Air Curtain Dust Removal Device Design and Dust Prevention Strategy in Heading Face

Dust protection is a safety guarantee of heading face. 'e previous model of air curtain research was ideal, and the dust removal effect was rarely studied in the actual dust-producing face.'is paper presents a method of air curtain dust removal (ACDR) in the actual heading face. 'e author designed an air curtain dust removal device (ACDRD). 'e law of total dust concentration, respiratory dust concentration, and respiratory dust ratio is obtained. 'e minimum outlet airflow velocity is analyzed using the flat-plane injection theory. 'e effect of the exhaust fan placement on the dust removal effectiveness is examined. Research indicates the following: 'e airflow speed at the upper, left, and right sides of the ∩-shaped slot is 17.39m/s, 12.04m/s, and 13.66m/s, respectively. 'e minimum dust removal speed of the air curtain is 5.48m/s.'e total dust concentration is the highest in the spot of roadheader operator, and the concentration of respiratory dust decreases sharply within 20m.When the indentation air duct is 2.1m away from the base plate, the dust-proof effect is better. 'e results can provide theoretical bases and methods for air curtain analysis of the heading face.


Introduction
Coal mine dust is one of the main hidden dangers restricting safe production. In China, 90% of underground coal mines face the problem of dust concentrations exceeding the limit [1]. e distribution of mine dust has the characteristics of wide spread in space and continuity in time. Normal ventilation system has not yet been formed in fully mechanized heading face. Dust concentration reaches up to 6000 mg/s 3 without dust removal technologies [2]. After automatic high-pressure dust removal technology, the concentration of breathing dust at the driver's spot of the roadheader reaches 100 to 150 mg/m 3 [3], which is more than 30 times the prescribed standard [4]. High dust concentration makes the working environment worse, threatens the health of workers, accelerates the wear of machinery and equipment, makes the visibility of space very low, and affects normal production [5].
Coal dust removal technology is divided into nine categories: dust removal, coal seam prehumidification dust removal, foam dust removal, water curtain dust removal, ventilation and dust removal, suction purification and dust removal, bonding dust removal, air curtain dust removal, and personal protection [6]. Due to technical conditions and the limitation of underground space area, underground excavation often uses wet dust removal methods, but those methods have the following disadvantages: water flow is easy to block, equipment and personnel work is not conducive, and water supply and drainage costs are increasing.
Air curtain dust removal (ACDR) technology has emerged in recent years [7]. It has the characteristics of transparent injection airflow, good sight of the driver, nonobstruction of transport, stable use, and so on [8,9]. Yucheng Li suggested that the air curtain spray airflow velocity should be 10 to 20 m/s, through the indentationextraction ventilation system, and the spray width should be 6 to 20 mm [10]. Liu Qiang analyzed the influence of radial pressure, axial pressure, and exhaust volume of the dust removal fan on the dust diffusion and pollution range of the air curtain device [11][12][13]. Wen Nie analyzed the effects of multidirectional air curtain dust removal on the fully mechanized heading face [14][15][16]. Hao Wang studied the effects of airflow quantity ratio parameters on air curtain dust suppression [17,18]. e operation of the ACDRD is carried out at the same time as the roadway excavation, in order to ensure good dust-proof effect, and the convenience of field operation should be considered. Only by the reasonable design of the ACDC system can it play its required role in the actual production. e airflow velocity of the air curtain fan, the structure size of the dust collector, and the dust pressure are the decisive factors influencing the dust removal effect. Previous researchers used numerical simulations and laboratory-like materials to study the effects of the width and distance of the air curtains in the dust removal unit on the dust removal effectiveness. However, the dust removal effect at spot of actual dust production in the subsurface is less studied. Subsurface space is limited, with multiple operation processes. Numerical simulation and similar material simulation research methods need to simplify the model; some of the factors are also ignored during model simplification process.
In this paper, an ACDRD is designed, and the minimum outlet airflow velocity of the air curtain device is analyzed using the flat-plane injection theory. rough the experiment of the dust concentration of the experimental heading face, the law of total dust concentration, respiratory dust concentration, and respiratory dust ratio is obtained. e effect of the exhaust fan placement on the dust removal effectiveness is examined. is study enriches the air curtain injection mechanism in a fully mechanized heading face.

