonwealth Scientific and Industrial Research Organisation, P.O. Box 883, Kenmore, Brisbane, QLD 4069, Australia d Department of Chemical Engineering and Safety, Bin Zhou University, Binzhou, China b
ARTICLE INFO
ABSTRACT
Keywords: Fully mechanized coal mining environment Dust from advancing support Negatively-pressured spraying collector Numerical simulation Engineering application
Combining experimental measurements and field tests, a negatively-pressured spraying collector was developed for the dust source of advancing support in fully mechanized coal mining environments based on the results obtained from the numerical simulation of wind-dust diffusion law under the action of dust collector using discrete particle model of Computational Fluid Dynamics (CFD) in order to reduce dust from advancing support. Simulation results showed that advancing support produced a dust band with high concentrations of up to 1,000 mg/m3 near the sidewalk around 0–9 m away from the wind leeward side of the advancing support which gradually diffused to 0–36 m. After the use of negatively-pressured spraying collector, the concentration of dust band at the leeward side of the advancing support was decreased to below 300 mg/m3 and diffusion extent shrank to 0–5 m from the leeward side of shearer; the form of dust mass near sidewalk also changed to low concentration dust band. After laboratory tests, ultrasonic atomizing nozzle was selected as the final nozzle and negatively-pressured spraying collector equipped with this nozzle was applied under the same water and air pressures of 0.4 MPa which gave effective range of 5.03 m and atomization angle of 71°. The developed negatively-pressured spraying collector was applied to 73down23 fully mechanized coal mining environment in Nantun mine and it was found that it effectively decreased dust concentration from advancing support in the operation zone near sidewalk. Average total and respiration dust-decreasing rates were up to 81.5% and 79.1%, respectively, which confirmed excellent dust reduction efficiency of the proposed device.
1. Introduction Coal dust seriously threatens the health of miners (Chen et al., 2018). In underground coal mining, maximum dust concentration can reach 2,500–3,000 mg/m3 (Yu et al., 2018) in the parallel operations of multi-working procedures such as coal cutting, advancing support, etc. (Yuan et al., 2018). In fully mechanized coal mining environments, dust production mainly comes from drum cutting and advancing support (Zhou et al., 2018).
Since 1950s, many researchers (Faeth et al., 1995; Grundnig et al., 2006; Mckenna et al., 2009) have systematically studied the law of dust diffusion with wind. In 1960, Hodkinson (Hodkinson, 1960) investigated the distribution of airflow velocity and dust concentration under the action of wind through experimental tests in wind tunnel and obtained the relationships between dust concentration field, airflow velocity field and dust concentration variation curve along wind direction. Courtney et al. (Courtney, 1986) studied the settling law of respirable dust in roadways and developed the equations of dust
Corresponding author at: College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, China. E-mail address: [email protected] (Q. Zhang). Address: College of Mining and Safety Engineering, Shandong University of Science and Technology, 579 Qianwangang Road, Economic & Technical Development Zone, Qingdao, Shandong Province, Postcode: 266590, China. 2 Address: Department of Chemical Engineering and Safety, Bin Zhou University, 391 Huanghe 5th Road, Bincheng Zone, Binzhou, Shandong Province, Postcode: 256600, China. ⁎
1
https://doi.org/10.1016/j.tust.2019.103149 Received 26 March 2019; Received in revised form 17 September 2019; Accepted 14 October 2019 0886-7798/ © 2019 Elsevier Ltd. All rights reserved.
