Search

In the present study, a numerical simulation method was adopted in order to examine the characteristics of dust dispersion during continuous dust release periods (CRP) and stop dust release periods (SRP). The purpose was to analyze the dust distributions and migration actions around road-header drivers in excavation roadways, and then determine effective dust control measures for underground coal mines. This study’s simulation results showed that the dust concentrations continuously increased, and then gradually reached a stability level during the CRP. During that time, the locations of the drivers were always at the intersection of the original migration dust and the backflow dust, and the drivers were invaded by these two strands dust. However, during the SRP, the dust concentrations gradually decreased under the actions of the roadway ventilation. Besides, obvious backflow phenomena were observed around the road-header during the SRP. The locations of the drivers were still within the backflow paths of the high dust concentrations. At the present time, dust separation and extraction systems have been implemented in coal mines, including vacuuming and air knife devices, which are designed to control the dust around the road-header drivers. The field applications of these systems were conducted in the 26 mechanized excavation faces of the Zhangcun Coal Mine. The results revealed that the use of these dust removal systems could effectively reduce the dust concentrations around the road-header drivers. In the present study, the dust removal rates during the CRP and SRP were determined to reach up to 88.7% and 94.6%, respectively. Therefore, the results of this research study provided effective theoretical guidance of the characteristics of dust distributions in coal mines, and introduced effective control methods for the hazardous dust concentrations around road-header drivers during the excavation process.

The ventilations of pressure cylinders is a widely used practice in coal mine roadways in China. It has been determined that the velocities of the ventilations will directly affect the diffusion of the dust particles which reduce the visibility of road-header drivers, and also tend to slowdown operating efficiency. In this study, a CFD-DPM method was adopted in order to examine the airflow flow and dust pollution behaviors in coal mine roadways under various ventilation velocities. The results showed that airflow fields could be divided into disordered regions of airflow flow fast and chaos; recirculation regions, where there were dust backflow; and stable regions, from the heading faces to the exits of the coal mine roadways. The airflow were observed to carry the dust particles along the roadway wall away from the pressure cylinders from the disordered regions into the recirculation regions due to the blocks caused by the road-headers. It was observed that the dust migration had slowed down in the areas behind the road-headers due to the sudden decreases in the airflow velocities. Then, major amounts of dust particles were carried by airflow into the stable regions. The remaining dust was carried to the exits of the pressure cylinders as a result of the dust backflow. As a result, areas of approximately 5 m behind of the road-headers, as well as the areas near the exits of the pressure cylinders, contained high dust concentrations. The dust concentrations near the exits of the pressure cylinders into which the dust particles had been attracted due to the strong entrainment effects, were found to increase with increasing of the ventilation velocities. However, this had only occurred when ventilation velocities had exceeded 17 m/s.

During mining operations, large amounts of coal dust escape from the heading faces. This dust presents a serious hazard to the health and safety of workers in the excavation roadways. In this study, a device was designed to capture the escaped dust in a timely way. The device was suitable for heading with forced ventilation and it could generate a fog layer with a certain thickness through the twice atomization process. The fog layer could cover the entire roadway section to filter and remove the coal dust escaping from the heading face. The performance of the proposed device was experimentally studied under different working conditions. The experimental results showed that, with the increasing of the water pressure (Pw), the particle sizes of droplets decreased while the coverage radius increased and its increasing rate gradually decelerated. Meanwhile, as the air pressure (Pa) increased, the coverage radius displayed an upward trend, and the particle sizes of the droplets decreased gradually. In this study, four different water-air pressure ranges were tested to cope with the different field conditions. Then, the proposed device and obtained test results were applied at Zhangcun Coal Mine in China. The application results indicated that the total and respirable dust suppression efficiency of the device has increased 24.8% and 43%, respectively, compared to the traditional water curtain. Also, the device was estimated to have saved approximately 25% in water consumption when compared with the water curtain method. Therefore, it is believed that the new atomization device would be ideal for replacing traditional water curtains for the suppression of hazardous dust particles in underground coal mines.