Methodology
2.1. Layout of ACDRD. An experimental heading face was arranged in a coal mine. e width of the roadway is 5.2 m, and the height is 3.8 m. As shown in Figure 1, an S200MJ roadheader is used to cut coal. e anchor machine is used to support mechanical excavation. e heading face is equipped with an SGW-40T scraper conveyor and a DSP-1080/1000 conveyor belt. e external spraying device of the roadheader switches on during excavation work.
As shown in Figures 2 and 3, an ACDRD is installed in front of the roadheader driver. e jet outlet of the ACDRD is 7 m away from the heading face. e forcing fan is installed at the end of the duct to absorb fresh air. e injection box is installed on a roadheader. e forcing fan is placed on a special trailer and moves forward as the heading face is pushed forward.
After installing the ACDRD, the roadway is equipped with an air duct and an exhaust fan (as shown in Figure 1). e airflow quantity of the exhaust fan is larger than that of the forcing fan. e distance from the extraction of the pipe to the heading face is 2.5 m. e pipe diameter is 600 mm. Considering factors such as equipment size, height of personnel, and height of air duct suspension, 2.1 m is used as the lower limit height of the air duct, and the upper limit height of the air duct suspension is 3.0 m.

Air Curtain Structure and Parameters.
ACDRD is the most important part of the dust removal system. It should occupy less space, with flexible and good operation, to avoid hindering the normal line of sight of the driver. ACDRD is divided into forcing module and exhaust module. e forcing module includes forcing fan, air curtain ejection device, and air curtain fan. As shown in Figures 4-6, the air curtain ejection device mainly uses a ▽-shaped injection box or a ∩-shaped narrow slot injection cavity to eject the airflow. At the same time, airflow velocity is adjusted by changing the slit width and the specific structure of the baffle. e exhaust module includes an exhaust fan and a dust removal fan. All components comply with relevant regulations to avoid safety hazards or deterioration of working conditions. According to the shape and layout position parameters of the S200MJ roadheader, the main design parameters of the ACDRD are shown in Table 1.

Test Site Layout and Dust Collection Device.
According to the actual situation of the experimental heading face, the measurement layout follows the "Sampling Specifications for the Monitoring of Hazardous Substances in the Air of Workplaces" [19], "Determination of Dust      Advances in Civil Engineering 3 Concentration and Dispersion" [20], "Determination of Dust in Workplace Air Part One: Total Dust Concentration" [21], and "Determination of Dust in Workplace Air Part Two: Respiratory Dust Concentration" [22]. e exact arrangement of measuring spots can be shown in Table 2. e AKFC-92A mine dust sampler (shown in Figure 7) is used for spot sampling tests. Part of the dust-laden air is extracted; all dust particles are captured on the filter membrane, and they gradually accumulate. After sampling, the total dust mass in the unit volume can be calculated from the increase in the weight of the membrane.
When measuring the concentration of inhalable dust, the separator captures larger particles onto a glass plate coated with silicone oil. Smaller particles can be captured by the cavity around the plate and retained on the membrane. After sampling, the mass of breathable dust per unit volume can be calculated from the increase in the weight of the membrane.