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the velocity field and pressure distribution of air in working face (Zhang et al., 2018). 3D steady-state incompressible N-S equation was used as the governing equation of airflow in coal face and k 2D model was used for turbulence model (Patankar and Joseph, 2001). Currently, these models are the most widely used engineering turbulence models in which only momentum transfer is considered and heat transfer is neglected (Qin et al., 2014). The specific forms of the equations as follows: Conservation of mass / continuity equation:
concentration distribution along centrally symmetrical roadways. Based on aerodynamic theory, Ni Guanhua (Ni, 2018, 2019) studied the causes and the influencing factors of the coal seam WBE are analyzed, and the critical micelle concentration (CMC), surface tension, viscosity and contact angle changes of the surfactants on the surface of coal. Liu Ronghua (Liu et al., 2000) studied the experimental characteristics of dust dispersion under the action of wind flow using similar simulation experiments. Wang Hetang (Wang et al., 2019b, 2019c) carried out an experimental comparison of the spray performance of typical waterbased dust reduction media, and carried out an investigation on the atomization characteristics of a solid-cone spray for dust reduction at low and medium pressures. Sun Biao (Sun et al., 2018, 2019) investigated the influences of turbulent airflow disturbance from coal cutting on pollution characteristics of coal dust, the airflow migration and the coal dust dispersion was numerically simulated, and the Venturi negative-pressure secondary dedust device was developed. Wang Pengfei (Wang et al., 2019e) studied the effects of structural parameters on the atomization characteristics and dust suppression performance of an air-assisted internal mixing atomizer. Thus, it can be seen that the current research on dust prevention and control in fully mechanized mining lacks the research on diffusion law and prevention technology of advancing support dust. Today, the main measure to decrease advancing support dust in fully mechanized coal mining environments is inter frame spray (Wang et al., 2018a). Cai Peng (Cai et al., 2019) determined the effect of air flowrate on pollutant dispersion pattern of coal dust particles at the fully mechanized mining face. However, due to high instantaneous mass concentration of advancing support dust, traditional spray dust control measures are not efficient (Wang et al., 2019d). For this reason, in this paper, we have proposed a method by high efficiency atomization to prevent and control dust production by advancing support and developed a negatively-pressured spraying collector to be installed on the top beam of hydraulic support for dust removal. In this method, on the one hand, water curtain formed by fog was used to seal dust on the sidewalk of support and the cable trench of working face. On the other hand, negative pressure was generated by spraying to suck foul air into collector for deep purification which could not only block the diffusion of dust to sidewalk, but also dedust and clean the air. This method addressed the disadvantage of traditional spraying in dealing with advancing support dust. In order to determine the effect of negatively-pressured spraying collector and guide its further development and application, this paper used FLUENT software based on discrete phase model (DPM) (Zhang and Li, 2015) to simulate and analyze the law of air-dust diffusion under the action of negatively-pressured spraying collector in the support of fully mechanized coal mining environments. Then selected the proposed nozzle to research a negatively-pressured spraying collector suitable for shifting process of fully mechanized coal mining environment by optimization of the atomization performance of nozzle. Field applications showed that the developed collector effectively prevented dust from spreading into the sidewalk of the support and sealed and collected the dust produced by advancing support. Cutting coal and moving support are the main sources of dust production in fully mechanized mining faces. Previous studies have mostly focused on cutting coal dust production and moving support dust production has been somehow neglected. However, moving support dust production greatly pollutes sidewalk and can easily cause occupational hazards to human body. As a spray device between hydraulic supports, the innovation of negatively-pressured spraying collector developed in this paper is that it can combine spray dust-fall and vacuum suction to effectively control the dust generated by moving support.
