The use of clean energy is an important part of promoting sustainable energy development. As a clean energy source, coalbed methane, during the mining process, the position of the nozzle can influence coalbed methane extraction efficiency by affecting the cracking effect of coal. To investigate the impact of nozzles on the effect of coal fracture, a test of simulated coal by high-pressure air blasting was executed using nozzles 100 mm, 200 mm, and 250 mm from the orifice. Based on the test results and theories of fracture damage mechanics, two damage fracture models were established for the nozzles located in the middle-upper and middle-lower of the blasthole, respectively. The fracturing process and increased permeability mechanism of the coal were revealed by these two models. The results show that: when the nozzle is 100 mm from the orifice, the high-pressure air impacts the blasthole wall first, similar to a uniform expansion. Multiple longitudinal cracks are formed penetrating the coal. The permeability of the coal seam is greatly improved. When the nozzle is 200 mm and 250 mm from the orifice, the high-pressure air first impacts the bottom of the blasthole. The bottom hole angle and apex hole angle first form horizontal cracks while longitudinal cracks only appear at the same depth as the blasthole. The nozzle is 250 mm from the orifice to form a compaction zone at the bottom of the blasthole. The crack density is small and the tangential depth is shallow, which is not conducive to coalbed methane mining. The results of the research offer a theoretical framework and point of reference for the use of high-pressure air blasting technology in the extraction of coalbed methane (CBM).

Understanding the dynamics of damaged rock masses and the evolution of internal fractures is beneficial to the construction of deep engineering projects. Dynamic tests on damaged granite were carried out using a split Hopkinson device which can apply a confining pressure. A group of damaged granites was CT-scanned and three-dimensional reconstructed using Avizo 2020.1 software. The results indicate that with increasing damage, the peak stress and peak modulus of the damaged granite decrease, while the peak strain increases. When the initial damage is consistent, all three parameters increase with the increasing confining pressure. Confining pressure alters the number and development direction of internal fissures in granite. Higher confining pressure results in fewer fissures, with their development direction shifting more towards the center of the sample and becoming straighter. The total volume of fissures within the rock samples, the volume of through fissures, and the maximum length of the fissures are decreasing with the increase in the confining pressure. In addition, the three-dimensional fractal dimension and the internal damage also decreased continuously with the increase in the confining pressure. This research provides valuable theoretical guidance for supporting and constructing surrounding rock in deep engineering projects.