Coal and gas outburst is still a major safety problem in the process of coal production in China. Correctly understanding of the migration law of outburst high gas and pulverized coal is an important basis for accurately predicting the occurrence time and possible scope of outburst. To reveal the airflow disturbance characteristics and coal-gas flow rule in coal and gas outburst process, outburst coal-gas migration simulations under different gas pressures were conducted using a self-developed visual outburst dynamic effect test device. The results showed that coal-gas flow state at the outburst port is divided into subcritical flow, critical flow, and supercritical flow state. The pulverized coal-gas flow migration in the roadway space can be divided into coal gas two-phase flow area, air compression area, and undisturbed area. Under the experimental conditions, the maximum propagation velocities of wave are 342.22~359.21 m/s, and the coal gas two-phase flow is far less than the propagation velocities of outburst wave, just 3.68~33.33 m/s. When the outburst energy is large, multiple compression waves can superimpose to form shock waves. The peak value of the wave does not necessarily appear in the first boosting range. The presence of pulverized coal leads to a faster attenuation of shock wave, but it makes a greater dynamic destructive force at the same speed.
Coal has always been China’s most important energy source. In 2019, coal consumption accounted for 57.7% of the country’s total energy consumption [
The number of mines, coal and gas outburst accidents, and the death tolls in recent years.
In the process of outburst, the adsorbed gas in the coal is desorbed quickly to generate a high-pressure gas flow. And continue to carry broken coal (rock) powder rapidly thrown out of the coal wall, producing a significant dynamic effect in the mining space. The strong compression wave generated by outburst cause casualties and damage roadway facilities. Additionally, the high-concentration gas that is emitted flows along the disturbance area and may even flow back into the air inlet lane, which may induce secondary disasters, such as gas explosions, and cause extensive casualties. For example, on November 21, 2009, after coal and gas outburst occurred in Xinxing Coal Mine in China, gas explosion was induced by gas countercurrent and electric spark, causing secondary damage. A total of 108 people died in the accident, 80 of them died of CO poisoning caused by gas explosion. On March 31, 2010, an extraordinarily serious coal and gas outburst occurred at the Henan National Coal Company, which caused an explosion and combustion of gas out of the wellhead, causing 44 deaths.
Most researches focus on the occurrence conditions of coal and gas outburst [
Otuonye and Sheng [
These studies conducted qualitative analysis mostly from a theoretical perspective and lacked quantitative data for support and verification. In recent years, with increasing demand for research on the mechanism of outburst catastrophe, some experimental devices for outburst coal-gas two-phase flow simulation with gas as the main power source have been developed, which provide basis for the study of outburst dynamic effects. Based on the principle of segmented pressure difference, an outburst simulation experimental device was developed [
Because of different experimental methods and purposes, some differences are evident in the experimental rules. In the observations of some scholars [
During the development stage of coal and gas outburst, the broken coal body is thrown out from the outburst hole to the roadway space under the action of high gas pressure flow. To observe the outburst wave and outburst pulverized coal movement characteristics in the roadway during the outburst process, a set of outburst coal-gas flow visual simulation experimental device was designed. The device consisted of a high-pressure gas cylinder, outburst chamber, visual simulation pipeline, sensors, and high-speed camera (as shown in Figure
The visual simulation experimental device of outburst coal-gas flow. 1. High-pressure gas cylinder; 2. outburst chamber; 3. pneumatic solenoid valve; 4. visual simulation pipeline; 5. sensors; 6. high-speed camera.
Main parameters of the device and the existing ones.
Technical parameter | Outburst cavity size/cm | Outburst opening mode | Simulated pipe type | Simulated pipe size/cm |
---|---|---|---|---|
Ball valve | Acrylic material | |||
Chen et al. [ | Mechanical damper | Metal | ||
Hu [ | Rupture of membranes | Plexiglass | ||
Jin [ | Mechanical damper | Acrylic material | ||
Sun et al. [ | Rupture disk | Metal, viewing window | ||
Zhou et al. [ | Rupture disk | Metal, viewing window |
To explore the migration law of outburst coal-gas two-phase flow in the roadway space, taking carbon dioxide as the experimental gas, the outburst simulation experiment was carried out under different initial gas pressure (0.1 MPa, 0.3 MPa, and 0.5 MPa) conditions. Before the experiment, the visual simulation pipeline was connected, and sensors and high-speed cameras were installed. The selected (3–10 mm) fresh coal sample into the outburst chamber was put, and it was manually tamped with the pressure plate. The coal density after tamping was about 1 g/cm3. Then, the experimental gas was filled and fully adsorbed. The pneumatic solenoid valve was used to open the outburst port to start the experiment. The data was monitored, and the outburst pulverized coal movement in real time was photographed. After the completion of the experiment, the coal samples that were thrown out in the roadway were collected and statistically screened.
The gas pressure variations in the outburst chamber and simulation pipeline with time under different gas pressure conditions are shown in Figure
Evolution curve of pressure in outburst chamber and simulation pipeline with time. (a) 0.5 MPa; (b) 0.3 MPa; (c) 0.1 MPa.
It can be seen from Figure
The peak pressure value of compression wave at different positions in the roadway.