xi
( ui ) = 0
(1)
N-S equations was expressed as:
xi
( ui uj ) =
( µ + µi )
xi
uj xi
+
p xi
ui xj
(2)
Standard k equation is the equation of turbulent kinetic energy, also known as k-equation, was written as: xi xi
(µ + ) (µ + ) + µi
( ui k ) = xi
xi
(
k xi
k xi
+ Gk C1 k
Gk
C2
2
k
(3)
k2
µi = Cµ uj
k
µi
( ui ) =
Gk = µi
xi
uj xi
+
ui xj
)
(4)
where Gk is turbulent kinetic energy variation rate due to shear force variation (kg/(m·s3), k is the turbulent kinetic energy of fluid (m2/s2), ε is turbulent dissipation rate (m2/s3), μ is laminar viscosity coefficient (Pa·s), μi is turbulence viscosity coefficient (Pa·s), p is effective pressure of turbulence in (Pa); ρ is air density (kg/m3), xi,j is coordinates along x, y and z directions (m), ui,j is velocity along x, y and z directions (m/s), and the values of Cε1, Cε2, Cμ, σε and σk were assumed to be 108, 1.44, 1.92, 0.09, 1.3 and 1.0, respectively. 2.2. Mathematical model of discrete phase In Fluent software, the trajectories of dust particles in discrete phase are solved by differential equations of particle forces in integral pull coordinate system (Ren et al., 2014). The force balance equation of discrete phase particles in Cartesian coordinates was as follows (Jinag et al., 2013): (taking X-direction as an example)
du p dt
= FD (u
FD =
Re =
uP ) +
gx (
)
p p
18µ CD Re 2 p d p 24
d |up
+ Fx
(5) (6)
u|
µ
a a CD = a1 + 2 + 32 Re Re
(7) (8)
where u is fluid phase velocity (m/s), up is particle velocity (m/s), dp is particle diameter (m), ρp is particle density (kg/m3), Re is relative Reynolds number, CD is drag coefficient, gx is gravitational acceleration along x direction (m/s2), FD(u-up) is unit mass drag force of dust particles (N), and Fx is other forces acting long x direction (Ν). In a certain range of Reynolds numbers, for spherical particles, α1, α2 and α3 are constant. Because the particle size of discrete phase was small and mass concentration was sparse, the particle was mainly affected by the drag force of fluid, followed by gravity. Other forces such as apparent mass force, Brownian force and Saffman lift were relatively small and were neglected.
2. Mathematical model of airflow-dust movement 2.1. Mathematical model of airflow The mathematical model of airflow was mainly used to determine 2
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3. Geometric model construction and numerical simulation parameter setting
arranged vertical to coal wall, the distance between adjacent nozzles was 0.5 m, and the nozzles were injected from roof to floor with the angle of 45°. Therefore, the air-water curtain formed by the nozzle injection generated a full-face closure between coal wall and hydraulic support in the coal mining space of fully mechanized coal face. On the pillars of hydraulic supports, three nozzles were arranged horizontal to coal wall, the distance between adjacent nozzles was 0.5 m, and the nozzles were injected from roof to coal wall with the angle of 45°. The air-water curtain formed by nozzle injection effectively captured and sealed small particle dusts which diffused from mining space to the front of support and sidewalk. The inlet and outlet of negatively-pressured spraying collector are shown in the enlarged part of Fig. 3. The design value of the air volume of collector was 3.5 m/s, average air velocities of inlet and outlet were 11.66 and 10.27 m/s, respectively.
3.1. Establishment of the geometrical model and mesh generation In 73down23 fully mechanized coal face in Nantun Coal Mine (located in Jining City, Shandong Province, China), air distribution capacity of working face was 2000 m3/min, the height and width of the intake airflow roadway were 3.5 and 4.2 m, respectively, cross-section area was 14.7 m2, and wind speed of intake airflow roadway was 2.27 m/s. The geometric model of numerical simulation was established by SolidWorks software and grids were divided by the integrated computer engineering and manufacturing code for computational fluid dynamics (ICEMCFD). The geometric model was composed of working space, shearer, hydraulic support and cable trough. Fully mechanized mining face was U-shaped and the sizes of intake and return airways were 10.0 m × 4.2 m × 3.5 m, with middle mining space of 90.0 m × 7.0 m × 3.5 m. Shearer mining height and cutting depth were 3.5 and 0.8 m, respectively. While cutting coal, coal cutter moved behind the advancing support and delayed the 3 operations of the rear drum of the shearer. Fig. 1 shows the diagram of equipment layout and air flow. The geometric model was divided into five parts, i.e. intake airway, advancing support area, coal-mining area, non-mining area and return airway. Mesh size (maximum size) was set at 0.8 for intake and returning airways, 0.3 for advancing support area, 0.25 for coal mining area, and 0.2 for non-mining area. The minimum size was the default value, which was generally assumed to be 1.55e-05 m to get unstructured mesh of geometric model, making numerical simulation results more accurate. Fig. 2 shows the distribution of finite element mesh after optimization where A is intake airway, B is coal mining area, C is return airway and D is shearer. Grid independence test was performed to eliminate the influence of mesh generation on the analysis results obtained from dimension control test of finite element model meshes. The density of mesh generation directly depended on the results of finite element analysis. Due to sparse meshing, the size of the whole model element was large and the number of elements was small resulting in inaccurate calculations. Mesh division was too dense, calculation accuracy was acceptable, but computer configuration requirements were very high, calculation time was too long, and it was not suitable for large-scale batch finite element analysis. Therefore, controlling the size and quantity of the grid was very important. Independence tests revealed that the simulation scheme described in Table 1 had the highest efficiency.
