Formation and propagation law of coal and gas outburst impact airflow

In order to fully understand the formation mechanism and propagation characteristics of coal and gas outburst impact airflow, based on the outburst test results under gas-solid coupling, combined with the theory of gas dynamics, the formation and migration model of outburst impact airflow is established in this study.It is further verified by numerical simulation. The results show that the outburst holes extend up to the top of the coal seam and produce layered cracks, and the development of layered cracks is directional. During the process of coal seam gas pressure and in situ stress drop,there is staged stability or recovery, indicating that the release of energy is completed in stages.The changes in coal seam stress are mainly concentrated in the stress relaxation zone, stress concentration zone and transition zone.Among them, the decreasing amplitude of the maximum principal stress is larger than that of the minimum principal stress. When the coal seam gas pressure is 2.0 MPa, the impact airflow velocity in the roadway can exceed 300 m/s, and the pulverized coal flow velocity driven by gas drag can reach 70 m/s. The impact airflow velocity is much larger than the pulverized coal flow velocity, and there is a phenomenon that the airflow is advanced and the pulverized coal lags behind. Therefore, the whole process is divided into a single phase airflow stage and a coal gas two-phase flow stage. The impact force on the section of the roadway shows uneven characteristics, and the impact force has increased sharply in some areas. As the distance increases, the strong impact force tends to expand from the center of the section to the outside. The flow state of impact airflow is directly related to coal seam gas pressure and outburst pore morphology. The impact airflow is jetlike, and the expansion wave zone and compression wave zone will appear periodically in the jet. At the same time, the jet cross section will periodically expand first and then shrink, causing the jet boundary to fluctuate up and down and be corrugated.During the migration process of impact airflow, the high speed jet zone, the smooth migration zone, and the recirculation zone can be divided according to its velocity law. There are vortices in the recirculation zone. The impact airflow is affected by the expansion wave and the compression wave, respectively, forming a wedge shaped vacuum zone and a cone shaped compression zone in the roadway.Its static pressure is in a negative pressure state and a positive pressure state respectively.