Coal gangue is a solid waste material which is generated during coal mining processes. Due to its low utilization value, it is normally discarded on gangue fields where it is stacked into very large hill formations. Spontaneous combustion is one of the major hazards which frequently occur during the long-term stacking of coal gangue. This study targeted the coal gangue stacks of the Chengzhuang Coal Mine, located in China’s Shanxi Province. The results of programmed heating tests helped determine that CO and C2H4 could be used as the index gases in the predictions of the self-heating degrees of coal gangue. Four of the typical coal gangue stacking modes which were often adopted by the Chengzhuang Mine were selected as the experimental objects of this study. The corresponding coal gangue models were piled in a field, and the self-heating degrees were measured with the passage of time. The entire experiment lasted for 60 days. The results showed that the loess isolation and loess stratified stacking methods could effectively suppress the self-heating processes. Also, the CO production and O2 consumption of the coal gangue were consequently reduced. The study also indicated that in the loess stratified stacking, the inhibitory effects on the spontaneous combustion of the coal gangue had increased with the increased thicknesses of the loess layers.

For combined piled foundation, passive loaded piles will be subjected to horizontal force when piled foundation is loaded, and horizontal force may cause the piles horizontal displacement, instability or even failure. With inclined bedrock nearby, the mechanical behavior of passive loaded piles will be more complex. In order to study the mechanical behavior of passive loaded piles under such condition, laboratory model test was carried out, during repeatedly loading/unloading cycles. Axial force, side friction, bending moment, and lateral soil pressure were analyzed. The study is based on model test, and more related studies are needed before being applied to practical conditions. The results show as follows: Axial force distribution of the piles changes with the increase of load and becomes stable under high load. This phenomenon is related to the expansion of potential slip surface in piled foundation. The passive loaded pile that is close to load boundary has only one neutral point which is located in the lower part of the pile, and its side friction has a relatively stable direction. On the contrary, the direction of side friction of the pile that is far from load boundary changes frequently. Bending moment curves of passive loaded piles vary with the change of the distance between the piles and load boundary, and they intersect at the lower part of the pile. Bending moment is basically negative above the intersection and gradually turns into positive below it. The maximum lateral soil pressure of passive loaded pile is triple that of the adjacent soil, indicating that passive loaded pile can effectively limit the horizontal deformation of piled foundation. But when the pile is too far from load boundary, it will lose the ability to limit the horizontal deformation of piled foundation.

Terahertz high-data communication creates numerous demands for terahertz wave amplitude modulation. Here, we experimentally demonstrate a high-speed terahertz wave modulator based on 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST)/Si hybrid structure. An externally modulated 808-nm pumping laser is utilized to generate photoexcited free carriers at the DAST medium. With the increase of illumination laser intensity, the modulation depth continues to increase in the frequency range from 0.23 to 0.35 THz. A dynamic amplitude modulation at 0.25-THz carrier wave shows that the our modulator provides a modulation speed of 1.26 MHz with a depth of up to 53% at an external pump laser irradiance of 3.5 W/cm2. Our present DAST/Si hybrid structure provides a practicable route to achieve effective signal modulation for terahertz communication system.