3.3. Numerical simulation parameters and boundary condition settings A steady state simulation with 1000 iterations was performed in this study with both reporting and profile update intervals of 1. The reflection of dust particles after contact with coal wall, that is, the particles rebound after meeting with coal wall and return to the fluid domain for migration. The partitioned grid files were input into FLUENT software and the boundary conditions of the developed geometric model and dust source parameters were set according to Table 2. 4. Analysis of numerical simulation results of airflow-dust migration law Analysis section was selected based on the height of working face and the direction of coal wall-hydraulic support. In this paper, the heights of 0.5, 1.5, and 3.0 m from the floor and the longitudinal section of pavement were taken to analyze the results obtained from wind-dust field simulation. The negatively-pressured spraying collector is not applied in Figs. 4–10(a) and applied in Figs. 4–10(b). 4.1. Numerical simulation of air flow law As shown in Fig. 4, when air entered from airflow roadway, velocity distribution as high in middle section and small on both sides. A small low speed area appeared on both sides of the corner when the air flow entered the advancing support zone from the corner. Because of the influence of hydraulic support, air flow was divided into two strands around both sides. The air flow on one side of the working face was larger which flew to the shearer and its speed was decreased because wind was blocked by the hydraulic support. The other flew along the side of the gob. In the non-mining area, the airflow in the working face mainly flew along hydraulic support-coal wall and that on the side of gob mainly came from wind leakage along support gap. After the application of dust collector, the velocity of airflow was increased and a wider high-speed wind belt appeared in the middle and rear of non-
3.2. Geometric modeling and mesh generation of negatively-pressured spraying collector The developed collector was applied on the top beam of hydraulic support as shown in Fig. 3. On the side of top beam, four nozzles were
Fig. 1. Geometric model of the fully-mechanized mining face. 3
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Fig. 2. Meshing model of the fully-mechanized mining face. Table 1 Geometric model meshing characteristics for optimum simulation conditions. Conditions
Number of grids
Number of faces
Number of nodes
Negatively-pressured spraying collector not used Negatively-pressured spraying collector used
469,220 482,248
1,026,426 1,059,069
121,586 127,152
Fig. 3. Layout of the developed negatively-pressured spraying collector. Table 2 Settings of boundary conditions and parameters of the dust source. Item
Name
Parameter
Item
Name
Parameter
Boundary condition
Inlet boundary type Inlet velocity /(m/s)
VELOCITY_INLET 2.27
Main parameters of dust source
Discrete phase model Turbulent diffusion ratio /(m2/s3) Outlet boundary type
ON 0.48 Outflow
Particle size distribution of dust Distribution index Mean diameter/(m) Max diameter /(m) Min diameter /(m) Dust output /(kg/s)
Rosin-Rammler 1.13 6.7e-06 28.5e-06 1.6e-06 0.012
mining area which promoted dust removal efficiency. As shown in Fig. 5, Whether or not a dust collector is used, the cross section of the sidewalk decreases and the wind speed increases under the influence of the shearer in the advancing support zone and coal mining zone; in the non-mining zone, the cross section of the sidewalk is larger than the advancing support zone, and the hindrance of the upright column and other equipment leads to the wind speed decreasing gradually. The difference is that after the application of dust collector, there were three wind masses with higher airflow velocities in the sidewalk area of 18 m, 40–45 m, 66 m on the leeward side of advancing support. This resulted in dust dilution and discharge from workface in the shortest time possible.