Minimum Dust Removal Airflow Velocity.
We choose a reasonable amount of airflow for the air curtain to reduce the impact on the working surface. e airflow of the ACDRD reaches the rock mass around the roadway and acts as a dust blocker. e airflow speed at the end of the air curtain is greater than the roadway. After being emitted from the air curtain, the airflow gradually decays. e airflow speed at the injection end must be ensured to remove the dust in the air curtain.
As shown in Figure 8, the overall structure of the injection is complex. Fluid characteristics are less affected by the external region near the injection outlet, which is the core area of the injection outlet. e initial segment (L 0 ) extends from the injection source to the end of the core area. Farther away from the initial area is the body part, where the injection fluid is fully mixed with the external area. ere is a shorter transition part between the initial part and the body parts. e transition part is negligible, so the injection is mainly divided into the initial part and the body part.
By experiments, Alberston obtained a general expression attenuation formula for the maximum axial average velocity of the injection [24]: where C 01 � 3.12-3.78. As different scholars use different reference coefficients, the experiment of air curtain injection slit was carried out, and the coefficient of semiexpansion thickness was obtained. e airflow speed on the upper and right sides is tested. e airflow speed on the left side is the same as that on the right side. e test data are displayed in Table 3.
We fit the test data according to the following equation [25]: e attenuation coefficient of the upper axial speed is 4.25, and the decay coefficient of the right axial velocity is 3.63. e axial mean speed attenuation coefficient on the right side is within the theoretical range. e attenuation coefficient of the above average axial speed is greater than the theoretical value. Considering the measurement position and measurement accuracy of airflow speed, it is acceptable to set a value greater than the theoretical value. e maximum time average speed decay formulas on the upper and both sides are, respectively, where u 0t , u 0l , u 0r represent the outflow speed on the upper side, left side, and right side, m/s; u mt , u ml , u mr represent the maximum axial average speed at the axial distance of the upper side, left side, and right side, that is, the lowest dustremoval airflow speed at the end of the airflow, which is 5.48 m/s; x t , x l , x r represent the distances of the upper side, left side, and right side wind outflow nozzles from the surrounding roadways, which are, respectively, 2.0, 0.7, and 0.9 m; and b s is the slit that can be adjusted to a half thickness of the maximum width, with a value of 0.011 m (the maximum width of the injection outlet can be 22 mm, which meets the maximum airflow demand). Separately calculated, u 0t � 17.39 m/s, u 0l � 12.04 m/s, and u 0r � 13.66 m/s. e upper, left, and right slots observe the effect of the ACDR at the lowest airflow speed of 2.04 m/ s. We use the following formula to obtain Q t , Q l , and Q r : where Q is the airflow at injection outlet, m 3  e actual airflow quantity of the outlet of the extraction network is 1224.4 m 3 /min, with some airflow being used for the heading face and the rest being provided for the ACDR. erefore, the actual airflow quantity of the heading face is 1075 m 3 /min. eoretically, the airflow quantity should be greater than 1075 m 3 /min, so that the outflow air will be deflected to the heading face. Figure 9 shows the effect of the air curtain. Plastic ribbon is tied at the outlet of the air curtain to facilitate observation of airflow direction and speed. After the ACDRD is installed in the experimental roadway, the distance between the outlet of the air curtain and the roadway walls on both sides is about 1.3 m. At this time, the airflow speed of the air curtain is about 3 m/s, which fully meets the requirements of the ACDRD. e airflow speed at a position 2.5 m away from the jet exit of the air curtain is measured. e airflow speed is generally around 2 m/s, indicating that this device can still form an effective air curtain in a wide roadway. 4 Advances in Civil Engineering

Dust Distribution in Roadway without ACDRD.
According to "Coal Mine Safety Regulations," the airflow speed has the following regulations: the airflow speed of coal roadways and semicoal roadways during excavation is 0.25 to 4 m/s. In this study, dust concentration values were obtained at a relatively constant speed of 1.15 m/s. e dust concentration test results are shown in Figure 10. e measurement spot is the X-axis; the dust concentration and the respiratory dust ratio are the double Yaxis.  3  20  25  35  40  50  60  65  70  80  90  95  100  110  120  Total dust

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Dust concentration at the front of the forcing fan is low and the dust forms a vortex on the other side of the fan, due to the blowing of fresh air. A narrow gap is formed between the roadway and the roadheader. A number of dust particles spread through the gap. As the air blows through narrow crevices, the airflow speed increases but the pressure decreases. e airflow creates a low-pressure zone, and then more dust particles are pressed into it. rough this narrow gap, dust particles move up and down from the middle of the roadway.
Within 40 m, large particles settle rapidly and the total dust concentration drops rapidly. Most large particles stabilize after 40 m. e fine dust is scattered in the roadway. e total dust concentration tends to stabilize. Within 20 m, the respiratory concentration drops sharply and is discharged under the effect of airflow. Within 20∼40 m, the airflow is very turbulent, and the exhaled dust is difficult to settle, so respiratory dust concentration decreases gradually. e respiratory dust spreads throughout the roadway, and the concentration tends to be stable after 40 m. Respiratory dust is suspended in the air; it decreases relatively slowly by the total dust concentration.
e change in inhalable dust concentration is related to the change in total dust concentration. Due to the effect of air turbulence, the total dust concentration in the driver's work area is reduced. e first inflection point occurs because the large particles settle faster. After the large particle deposit is completed, the smaller particles of breathing dust become the main sediment and a second inflection point occurs. After that, the respiratory dust spreads to the roadway. e large particle dust continues to precipitate, with a third inflection point. With the continuous settlement of large particulate dust, the total dust concentration continues to decline, but the concentration of respiratory dust remains unchanged, and the curve begins to rise gradually. e curve changes gradually with the deposition of large particles after 90 m. e trend of total dust concentration and respirable dust concentration is almost the same. At a distance of 120 m from the heading face, the dust concentration in the roadway still does not meet the standards set out in the "Coal Mine Safety Regulations." Certain dust removal techniques should be used to improve the working environment.

