Mechanism and mathematical model of the continuous process of gas jet breaking coal

Aiming at the defects of water seal inhibiting gas desorption and reducing coal permeability during the application of hydraulic anti-reflection measures in soft and low-permeability coal seams,a technical method for perforating coal with high-pressure gas jets to break coal and increase permeability is proposed,at the same time in order to understand the characteristics of air jet punching coal. Based on the aerodynamic theory and rock mechanics,the mechanism of the full continuous process of gas jet breaking coal is analyzed,the corresponding mathematical model is proposed and the criteria for determining coal breaking capacity and erosion pit characteristics are established. It is concluded that the high pressure gas accelerates through the Laval nozzle,the axial velocity decreases and the radial section expands under the shear action of the air medium. When the gas jet impact applies on the coal body,the broken core and the extended crack are formed,and the failure occurs under tensile action,which induces the plastic fracture pit. Calculations of the capability of gas jet breaking coal are conducted for 3 different strength coals and 1 sandstone cases. Results show that the pressure at the stagnation point of gas jet increases with the increasing inlet pressure and nozzle diameter. Under the condition of low inlet pressure,the gas jet can cause erosion damage to the coal with lower strength,and when the inlet pressure reaches 16 MPa,the erosion and failure of sandstone is achieved. With the increase of jet pressure,the axial depth and radial length of the broken pit are increased,while the increment of axial depth is much larger than the increment of radial length. The shape of the broken pit is conical initially but changes to rugby shape gradually.