4.2. Numerical simulation of dust diffusion law In this paper, y = 0.5, 1.5, and 3 m sections as well as the longitudinal section of pavement along height direction were selected according to the airflow velocity distribution map to carry out more detailed dust concentration analyses. In Figs. 6–8, it was seen that dust concentration distribution was very complicated but had certain regularity. As shown in Fig. 6, before the application of dust collector, dust mainly deposited on the floor near the bottom of the shearer. In nonmining area, only a small amount of dust was distributed along the cable trough and coal wall. After the application of dust collector, the 4
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Fig. 4. Comparison of air velocity distribution in workface (y = 1.5 m).
Fig. 5. Comparison of airflow velocity distribution in pavement.
high concentrations of dust mass near the leeward side of shearer was almost disappeared, but low concentration dust of 150 mg/m3 was distributed in a certain area in the second half of non-mining area and return airway. In all, after the application of dust collector, dust concentration near the floor was decreased and the average concentration of dust was decreased from 316.5 to 96.0 mg/m3. As can be seen from Fig. 7, when collector was not applied, the main
area of dust accumulation at the height of respiratory zone was 0–16 m on the leeward side of advancing support which was diffused along coal wall, support and sidewalk to the side of gob, with concentrations of as high as 1000 mg/m3, which seriously threatened the health of workers. High concentration dust mass mainly gathered in front drum, driver and hydraulic support. Dust distribution was lower in the second half of non-mining area. After the application of dust collector, dust 5
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Fig. 6. Comparison of dust concentration distribution in workface (y = 0.5 m).
Fig. 7. Comparison of dust concentration distribution in workface (y = 1.5 m).
concentration was greatly decreased with maximum concentration of only 300 mg/m3 and diffusion range was reduced to 0–4.5 m on the leeward side of shearer. The dust form near the sidewalk was also changed from a large area of high concentration agglomeration to a low concentration band. In non-mining area, only low-concentration dust of no more than 150 mg/m3 was spread along coal wall. Under the action of negatively-pressured spraying collector, the average dust concentration of the whole working face was decreased from 198.83 to 61.74 mg/m3. From Fig. 8, it can be seen that dust near the roof was mainly concentrated at 0–22.5 m on the leeward side of advancing support. High concentrations of dust was mainly spread along support column and coal wall, which caused dust concentration on the top of sidewalk to increase to as high as 1500 mg/m3. After the application of dust collector, the range of high-concentration dust cluster near advancing support was decreased and only a small amount of low-concentration dust was settled near the floor such that the amount of dust 22.5 m
away on the leeward side was very small. As shown in Fig. 9, dust particles generated by advancing support were diffused with wind, with relatively smooth trajectories. Under pressure, a large number of droplets were sprayed from nozzles and negative pressure entrainment effect was created. Due to the presence dust removal cover which formed a positive pressure field in ejection cover accelerating the diffusion of droplets and increasing dust removal efficiency ultimately resulting the formation of a negative pressure field in throat tube. Therefore it can effectively inhale the dust generated by moving support from negative pressure suction port. In another word, after using dust collector, the running speeds of dust particles were obviously increased and trajectory variation became larger; consequently, the settling speed of larger particles was decreased. As shown in Fig. 10, the dust generated by advancing support moved along the wind direction and gradually settled under the action of wind current with continuous increase of speed. Under the shearer and space between shearer and cable trough, a large number of low6
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Fig. 8. Comparison of dust concentration distribution in workface (y = 3.0 m).
Fig. 9. Comparison of dust concentration particle distribution in workface.
speed dust particles were accumulated. In non-mining area, most of dust particles settled on the floor and the remaining particles were spread in the whole roadway section. After the application of dust collector, the number of dust particles in coal mining area was obviously decreased and low-speed dust particles which were collected near the shearer were disappeared. Only limited dust particles with higher speeds migrated with the wind.
better controlled dust production by advancing support. Therefore, in this paper, an efficient nebulizer and negative-pressure entrainment dust collector capable of effectively dealing with high concentration dust mass has been developed to control dust production by advancing support. As a spray device between hydraulic supports, negatively-pressured spraying collector was capable of combining spray dust-fall and vacuum suction to effectively control the dust generated by advancing support. Therefore, the efficiency of the method highly relied on its atomization angle, effective range and droplet size.
