China's energy structure determines that coal makes a significant contribution to economic development1,2. At present, under the carbon neutrality target, coal is still the cornerstone of China's energy, and the energy transformation cannot be separated from the strong support of coal. In 2021, China produced about 4.13 billion tons of raw coal, more than half of the world's total output, accounting for 56% of China's primary energy consumption. The geodetic structure determines that the geological conditions of China's coal fields are complex, and underground mining is the main coal mining method. Coal and gas outbursts (CGO) have always threatened the safe production of coal mines in China3–7. In 2021, six CGO accidents happened in coal mines in China, killing 24 people, with year-on-year increases of 200% and 60%, respectively.
During outburst process, high-pressure gas engulfs the fractured coal body and quickly rushes from outburst hole to mining space or roadway, causing serious casualties and damage to the mining facility. The main outburst hazards are gas suffocation, coal powder burial, and coal-gas two-phase flow impact8. In the aftermath, an expert investigation team usually conducts inverse analysis of the outburst intensity and the causative conditions based on the roadway damage, ventilation facility damage, outburst coal ejection distance and accumulation, outburst coal powder particle size, and sensor record data at the outburst site. That is, the parameters of the disaster scene are used to deduce the disaster process, determine the causes of the accident, and provide guidance for the next step in disaster prevention and control.
At present, such deductions are mainly based on theoretical results. The energy model of CGO can satisfactorily explain the relationships of the geo-stress, methane, physical and mechanical properties of coal. It assumes that the accumulated energy of the outburst is mainly composed of elastic energy and the gas internal energy, and the gas internal energy is usually two to three orders of magnitude greater than the elastic energy9,10. However, there is no consensus by researchers regarding how much gas involves in the outburst (GIO). Wen et al. concluded that only part of the free gas flowing from the pore spaces into the fractures involved in the outburst energy11. Experimental studies have shown that the amount of free gas alone cannot provide all the energy for coal rejection or crushing. Hu and Wen believed that some adsorbed gas also participated in outburst work through desorption9. Tu et al. studied the relationship between the amount of gas involving in the outburst and the particle size and diffusion coefficient of coal12.
To explore the mechanisms by which gas and the other main controlling factors influence on CGO, scholars conducted a series of one-dimensional and three-dimensional coal and gas outburst simulation experiments and investigated the impacts of various factors on the outburst intensity13–18. The multi-physical field coupling evolution law of stress, air pressure and temperature in the process of outburst occurrence was obtained through large-scale similarity simulation experiments19–25. However, the outburst ports in such experimental devices are all designed to be open, and the outburst coal powder ejects directly into the open space, ignoring the friction and collision between the coal powder particles in the confined space of the roadway, and as a result, these experiments do not simulate the actual coal powder migration processes. Whether the current research conclusions are universal still needs to be further explored26.
The ejection and crushing of outburst coal is the main pathway of energy dissipation during the outburst process. In recent years, more and more CGO test devices connected to simulated roadways have been developed, providing an effective means of studying the of influence of gas on outburst coal powder migration and distribution. Chen first developed a set of CGO test systems with roadway27. So far, the study on the outburst shock wave induced by CGO has begun 28–29. Since the entire roadway is welded with steel pipes, it is impossible to observe the migration process of coal and gas flow. The test systems developed by Zhou et al. included observation windows in local roadways to allow for observation of the two-phase flow in the roadways30. Using these devices, Sun et al. observed the movement of the coal powder, an outburst coal migration model was established31. Xu et al. conducted systematic research on the impact dynamics of coal and gas two-phase flow and observed that in L-shaped roadways, the mass of the outburst coal powder exhibited an increase-decrease-increase-decrease distribution pattern along the roadway and piled up in large quantities at the front of the right-angle corner32–33. Zhang et al. obtained a linear relationship between the crushing rate and relative outburst intensity, and the slope of the fitted line can be used to characterize the hazard posed by the outburst34. The simulation test systems made of transparent materials has been developed successively, allowing for visual observation of the two-phase flow in the roadways35,36. A preliminary study conducted by Liu et al. revealed that the ejected coal accumulated in roadway approximately exhibits a normal distribution, and the maximum flow rate of the coal particles observed 3.6 m from the outburst port is 25 m/s37. Jin et al. concluded that the coal powder flow in the roadway is mainly composed of four types of flow patterns: suspension flow, stratified flow, dune flow, and plug flow. However, the length of the roadway was designed to be short during their experiments, which may influence the development of coal and gas flow38.
From the point of view of energy, the CGO process is actual a process of energy conversion. The ejection and crushing of the outburst coal is the main pathway of energy dissipation in this process39–41. Therefore, it is essential to make a quantitative analysis of the mechanism of outbursts by studying the motion process and crushing characteristics of coal powder under the effect of gas, especially in the restricted space of the roadway. In this study, an improved visualized CGO simulation test system was used to conduct outburst simulations under different gas pressure conditions, to analyze the migration form, movement speed, mass and particle size distributions of outburst coal powder in the visualized restricted space of the roadways, and to investigate the amount of gas involving in the outburst (GIO) from the perspective of energy, thus providing a basis for better comprehension of the outburst mechanism.