| The coal production has fallen for two years in our country, and drop increases, coal prices have been falling which result in heavy losses among most of the mines. However, The status of the coal in the energy structure in our country is still unshakable, along with that many coal and gas outburst mines exist in our country, the high cost of coal mining, and great difficulties in gas extraction and pressure relief, all of which are serious threats to the miner’s life and development of coal mine. In the current grim situation, how to eliminate the outburst danger in the form of security and economic has become the urgent request.From the point of coal and gas outburst mechanism, the gas extraction is a very effective measure of outburst prevention. If there is a pressure relief for the mining of coal containing gas in advance,it effectively improves the effect of gas extraction. Therefore, to develop the research and application of low permeability coal seam pressure relief anti-reflection strengthen extraction can help us to find an effective solution of coal and gas outburst accidents and preventive measures. Hydraulic-cutting measure is a promising technology and get more research. Formed by high pressure water jet cutting coal, the size of a certain slot, so that slot around coal occurrence within a certain range of movement, improve coal body fracture, reduce the stress of coal and gas from escaping to communication channels, to enhance gas extraction effect and the purpose of eliminating outburst danger. However, from the current application point of view, there are still breaking mechanism of coal flawed, difficult to determine the scope of the slot, time-consuming process and other issues, it restricted the application and development of this measure.Based on the above analysis, the main contents of this paper are as follows:(1) Based on coal and rock damage mechanism, researched on the high-pressure water jet and structural properties of coal and coal-breaking theory, applied dimensional analysis method to study the water jet from the nozzle axis at different jet velocity; based on Huygens wave theory, research the impact of high pressure water jet coal process, built the mathematical models of water jet cutting coal maximum depth.(2) based on rock mechanics and elastic-plastic mechanics, studied the stress-strain andfailure laws of coal body around coal slit formed by water jet and the influence of coal pressure relief effect from each parameter of slot and its variation, using numerical simulation software FLAC3 D. Study on the synergistic effect of relieving slotted between a plurality of cloth under the hole way. Finally, built mathematical model of a hydraulic cutting coal extent of damage.(3) Based on computational fluid dynamics theory, using FLUENT software,did numerical simulation for high-pressure water jet flow field variation, and water jet nozzle geometry parameters on jet numerical simulation studies; on the basis of a large number of numerical simulation, the establishment of the mathematical model of water jet nozzle considering the loss.(4)Based on theoretical and numerical simulation research, established the mathematical model of slot coal area of damage which synthesized nozzle parameters, slot parameters, device parameters.(5)Using of high pressure water jet nozzle independent design, through the field test to study the parameters of the water jet nozzle and the pump pressure to the influence of cutting coal effect; the results of numerical simulation and field trials to optimize the drill integration of drill cutting parameters; studied the basic parameters in process of hydraulic-cutting; field test and adopted drop method to examine the scope of the relief of hydraulic-cutting holes; designed hydraulic cutting hole program.(6)Examined the effect of gas drainage; by examining the content of the residual gas and residual gas pressure, measures for the hydraulic-cutting verification for effectiveness of strengthening drainage and eliminate sudden.After the high-pressure water jet passing the nozzle, due to the structure of the nozzle, the jet speed increase, with the water jet away from the nozzle, there are different variations. Depending on the distance between the nozzle axis at a location, the water jet is divided into close segments, core segment, fracture segment and droplets segment, since the air is mixed, keeping the nozzle exit velocity portions geted smaller and eventually disappear; studying relationship of the nozzle diameter, water pressure and water jet flow, when it kept the same water pressure, nozzle diameter may be greater narrowing jet exit velocity, when it kept the same nozzle diameter, it is necessary to increase the pump pressure in order to improve water jet speed; studies two kinds of broken coal hypotheses currently getting more recognition, which made a good description for some phenomena in water jet impingement process, but also have some limitations. Analysis of the different stages of coal damage, due to the compressive strength of coal is far greater than the tensile strength and shear strength of coal and therefore tensile failure and shear failure process can generate more damage results. Study the impact of coal pressure from the water jet when it is perpendicular to the incident coal wall on the basis of momentum theorem, indicating that the greater speed, pressure caused by the impact of coal is also greater. The continuous water jet is divided into numerous small section of water, the water column were analyzed by small water hammer pressure, according to the Rayleigh dimensional analysis method, obtained the speed of the jet from the different locations of nozzle; according to Huygens wave