5. Development and performance measurement of negativelypressured spraying collector
5.1. Technical principle of negatively-pressured spraying collector
Through numerical simulations, it was found that dust concentration in coal mining area, near the roof of non-mining area and the respiratory zone were higher due to the advancing support in fully-mechanized coal mining environments, but the developed negative-pressure collector
As shown in Fig. 11, negatively-pressured spraying collector produced a high speed air-water jet from nozzle and a negative pressure in the tapered section to inhale dust-laden air from suction nozzle. The 7
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Fig. 10. Comparison of dust velocity particle distribution in workface.
inlet on the back part of the collector was used to suck dirty air flow in the sidewalk area of advancing support to purify. 5.2. Comparison of the atomization performance of nozzle Based on the experimental platform of Doppler laser interference spray and dust reduction (as shown in Fig. 12), the nozzle of negatively-pressured spraying collector was compared through atomization characteristic parameter experiments. To fully investigate the structural characteristics and atomization efficiencies of different types of nozzles (Dehkordi et al., 2017), two types of nozzles were selected as alternatives for experiments. The types and diameters of the selected nozzles are shown in Fig. 13. Taking into account the requirements of negatively-pressured spraying collector regarding particle size distribution and range of nozzle to fully investigate the structural characteristics and atomization efficiencies of different types of geomantic nozzles (Wang et al., 2018b), ultrasonic atomizing nozzle and air blast atomizer were selected for the experiments. Because droplet size can easily be controlled in ultrasonic atomizing nozzles, it can produce smaller droplets which result in dust reduction. Air blast atomizer is a two-phase flow nozzle (Prostanski and Dariusz, 2013) and has the advantages of stable atomization quality and non-blocking characteristic.
Fig. 11. Schematic diagram of the principle and structure of negatively-pressured spraying collector.
developed negatively-pressured spraying collector was installed on the top beam of hydraulic support and was arranged along the length and width directions of advancing support. Dust was collected by spraying (Zhou et al., 2014; Wang et al., 2019a) and negative pressure suction
Fig. 12. Laser Doppler spray dust suppression experimental device. 8
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Fig. 13. Practicality and structural parameters of alternative nozzles.
Fig. 14. Effective range contrast of nozzles.
Ultrasonic atomizing nozzle is an electronically controlled nozzle. Its greatest advantage is that it can obtain excellent atomization quality at very low liquid transfer rates and generate small and uniform droplets. Air blast atomizer breaks up liquids using air kinetic energy and transforms them into droplets. The main characteristic of this type of nozzle is that it can mix air and water to produce small droplets with relatively low pressures. Experimental equipment was consisted of a three-phase coupling test platform (Zhou et al., 2017b) of air flow-droplet-dust independently developed by Shandong University of Science and Technology. Total platform length was 15.5 m. The main components of the platform were PDI-200MD laser phase doppler interferometer (Manufacturer: Artium Technologies Inc., Droplet size range capability:
0.5 μm to 2000 μm or greater, Measures in droplet number densities: 100,000/ml) and ASA signal analysis system, Topas SAG-410 dust aerosol generator (Manufacturer: Topas Gmbh, Dresden, Germany, Frequency: 67–63 Hz), transparent simulation roadway, frequency conversion ventilation device, and Aero Trak APC 9303 (Manufacturer: TSI Incorporated, Size range: 0.3–25 μm, Flow rate: 2.83 L/min). Main parameters were as follows: droplet size measurement range of 0.3–7000 μm, total droplet volume measurement range of 100–106/ cm3, velocity measurement range of −150 ~ 300 m/s, dust concentration detection range of 0–10000 mg/m3, and spray pressure of 0.1–10 MPa. The equipment could perform different types of experimental analyses under coupling conditions of spray pressure, air supply pressure, airflow velocity and dust concentration. 9
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Fig. 15. Atomization angle contrast of nozzles.