Coal spontaneous combustion is a natural hazard during mining. In China, the longwall gob area is the main places that are prone to coal spontaneous combustion due to excessive residual coal in the gob and severe air leakage during underground mine ventilation. Injecting CO 2 into gob has many advantages to prevent fire disaster. However the characteristics of coal low-temperature oxidation after CO 2 injection were rarely studied. In this paper, a temperature programmed test system was adopted to simulate coal spontaneous combustion at low-temperature stage before and after CO 2 adsorption. Compositions and concentrations of gases produced by coal samples from Yiyuan mine in China at different temperatures were analyzed. It is found that with the increase of the temperature, the gas concentration of CO 2 generated from coal before CO 2 absorption, expressed the trend of exponential growth. While the CO 2 concentration after CO 2 absorption, showed the distribution of “V” type on the whole, that is, the CO 2 concentration firstly dropped to the minimum value and then rose up gradually. Before reaching to the temperature corresponding to the minimum concentration of CO 2 , the CO 2 concentration was significantly higher comparing with that before CO 2 absorption. While over that temperature, with the temperature increasing, the CO 2 concentration was obviously lower than that before CO 2 absorption. During the heating oxidation of coal, the gas concentrations of CO and C 2 H 4 showed the increasing tendency with the temperature increasing. However, the initial temperature of CO and C 2 H 4 detected firstly during the coal oxidation process after CO 2 absorption was obviously lower than that before CO 2 absorption, what's more, the concentrations of CO and C 2 H 4 were also obviously lower than that before CO 2 absorption at the same temperature, showing an obvious “hysteresis” phenomenon of inhibiting coal oxidation. The experimental results provided the basis of a method to control the spontaneous combustion of residual coal by injecting CO 2 into the gob. The field test showed that the index gas concentration of CO was reduced sharply and coal spontaneous combustion in the gob was controlled effectively.

We numerically demonstrate a compact multichannel terahertz demultiplexer which is composed of three photonic crystal directional couplers. By using plane wave expansion, the transmittance of the designed photonic crystal directional couplers are analyzed. Then, the performance of the demultiplexer based on the photonic crystal directional couplers is investigated with the aid of finite-difference time-domain method. The transmittances at frequencies of 0.57THz, 0.52THz, 0.54THz, and 0.6THz are 98%, 92%, 94%, and 88.2%, respectively. We confirm that the device can split four different terahertz wave frequencies into different output ports with high transmittance efficiency. In addition, the total size of the our terahertz demultiplexer is about $5.2\mathbf{mm}\times 5.6\mathbf{mm}$ , which can be easily integrated in terahertz circuit systems. This advice provides a new method to manipulate terahertz wave.

Nowadays, there is much interest in body-worn textile antenna which is made of common clothing and operates at wireless communication bands. In this letter, a novel wearable textile antenna is designed to operate at Industrial, Scientific and Medical (ISM) bands of 2.45GHz and 5.8GHz for future wireless systems. The performance characteristics of our present wearable antenna are studied in free space and on human-body environments. The wearable textile antenna is compact with omnidirectional radiation patterns and the total size is of $42\text{mm}\times 13\text{mm}$ . Due to its small size, the proposed antenna may have practical applications in the smart wearable textile fields.

A novel terahertz wave modulator based on CsPbBr3 perovskite quantum dots heterostructure is proposed. An external modulated 450 nm pumping laser is utilized to generate photoexcited free carriers at the CsPbBr3 perovskite quantum dots heterostructure medium. We measured an amplitude modulation of the terahertz transmission in the frequency range from 0.23 to 0.35 THz with various laser intensity irradiances. In addition, dynamic amplitude modulation at 0.27 THz carrier wave show that the modulator provides a modulation speed of 2.5 MHz at a external pump laser irradiance of 2.0 W/cm2. Our CsPbBr3 perovskite quantum dots heterostructure can high speed modulation and can be used for terahertz modulation in addition to photovoltaics application.

Lately, programmable metasurfaces have been proposed by adding a PIN diode between two metallic patches in microwave region. However, the pin-diode switches can only operate at relatively lower frequencies (i 60 GHz), the conventional electronic method of device design used in the microwave band cannot easily be extended to the terahertz frequency regime. Here, we describe a new design principle for the actively tunable terahertz wave coding metasurfaces using vanadium dioxide embedded metallic patch. The radar cross section (RCS) reduction and radiation patterns of our new coding scheme can be altered using the metal–insulator transition of vanadium dioxide at around 68 °C. To the best of our knowledge, this is the first time that an active control of programmable coding metasurfaces was demonstrated at terahertz regime.