theory, studied laws of effect between water hammer pressure wave and surface coal, build mathematical models of a high-pressure water jet cutting coal maximum depth, provide a method for quickly calculate depth of the kerf based on different coal parameters and device parameters, provides a reference foe numerical simulation and field test paper later.FLAC3D numerical simulation software stands for Fast Lagrangian analysis method,which is quite suitable for the mechanical properties of the material body of rock and soil, especially in terms of plastic flow simulation features. Based on this,the paper build numerical model to study the law of the destruction of the coal when water jets work on it. The study area for coal and gas outburst coal seam, where the gas drainage is difficult.Collection of geological data and other parameters of the study area, based on the meeting the basic accuracy of the conditions and computer hardware, established numerical simulation model and hydraulic-cutting model, by excavation of coal in the slot, do simulation of coal body around slot stress- strain and plastic zone variation. After excavation slot, stress suffered by coal body around slot in the X-axis direction gradually becomes smaller in X, Y-axis direction and the symmetrical distribution center is located at the point of the groove joints, which is similar to Z-axis direction stress distribution laws. In the Y direction on the section, stress distribution laws was "butterfly" shape. Since hydraulic-cutting out of slot depth is much greater than its width, when distance from the center point of the slit is same, the stress level is much greater than unloading Y direction uninstall degree in X and Z directions; the slot parallel to the drill direction of the coal hole in a plastic destruction is far greater than the area perpendicular to the direction of the borehole in the area of coal plastic failure, and all in the area of coal plastic relief situation are good.Every parameter of hydraulic-cutting have different degrees of impact on water jet breaking effect of coal, here to selected part of split seam parameters for numerical simulation as follows: nozzle outlet pressure, ground stress, slot depth, slot width and slot spacing.(1) Nozzle outlet pressure: setting pressure gradient as follows: 5MPa, 10 MPa, 15 MPa, 20 MPa, 25 MPa, 30 MPa, 35 MPa, 40 MPa. Stress unloading of coal body around slot in all directions became larger, it show that the Y direction is larger than the other two directions stress relief under different pressure gradients, stress relief was in direct proportion to the pressure gradient. With the increase of water jet pressure, high stress areas of coal body gradually move toward deep, in process of water pressure from 5MPa to 40 MPa, the growth rate of coal in plastic area was slow down after trend, therefor, considering all of this, the hydraulic-cutting seam water pressure is confirmed as about 30 Mpa.(2) Ground stress: due to the linear relationship between stress and depth, the seam depth replaces ground stress. Setting depth gradient as follows: 200 m, 400 m, 600 m, 800 m, 1000 m. The numerical model figure which shows the relationship between the maximum value in the X, Y, plastic zone and the maximum stress in the Z direction indicated that the slots around the coal is also increasing in the range of relief in all directions as the depth increases. The maximum stress in all directions were greater than the maximum plastic zone, indicating that in these areas, although coal was no damage, stress still decrease. It also shows that the measure is particularly suitable for high stress areas.(3) slot depth: setting depth gradients as follows: 1m, 1.5m, 2m, 2.5m, 3m. With slot depth increases, the maximum value of the plastic zone in all directions are larger, which almost coincides with the maximum value of the plastic zone in X and Z directions; moreover, stress relief in all directions and the maximum stress unload volume are constantly changing large, and stress unloading volume exponentially grew. As for the case of the coal body displacement change, along with the depth of the kerf increases, the coal displacement in both sides of slot gradually become larger and the coal displacement in both sides of slot is about the same size. In conclusion, slot depth has great impact on pressure relief, it is recommended where possible, to maximize the kerf depth.(4) slot width: setting the slot width gradient as follows: 0.1m, 0.2m, 0.3m, 0.4m. Increasing slot width increased the degree of coal relief around slot, so it made range of movement from coal to slot become larger, coal deformation greater extent of damage. Further the range of plastic zone grew rapidly, when slot width is 0.1m, the plastic failure region corresponding to the range is 0.9m, when the slot width is 0.4m, the plastic zone expanded to 3.4m, exponential growth. It showed that increasing slot width has an more obvious effect on coal pressure relief, but increasing slot width need to increase the width of the slotted nozzle outlet diameter which result in reducing water velocity.(5) Slot spacing: setting slot spacing gradients as follows:2m, 3m, 4m, 5m. When slot spacing setting is small, there is duplication between regional relief slots. With increasing slot spacing, pressure relief area among slots is gradually reduced until to 5m, coal in intermediate position of slot has not been affected on both sides of the slot, in which the effect of pressure relief is bad. When slot spacing increases within 2m ~ 5m, the volume of plastic zone increases at first and then decreases, while the Y direction stress unloading volume shows a gradual increasing trend, both show substantially linear.