Dust Removal Effect of ACDRD.
e processing of the heading face has not changed, but the ACDRD is activated. e dust concentration distribution in the roadway was tested under the two conditions of 2.1 m and 3.0 m from the base plate. e results are shown in Figure 11. e dust concentration in the working face has decreased due to the action of the exhaust fan. e vortex becomes smaller and moves toward the exhaust fan. e exhaust fan takes away a large number of dust particles.
When the position of the fan outlet is 2.1 m and 3.0 m from the base plate, the dust concentration in the aisle is of the same order of magnitude. e dust-proof effect of the fan at 2.1 m from the base plate is better than that at 3.0 m.
According to the movement trajectory of the dust particles (as shown in Figure 12), it can be seen that part of the dust will be turned back after being injected by the air curtain. After installing the ACDRD, the dust concentration of the entire roadway is significantly reduced, indicating that the ACDRD has played its role. Some high-speed dust particles will spread through the air curtain to the entire roadway.

Discussion
e air curtain is generally used to regulate the airflow in the underground tunnel, and it is also used in the dust removal of the fully mechanized coal mining face. Although the theoretical results of ACDR for heading face have continuously emerged in the past few years, devices that can be used on workplace are relatively rare. Hao Wang had studied the principles and methods of air curtain isolation of respirable dust and successfully applied ACDR technology to rock tunnel's fully mechanized working face [17]. e ventilation method and space size of the heading face and the fully mechanized face are different, resulting in the ACDRD of the fully mechanized face not being used directly in tunneling. Baoshan Jia developed an ACDRD [26], but restricted by the technical safety of the equipment and the   dust exhaust system, the equipment could not be massproduced and widely promoted. Professor Li proposed a comprehensive digging dust prevention method for dust collection by long-pressure and short-exhaust ventilation systems and built an ACDRD [10]. After the air curtain equipment is added to the fully mechanized roadway, it is not conducive to the work of the driver and also causes inconvenience to the on-site construction management. e ∩-type jet device and jet box were developed in this article form an organic whole, and the device occupies a small space. When the placement position is appropriate, it basically does not affect the operation of the roadheader driver. In addition, the ACDRD proposed in this paper is based on dry dust collection. Without changing the original tunneling ventilation system, the application of the principle of ACDR can achieve the effect of suppressing the spread of dust.
Affected by machining accuracy, the actual size of ACDRD is slightly different from the design size. ere is a certain deviation in the coupling degree with the roadheader. At the same time, the entire air curtain dust removal device has a certain air leakage. In practical applications, the equipment needs to be modified and reinforced to reduce air leakage and improve the dust removal effect.

Conclusions
(1) Using the principle of flat free turbulence injection, the axial maximum speed attenuation coefficients k 2 on the left and right sides are 4.25 and 3.63; the airflow velocity attenuation of the ACDRD at the upper, left, and right outlets of the "∩" slit is 17.39 m/ s, 12.04 m/s, and 13.66 m/s, respectively; the minimum airflow velocity of the air curtain is 5.48 m/s; and the airflow quantity of the ACDRD is 149.4 m 3 / min. (2) e total dust concentration is the highest in the operator's operation of the roadheader; the farther away the heading face, the lower the total dust concentration. A narrow gap is formed between the roadway and the roadheader, and a number of dust particles spread through the gap between the roadheader and the walls. (3) e trend of total dust concentration and respirable dust concentration is almost the same. Within 20 m, the respiratory concentration drops sharply and is discharged under the effect of airflow. e relative total dust concentration decreased slowly. Within 20∼40 m, the airflow is very turbulent, and the exhaled dust is difficult to settle, so respiratory dust concentration decreases gradually. e respiratory dust spreads throughout the roadway and the concentration tends to be stable after 40 m. Respiratory dust is suspended in the air; it decreases relatively slowly by the total dust concentration. (4) When the airflow outlet of exhaust fan is 2.1 m away from the base plate, the dust removal effect is better than that at 3.0 m from the base plate.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e author declares no conflicts of interest regarding the publication of this paper.   Advances in Civil Engineering 7