Study On The Mechanism Of Coal Failure By Drilling Of High Pressure Water Jetting

The coal and gas outburst, which is a kind of serious project disaster in the process of coal mining, is a major problem that urgently needs to be solved to guarantee the coal mine safety in production. The basic measure to control coal and gas outburst is gas drainage, but the construction of drilling holes is still the difficult project problem until now. However, using the drilling bit of high pressure water jet to carry out the construction of drilling holes and gas drainage has a great superiority with which the traditional mechanical drilling bit is unable to compare. This technic can be studied deeply as a new way and technical approach. As the theoretical principle of technology research of the high pressure water jet drilling holes, the study on mechanism of high pressure water jet breaking coal and drilling holes, which has the important theoretical value and project suitable value, can basically solve several problems of gas drainage and holing construction such as perfecting the breaking rock mechanism of the high pressure water jet, developing the jet drilling bit suiting drilling holes in the coal bed, obtaining the key technology of water jet drilling holes .Therefore, this dissertation concentrates on the task of the mechanism of the high pressure water jet breaking coal and the jet drilling bit development which is the key technology of water jet drilling holes in the coal bed to research from following several aspects and drew the following conclusions:First, in this dissertation, the structural characteristics of high pressure water jet were researched. The jet problems of jet drilling holes referred in the coal bed belongs to the turbulent impact jet flow problem were pointed out and the turbulent jet flow basin structure, the axial velocity and the rules of axial pressure changing were analysed. The variation regularity of jet flow during drilling hole had been mastered.Next, (in the dissertation,) the process and mechanism of the high pressure water jet breaking coal were studied based on the analysis of the coal petrographic stress field under the high pressure water jet flow impact with dynamic load function.Through the description of the high pressure water column impact process, the author constructed mechanics calculating model of the jet flow impacting coal-bed by spherical wave theory and obtained the analytic solution on the coal petrographic stress field that producing under the impact moving load stimulation, then divided the water jet flow breaking coal process on the time level and analyzed the coal damage and destruction way at various time as well as the function of water jet flow in the process of breaking coal. After analyzing and studying, the author drew such conclusions: in the process of the high pressure water jet flow breaking coal and drilling holes, the preliminary jet flow impact moving load to the coal damage and destruction effect are given priority and influence ofd quasi-static pressure in the later perio is limited, in which the major effects of the shear action of the stress wave and the stretch action of the unloads are prior in the jet flow impact moving load were analyzed in operating process.Then, this dissertation established the numerical computation model with the non-linear dynamic finite element method, achieving the fluid - structure coupling computation with penalty function coupling by using technologies such as the finite element divergence, the single-point Gauss integral, the hourglass control, the explicit centre difference etc., and obtaining the similar conclusions with the theoretical analysis that the jet flow impact function having gradualness, the stress wave weakening extremely rapidly, the unloading effect in the coal causing the tension failure and so on. All these conclusions indicated that the numerical computation model was reasonable and the computational method was feasible, and the computed results could well re-appear the dynamic evolution process of jet flow breaking coal which the experimental method couldn't complete and the mechanism of jet flow breaking coal. The results also identify and further reveal the theoretical research findings of jet flow breaking coal.Finally, taking the mechanism of the high pressure water jet flow breaking coal and the numerical simulation research results as the foundation, the dissertation developed and designed the offset type auto-gyration jet bits and impeller type gyration bits which can be used to drilling holes in the coal bed with the high pressure water jet flow, and carried on the field tests by using the developed jet bit in the Xinzhuangzi coal mine of HuaiNan mining industry (group) company. The experiment indicated that the developed offset type auto-gyration bits and impeller type gyration bits have well abilities for breaking coal and drilling efficiency. Research findings that of high pressure water jet flow breaking coal by theoretical research and numerical computation have been provd.