Gas pressure sensor | Distance from outburst port/m | Peak pressure of compression wave/kPa | |||||
---|---|---|---|---|---|---|---|
0.5 MPa | 0.3 MPa | ||||||
p1 | 0.5 | 14.595 | 11.395 | 3.125 | 9.99 | 10.09 | 5.49 |
p2 | 2.5 | 14.504 | 10.804 | 2.904 | 10.21 | 9.69 | 4.983 |
p3 | 5.5 | 11.29 | 6.32 | 1.44 | 9.27 | 6.81 | 2.854 |
p4 | 7.5 | 3.512 | 1.982 | 0.769 | 3.22 | 1.79 | 0.77 |
It is assumed that the air velocity in the roadway before the outburst is zero relative to the initial pressure of the outburst flow. At this time, the wave front air pressure is
The propagation speed of outburst wave under different initial gas pressure.
Distance from outburst port/m | ||||||
---|---|---|---|---|---|---|
0.5 MPa | 0.3 MPa | 0.1 MPa | 0.5 MPa | 0.3 MPa | 0.1 MPa | |
0.5 | 31.15 | 20.72 | 3.68 | 359.20 | 352.66 | 342.22 |
2.5 | 31.15 | 19.00 | 2.63 | 359.21 | 351.59 | 341.58 |
5.5 | 26.57 | 14.98 | 1.32 | 356.31 | 349.10 | 340.79 |
7.5 | 5.83 | 2.92 | 0.61 | 343.52 | 341.76 | 340.37 |
The flow pattern of the outburst coal-gas two-phase flow uses a high-speed camera. According to the theory of pneumatic conveying, following are the types of flow states of outburst coal-gas two-phase flow: suspension flow, dune flow, stratified flow, and slug flow [
The evolution process of two-phase flow in the first section during outburst (0.3 MPa). (a) 0 ms; (b) 50 ms; (c) 110 ms; (d) 250 ms; (e) 400 ms; (f) 500 ms; (j) 600 ms; (h) 800 ms; (i) 1000 ms; (j) 1400 ms.
The average velocity of pulverized coal flow in different positions of roadway during outburst can be obtained by image measurement method (as shown in Figure
Relationship between pulverized coal flow velocity and position in outburst process.
The outburst strength (outburst coal quality) was different under different conditions. The mass of outburst coal under the test initial gas pressure conditions of 0.5 MPa, 0.3 MPa, and 0.1 MPa was 1.0486 kg, 0.8523 kg, and 0.4749 kg, respectively. The occurrence and development of outburst is the result of energy accumulation and dissipation. When the elastic energy and gas internal energy stored in the outburst coal seam reach a certain level, an outburst is possible [
From the experimental results, under 0.3 MPa and 0.5 MPa gas pressure conditions, it was evident that multiple compression waves and expansion waves were generated in the roadway after the outburst, but this may have been caused by the fact that the total energy involved in the outburst was small (the maximum is 1.82 KJ), and the compression waves were not superimposed to form a shock wave.
The outburst shock wave was generated, however, when the coal and gas outburst dynamic simulation test device was used to conduct the outburst coal-gas flow simulation experiment under the condition of 0.35 MPa [
Results of large-size outburst coal-gas flow simulation test. (a) Curves of gas pressure of the whole outburst process; (b) curve of pressure in roadway in the first 100 ms of outburst.
According to the above analysis and the experimental conditions, the model of outburst wave in the roadway can be established (as shown in Figure
The model of outburst wave in the roadway.
Coal and gas outburst is a continuous process, usually lasting from a few seconds to tens of seconds. The gas pressure in the outburst hole during the whole process is not constant, exhibits pulse characteristics, and generally shows an exponential decay trend [
The coal and gas outburst process is accompanied by large amount of pulverized coal migration and accumulation, which make the formation and propagation of the later outburst shock wave in the roadway environment contain a high concentration of coal dust. The particle volume concentration of pulverized coal is significantly higher than other explosion shock waves in coal mine. The concentration of pulverized coal particles has a significant influence on the propagation characteristics of the outburst shock wave. As the particles interact with the shock wave, they will absorb a lot of energy, thereby speeding up the attenuation of the shock wave, reducing the destructive effect of the shock wave overpressure, and causing the shock wave propagation distance to be affected. As shown in Figure
To reveal the airflow disturbance characteristics and coal-gas flow rule in coal and gas outburst process, outburst coal-gas migration simulations under different gas pressures were conducted using a self-developed visual outburst dynamic effect test device; the following conclusions can be drawn:
The outburst coal-gas flow experienced four stages and the migration of outburst coal lagged behind the initial airflow disturbance in the roadway space. Under the experimental conditions, the maximum propagation velocities of wave are 342.22~359.21 m/s, and the coal gas two-phase flow is far less than the propagation velocities of outburst wave, just 3.68~33.33 m/s When the outburst energy was large, the latter compression wave caught up with the previous one, and multiple compression waves in the roadway superimposed to form a shock wave. The gas pressure in the roadway near the outburst port increased in a very short time and lasted for a period of time in the initial stage of outburst (the first 100 ms) The peak value of the outburst disturbance wave generated by the outburst did not necessarily appear in the first boosting range. The outburst coal particles accelerated the attenuation speed of the shock wave. The presence of pulverized coal increased the mass of high-speed air flow after the shock wave, which made its dynamic pressure destructive force increase under the same movement speed
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This study was financially supported by the National Natural Science Foundation of China (51774319, 51974358, and 51874348) and Natural Science Foundation of Chongqing, China (cstc2019jcyj-msxmX0531).