It is known that high dust concentrations are severely hazardous to health of miners and the safe operations of coal mines. The results of the suppressions of coal dust via chemical dust suppressant methods have been of critical significance, and these methods have been widely applied in coal mines. In this research study, a type of complex dust suppressant composed of surfactants, synergists, and cellulose was prepared. The prepared suppressant was characterized by a high wetting ability and adhesive capacity. The results of the performance tests of this study’s proposed complex dust suppressant showed that its dust sedimentation time was only four seconds, which was observed to be much shorter than that of water alone. Also, the proposed suppressant displayed a viscosity which was 25 times that of water. In this study’s field tests, the proposed complex dust suppressant was used for dust control in a mining roadway of the Zhangcun Coal Mine. The field test results revealed that the average suppression efficiencies of the total dust and respirable dust had reached 89.2% and 87.7%, respectively. Furthermore, a 44.5% increase in the control of the total dust had been observed, and a 65.6% increase in the control of the respirable dust had been achieved with the proposed method, when compared with the results of the previously used water curtain technology.

Image annotation generates a set of semantic labels that describe the contents of an input image. Recently deep learning techniques have achieved significant success in many areas of image processing. In this paper, we present a multi-label image annotation method that combines unsupervised object hypotheses generation and deep neural network. Given an image, object hypotheses are generated in an unsupervised manner. Then we extract the image features for each hypothesis with a deep neural network model. By combining the features of all hypotheses, we get the features of the entire image. Finally, we calculate for each label the probability of that the label is correlated with the given image. It can be trained in an end-to-end way using the standard backward propagation algorithm. Experimental results on multiple benchmark datasets show that our method is better than the state-of-the-art ones.

The digital coding metasurfaces need several kinds of meta-particle structures to obtain corresponding electromagnetic wave responses and require time-consuming optimization. In this paper, we present train-symbol-shaped meta-particles with various orientations utilizing Pancharatnam–Berry (PB) phase to achieve 1-, 2-, and 3-bit digital coding metasurfaces. Terahertz wave scattering patterns of the coding metasurfaces with regular and random sequences are given and discussed. They have strongly suppressed backward scattering with approximately −13.5 dB radar cross section (RCS) reduction in a wide band range from 0.85 THz to 1.6 THz. The proposed digital coding metasurfaces provide a simple way and new opportunities for manipulating terahertz wave scattering with polarization independence.

A multifunctional frequency coding metasurface is proposed in terahertz frequency band. Different from the previous coding metasurface with a fixed phase difference of the neighbor coding unit cells in the whole working frequency band, the proposed frequency coding metasurface has the linear change phase difference in the operating frequency band with different phase sensitivity. It can flexibly control the reflected terahertz waves to the numerous directions by changing the working frequency without redesign the coding metasurface structure. Additionally, we demonstrate the polarization insensitive properties of our frequency coding metasurface by using 1-bit frequency coding metasurface with the periodic sequences of “0-0, 0-1” along the x-direction, 2-bit random frequency coding metasurface and non-periodic frequency coding metasurface. Our frequency coding metasurface provides a novel and flexible way to manipulate the energy radiation of terahertz wave and has tremendous values in terahertz systems.

We design a coding metasurface based on Pancharatnam-Berry (PB) phase to manipulate terahertz waves, which is simple and flexible. Compared with the previous design of the coding metasurface, the present coding particles can be obtained by using a same size meta-particle with various orientations instead of designing multiple structures or changing specific size parameters. The PB coding metasurfaces composed of U-shaped particles with pre-designed coding sequences can generate multi-bit coding in the terahertz frequencies and control the reflected terahertz waves to the various directions. Both simulation and theoretical calculation scattering patterns of the designed PB coding metasurfaces demonstrate the expected manipulations. Additionally, the bandwidth of radar cross section (RCS) reduction of approaching −15 dB is 1.05THz (range from 0.9THz to 1.95THz). We believe that the proposed design provides a more flexible way for the manipulation of reflected terahertz waves.