(6) Study the synergistic effect of relieving the law among slots. When the distance to slot drilling is 3m, hydraulic-cutting pressure relief effect is obvious for drainage borehole, it can improve effect of gas drainage. Slot drilled alternately arranged in parallel arrangement is more significant than the role of the media for relief.Based on the foregoing numerical simulation about slot parameters on the influence of coal broken laws, select slot stress, slot width and depth as parameters, continue making numerical simulation increasing a number of parameters cross. Get the plastic zone range of different combinations of parameters, using MATLAB to fit and obtain a range of plastic zone mathematical model containing three variables which provided a method quickly determining the extent of damage in all directions of slot at the scene.Using numerical simulation software FLUENT to study the high-pressure water jet flow field and variation; making the numerical simulation about water jet cutting nozzle geometric parameters on the influence of the jet. Built the numerical model of the nozzle, obtained the water velocity profile on the cross section at different positions from the axis and the maximum speed of the water jet waas in position of the axis. The lowest was in section 6D position; In terms of the jet outside of the nozzle, the area where turbulent kinetic energy is larger was in the core area around on both sides, while the area where turbulent kinetic energy is smaller located in the core area.Based on the numerical simulation of the above, the paper studied the influence ofthe various geometrical parameters on parameters of water jet speed and other parameters.(1) Shrinking angle: setting shrinking angles as follows: 13°, 30° and 60°. As can be seen from the jet velocity contour map, the maximum speed of the nozzle in three different shrinking angle did not vary widely. When the shrinking angle is 13°, the jet expediting time in the contraction section was longest and the constant jet speed core was longest.Considering all of above, water jet broken coal better when the shrinking angle nozzle was 13°.(2)Nozzle exit diameter: setting nozzle exit diameters as follows: 1.4mm, 1.6mm, 1.8mm, 2.0mm. When the nozzle exit diameter was 2mm, water jet has the maximum axial velocity and jet velocity decay was slow. The larger the nozzle exit diameter, the greater the pressure on its axis, both are almost linear relationship.(3)Straight length: setting straight length as follows: 4mm(2D), 8mm(4D), 12mm(6D), 24mm(12D). When the length of the axial velocity of the jet and dynamic pressure in straight column reached 24 mm, it reached the limit state. When length of straight column section is 2D, water jet had the largest core area. In conclusion, the length of the straight column section should be 4D or 6D.In order to fit the model more accurately, adding 6 shrinking angles as folloes: 0 °, 20 °, 50 °, 90 °, 120 ° and 180 °. Adding 7 water pressure as folloes: 5MPa, 10 MPa, 15 MPa, 30 MPa, 40 MPa, 50 MPa and 60 MPa. Through numerical simulation, the paper obtained the relationship between the nozzle parameters and the nozzle exit velocity. Taking the hydrostatic pressure loss into account, in order to reduce errors, built mathematical model of the influence of nozzle geometric parameters and water jet pipe parameters and other parameters on the nozzle exit velocity, considering the frictional pressure loss and local pressure loss. This provides a reference for similar studies for water jet nozzle and field tests below.Based on the above findings, synthesizing the earlier research, the paper obtained the mathematical model of the influence of high pressure water jet breaking coal which combined effects of cutting-slot equipment, nozzle parameter and coal parameters. The site provides a method of quick determination of the extent of hydraulic-cutting damage.Using self-designed nozzle, the paper studied the the crushing effect of nozzle parameters and the pressure on the coal through field tests,acquired the mine the nozzle parameters of hydraulic-cutting measures which was suitable for the mine, improved the existing mine water jet nozzle; investigated hydraulic-cutting equipments and studied sealing technology; construction of hydraulic cutting processes were studied; used the pressure drop method to investigate effective influence radius of hydraulic-cutting based on field test which provides a reference for hydraulic-cutting hole program.The paper investigated change of gas emission during before and after construction of hydraulic-cutting, a sharp rise in the amount of gas emission during slotting, and later fall; tracking studied for gas drainage volume index, after hydraulic-cutting, the gas concentration, scalar, drainage volume has increased to varying degrees. Hydraulic-cutting drilling drainage effect is far better than the normal drainage holes. Based on different distance away from the hydraulic-cutting hole,the degree of influence of hydraulic-cutting measures on ordinary drainage holes was different, some drilling increased significantly.Using examining the sudden elimination of indicators to determine the effect of hydraulic-cutting measures relief, seclected residual gas content and residual gas pressure as eliminating sudden checksum index. Arranged check hole and measured residual gas content with direct method and result showed gas content of four holes were not overweight. Investigated by the indirect method residual gas pressure, the result was far below the critical value which prove that there was no danger of outburst in the region and verified the validity of hydraulic-cutting measures. |
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