| Coal mine gas has become a major hazard to mine coal safely and efficiently in our country, and hydraulic slotting technology is one of the main methods to release pressure and enhance permeability of the coal seam, to strengthen the gas extraction and prevent coal mine gas accidents with characteristics of obvious effect, strong practicability and so on. However, the hydraulic slotting technology has some problems at present, such as low jet cutting and breaking coalability, high operating system pressure, poor stability of system equipment and so on. There fore, based on the existing hydraulic slotting technology, this thesis put forward the high-pressure gas-liquid two-phase jet slotting technology, and studied the characteristics of the transient dynamic impact of high-pressure gas-liquid two-phase jet, the performance and the mechanism of high-pressure gas-liquid two-phase jet breaking coal, using the methods of equipment research and development, the oretical analysis and laboratory tests. The main research results are as follows:(1) The basic structure of high-pressure gas-liquid two-phase jet and the characteristics of its transient dynamic impact were studied. Besides, the mathematical model of high-pressure gas-liquid two-phase jet was established, and the calculation formula of the jet impact dynamic pressure was deduced, and the main factors affecting the jet impact dynamic pressure were analyzed. The study found that the high-pressure gas-liquid two-phase jet can form local pulse jet on the surface of impacting area, and produce the vertical and horizontal impulse.The main factors influencing the impact dynamic pressure of the high-pressure gas-liquid two-phase jet are the air rate, the initial pressure of the jet, the stand off distance and the nozzle diameter.(2) The high-pressure gas-liquid two-phase jet slotting technology test system was designed and developed, and using this test system, the main factors affecting the high-pressure gas-liquid two-phase jet breaking coal properties were studied. It was found that the water pressure(Pw) and the air rate of jet are the key parameters affecting the performance of the high-pressure gas-liquid two-phase jet breaking coal, and the stand off distance(S) and the nozzle diameter are the relatively minor ones. If the performance of the high-pressure gas-liquid two-phase jet breaking coal is the best and the most efficient, the jet parametersshould be ensured that the Pw?12MPa, the air rate is in the range of 30% to 35%, the S is from 200 to 250 mm and the nozzle diameter is about 3.9mm. In addition, the porosity of coal also has a great influence on the performance of high-pressure gas-liquid two-phase jet breaking coal, and the greater the porosity of coal, the higher the efficiency of jet breaking coal.(3) The contrast experiments on the performance of the high-pressure gas-liquid two-phase jet and water jet breaking coal under the condition of submerged and non-submerged jet respectively was carried on, and the comparative analysis from the perspective of jet impact energy and impact frequency was carried on. Experiments showed that the performance of high-pressure gas-liquid two-phase jet breaking coal is generally better than that of pure water jet in the condition of submerged and non-submerged. Compared with the pure water jet, the average diameter of the jet punching hole increased by nearly 2 times, the average depth of that increased by about 25%, the time of breaking samples decreased by about 75%, and the broken threshold pressure of samples was reduced by 25% to 50%. At a higher jet pressure condition(P?14MPa), the air rate affects the performance of high-pressure gas-liquid two-phase jet breaking coal significantly, which makes the high-pressure gas-liquid two-phase jet have the best effect to break coal in the range of 30% to 40%, and makes the jet impact energy and impact frequency greater than the pure water jet. The maximum impact frequency is approximately two times as that of the water jet.(4) The stress distributionin coal were analyzed when the jet impacted coal samples, besides the propagation of macro cracks on the surface and the dynamic crushing process of coal samples were tested. The study found that stress waves consisting of longitudinal wave, transverse wave and Rayleigh wave are generated when the high-pressure gas-liquid two-phase jet impacts coal body, and the stress waves spread into the coal body in a hemispherical direction from the center of jet contacting point. According to the macroscopic breakage characteristics of coal samples and the high-speed video analysis of coal crushing process, the internal fracturesin coal samples can be divided into three types of circumferential cracks, radial cracks and the fan-shaped fractures, and the process of high-pressure gas-liquid two-phase jet breaking coal has three different stages:(i) the initiation stage of the micro cracks,(ii) the expansion stage of the micro cracks, and(iii) the propagation stage of the macro fractures. The step-type fragmentation of coal samplesis caused by the combined effect of the stress wave generated by the dynamic impact of jet and the coupling effect of gas-liquid-solid in the coal fractures.The study results has important theoretical and practical significance for understanding the characteristics and microscopic mechanism of coal breaking caused by the high-pressure gas-liquid two-phase jet, optimizing the key parameters, improving and perfecting the technology of hydraulic slotting to release pressure and increase permeability of coal seam.During the study, 1 relevant Chinese core paper, 2 authorized national invention patents, and 3 utility model patents were published. |
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