Minimum wind flow of air directional drilling technology in soft-fragmentized coal seam

In order to solve the problem that it is difficult to determine the range of the minimum wind flow of air directional drilling technology in soft-fragmentized coal seam, in this paper, loading of cuttings in air drilling is analyzed, and the critical incipient velocity of cuttings and the minimum wind flow under different grain sizes and different dip angles is calculated, and the distribution and variation of air flow rate in the borehole are analyzed by numerical method. The results show that the critical starting velocity of cuttings is maximum when the inclination of the borehole is about minus 30°, and the critical starting velocity of cuttings carrying rock is consistent with the minimum air flow rate under the conditions of different inclination angles and different grain sizes. In the radial direction, the air velocity is the largest at the center of the annulus section, and gradually decreases toward the wall of the hole on both sides and the wall of the drill pipe or the wall of the drill pipe and the inner wall of the drill pipe. In the axial direction, the air velocity increases gradually from the hole mouth to the bottom of the hole, and an obvious high-low velocity zone is formed at the bottom of the hole. When the air flows through the joint of the drill bit and the drill pipe, a sudden expansion effect is produced, which causes the air velocity to suddenly decrease, forming a obviously high-low velocity laminar flow zone, subsequently, the air velocity in the annular gap between the drill pipe and the hole wall tends to be stable.