was set at coordinate origin, the direction of injection was along x axis, and the direction perpendicular to x axis was considered as y axis. Nine measuring points were selected in the experiment as shown in Fig. 16. The coordinates of these seven points along with their names were A (1, 0), B (2, 0.8), C (2, 0), D (2, −0.8), E (3, 1.6), F (3, 0.8), G (3, 0), H (3, −0.8), and I (3, −1.6), and the atomization performance of the two nozzles were measured. When both water and air pressures were 0.4 MPa, the range and atomization angle of the nozzle were moderate and droplet size and velocity distribution were measured (Anubhav et al., 2015). Fig. 17 shows the contrast diagrams of particle size distribution at different measuring points of nozzles and Table 3 compares average particle sizes ¯ 50 , D ¯ 90 and D ¯ 100 represent the ¯ 10 , D produced by nozzles where D characteristic diameters of droplets; that is, droplets with lower diameters accounted for 10, 50, 90 and 100% of total droplets. The change trend of droplet size distribution was similar for two nozzles. The droplet sizes of nozzle No. 1 were smaller than those of nozzle No. 2. For example, the average median diameter of nozzle No. 1 was 41.1 μm while corresponding value for nozzle No. 2 was 48.4 μm. Therefore, the atomization efficiency of nozzle No. 1 was higher in which could result in better dust suppression. The radial and axial velocities of the two nozzles are shown in Fig. 18. The radial average velocities of nozzles No. 1 and No. 2 were 1.781 and 1.611 m/s, and their axial average velocities were 24.721 and 18.605 m/s, respectively.
Fig. 16. Location of spray points.
5.2.1. Measuring macroscopic atomization characteristics of the nozzle Macroscopic atomization parameters of nozzles are important indicators of the atomization efficiency of nozzles (Zhou et al., 2017a). In this paper, effective range and atomization angle of nozzles No. 1 and No. 2 under air pressures of 0.3–0.6 MPa and water pressures of 0.3–0.6 MPa have been measured, as show in Fig. 14 and Fig. 15, respectively.
5.3. Performance test of negatively-pressured spraying collector
5.2.2. Measuring microscopic atomization characteristics of the nozzle Particle size and velocity of sprayed droplets of wind-water two phase nozzle were measured at micro level by Doppler laser interferometer (Ren et al., 2011). The nozzle was installed in the experimental device of mine roadway, as shown in Fig. 12. This device was consisted of four stages, namely air inlet stage, diffusion stage, experimental stage and contraction stage. The experimental device was cuboid with dimensions 6.0 m × 3.0 m × 3.0 m. The position of nozzle
For the acquisition of macro-parameters of the nozzles, the measurement of the effective ranges of nozzles was relatively simple such that once the atomization field of nozzles was stabilized, it could be measured directly by a tape ruler. Atomization angle was obtained by photographic method and then the negativity of the recorded images were processed by Photoshop software to obtain more accurate atomization angles.
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Fig. 17. Comparison of droplet size distribution produced by nozzles at different measuring points.
Fig. 18. Comparison of axial and radial velocities of nozzles at different measuring points.
The experimental results showed that the macro and micro atomization parameters of nozzle No. 1 were better than those of nozzle No. 2. Therefore, nozzle No. 1 was considered to be more effective in preventing dust from spreading to sidewalk along length and width directions of hydraulic support. When both water and air pressures were 0.4 MPa, the effective range and atomization angle of negatively-pressured sucker collector on nozzles were 5.03 m and 71°, respectively. The average wind speed of ejector hood outlet and average air suction volume were 12.53 m/s and 3.76 m3/min, respectively.
