Mechanism and Key Parameters of Coal Fragmentation by Supersonic Gas Jet Impact: Numerical and Physical Simulation Research

To address the challenges of hole collapse and plugging in soft coal seams resulting from water jet–enhanced extraction technology, this paper investigates the continuous process mechanisms, jet characteristics, and comprehensive relationships among key parameters of enhanced gas extraction induced by gas jet impacts. The analysis employs theoretical evaluation, numerical simulations, and physical testing methods. Examination of the gas jet’s development through various stages reveals that the coal fragmentation process consists primarily of three stages: the free jet stage, the impact jet stage, and the wall jet stage. The key parameters affecting coal-breaking capacity include jet pressure, target distance, and nozzle size. The findings indicate that coal-breaking capacity increases with higher jet pressure, while the optimal target distance for effective jet impact cracking is typically around 7–8 times the nozzle diameter. Although nozzle size has a limited effect on erosion and fragmentation, an increase in nozzle size results in a gradual enhancement of fragmentation, stabilizing at approximately 2.75 MPa. These insights significantly advance on-site enhanced gas extraction technology from coal seams through the impact of supersonic gas jets. Furthermore, the research findings offer a novel approach and theoretical foundation for local pressure relief and enhanced gas drainage in coal seams.