measured several times in different time periods. While doing this, shearer was not open and the only dust source was advancing support. Therefore, the collector effect of the dust collector for dust of advancing support is obtained. A total of 6 dust sampling points were selected with the specific distribution described below: #1, #2, #3, #4, #5, #6 sampling points located at 10, 15, 20, 25, 30 and 35 m behind advancing support, respectively. When collecting dust from advancing support in fully-mechanized mining face using filter membrane method, the collecting time of each dust sample was not less than 5 min. Dust filter membrane had to be packaged, numbered and recorded clearly at the end of each sample collection cycle. The results of engineering application showed that, after the application of collector in hydraulic support, the average dust fall rate of total dust and respiratory dust were 81.5 and 79.1%, respectively, as shown in Figs. 20 and 21. It was seen that negatively-pressured
6. Engineering applications The developed negatively-pressured spraying collector was applied to 73down23 fully-mechanized mining face on Nantun coal mine (Shandong province), as shown in Fig. 19. Dust concentrations before and after turning the collector on were
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Fig. 19. Engineering application of the dust collector.
Fig. 20. Comparison of total and respirable dust concentrations before and after using negatively-pressured suction collector.
spraying collector had played a positive inhibitory role in removal link and its dust reduction effect was remarkable.
(2) By using the developed spraying collector, high concentration dust cluster generated near the floor was transformed into a low concentration band, dust diffusion range in respiratory zone was decreased, dust was mostly dispersed in front of mining area and nonmining area was in low concentration state, and the range of high concentration dust cluster near the roof was decreased. (3) Ultrasonic atomizing nozzle with diameter of 2.0 mm had large atomizing angle, long range and good droplet effect under water and air pressures of 0.4 MPa, therefore, it was considered as suitable for negatively-pressured spraying collector. After the use of the developed collector in fully mechanized mining face, dust concentration in the wind side and pavement area on the leeward side of the advancing support was obviously reduced. The total dust and exhaled dust fall rates were 81.5 and 79.1%, respectively, and dust removal efficiency was excellent.
7. Conclusion According to the numerical simulation results and atomization experiment of nozzle, a negatively-pressured spraying collector for the dust produced in advancing support was developed and applied in field. The following conclusions were drawn: (1) After the use of negatively-pressured spraying collector, wind speed in the non-mining area of fully mechanized mining face was increased, especially at 20, 30, 40 and 75 m distances on the leeward side of advancing support, where there are large air masses or wind belts, which makes the dust expedite the dilution and discharge, reduce its concentration and narrow its diffusion range.
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Fig. 21. Comparison of simulated and measured dust concentrations. Table 3 Comparison of mean droplet size of nozzle. Average particle size (μm)
No. 1 nozzle
No. 2 nozzle
¯ 10 D ¯ 50 D ¯ 90 D ¯ 100 D
17.4 41.1 66.4 76.7
21.0 48.4 77.3 91.6
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Declaration of Competing Interest The authors declare that there are no conflicts of interest regarding the publication of this paper. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant no. 51774198, 51474139, 51904032, 51904171), the National Key Research and Development Program of China (Grant no. 2017YFC0805202), the Outstanding Youth Fund Project of Provincial Universities in Shandong Province (Grant no. ZR2017JL026), the Qingdao City Science and Technology Project (Grant no. 16-6-2-52-nsh), the Qingdao Postdoctoral Applied Research Project (Grant no. 2015194). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2019.103149. References Anubhav, Sinha R., Surya, Prakash, Mohan, A.M., Ravikrishna, R.V., 2015. Airblast spray in crossflow–Structure, trajectory and droplet sizing. Int. J. Multiph. Flow 72, 97–111. Cai, Peng, Wen, Nie, Dawei, Chen, Zhiqiang, Yang Shibo Liu, 2019. Effect of air flowrate on pollutant dispersion pattern of coal dust particles at fully mechanized mining face based on numerical simulation. Fuel 239 (2019), 623–635. Chen, Lianjun, Guoming, Liu, Weimin, Cheng, Zhaoxia, Liu, 2018. Development of cement dust suppression technology during shotcrete in mine of China-A review. J. Loss Prev. Process Ind. 55, 232–242. Courtney, W.G., Cheng, L., Divers, E.F., 1986. Deposition of respirable coal dust in an
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