| With the increase of coal mining depth,the difficulty of coal mining is increasing day by day.Because the traditional 121 mining method needs to set up roadway coal pillars,the waste of resources is serious.Under this background,the 110 mining method came into being.The110 mining method can effectively improve the coal recovery rate and reduce the amount of roadway excavation.Safe and fast cutting of roof rock is the key of self forming roadway technology without coal pillar pressure relief.At present,the main seam cutting timing includes shaped charge blasting seam cutting technology and hydraulic fracturing seam cutting technology,but these technologies have great hidden dangers in the application of high gas mines and goaf overlying the working face,while the abrasive water jet cutting technology has the characteristics of no open fire,easy operation,safety and stability,high controllability,good cutting efficiency and wide application range,which can achieve a better alternative role in the above environment.In the abrasive water jet,the post mixed abrasive water jet has better economic benefits and can better adapt to the complex underground environment.However,the current post mixed abrasive water jet system has low timeliness and shallow cutting depth in cutting hard and thick roof,so it is difficult to be competent for the cutting of hard and thick roof.To solve this problem,this paper selects the limestone with high hardness as the target material,deeply explores the cutting principle and process of ultra-high pressure post mixed abrasive water jet by means of theoretical analysis,analyzes the relevant factors affecting the cutting effect of ultra-high pressure post mixed abrasive water jet through the combination of numerical simulation and experimental research,and optimizes the parameters of ultra-high pressure post mixed abrasive water jet based on the analysis results.At the same time,the application process parameters are optimized and adjusted based on the field test.The main results are as follows:(1)Based on the theoretical analysis,the selection range of target distance during abrasive jet cutting in the core section of the jet is clarified.Through the law of energy conservation and Bernoulli equation,the water jet velocity model at the outlet of the abrasive jet nozzle and the acceleration model of abrasive particles are established.The model is solved by the integral iteration method and verified by fluent numerical simulation.The results show that the velocity of abrasive particles will increase to a range with the increase of moving distance,but it is still less than that of water jet.(2)By observing the section characteristics of abrasive jet,the tailing phenomenon on the section is found,and the formation mechanism of tailing phenomenon is analyzed in detail by mixing non-uniformity theory and step theory.The results show that there are deficiencies in using the uneven mixing of abrasive particles and water jet in abrasive jet to explain the generation mechanism of tailing phenomenon.From the perspective of step theory,it is concluded that the energy of abrasive particles and water jet is not unified.When encountering cutting steps,the water jet will deflect greatly,while the deflection angle of abrasive particles is small.With the movement of abrasive jet,Abrasive particles will constantly impact the cutting step,and a "incomplete cutting belt" will be formed in front of the step,resulting in the tailing phenomenon.(3)The physical model of the post mixed abrasive water jet nozzle is established based on cfd-fem,and the coupling analysis of the mixing of water jet and abrasive particles is carried out.The results show that under the same working conditions,the velocity of abrasive particles gradually increases with the increase of the diameter of the water nozzle in the post mixed abrasive water jet nozzle,and the velocity of abrasive particles gradually decreases from the axis to the edge on the outlet section,And it is concentrated near the outer edge of the jet.The change of the position and angle of the abrasive inlet will affect the movement time and trajectory of the abrasive particles in the nozzle.Under the influence of a single factor,the outlet velocity of the abrasive particles is the largest when the distance between the abrasive inlet and the water nozzle is 3mm and when the angle of the abrasive inlet is 60 °.Based on the impact index K,which represents the impact strength of abrasive particles,an orthogonal experiment is designed for variance analysis.It is concluded that the weight affecting the impact index k of abrasive particles is abrasive inlet angle > abrasive inlet position > water nozzle diameter,and the best nozzle structure parameters are water nozzle diameter 1mm,abrasive inlet position and angle 0mm and 60 ° respectively.(4)Through the cutting experiment of limestone target with ultra-high pressure post mixed abrasive water jet cutting device,the effects of system pressure,abrasive flow,nozzle transverse velocity,jet target distance and jet cutting angle on the cutting depth of abrasive jet are analyzed respectively.The results show that the cutting depth of abrasive jet will increase with the increase of system pressure,but the growth rate of cutting depth decreases under high pressure;When the abrasive flow rate increases,the cutting depth will also increase.When the abrasive flow rate exceeds 0.6kg/min,increasing the abrasive flow rate will reduce the cutting depth;With the increase of nozzle transverse velocity,the cutting depth of abrasive jet will gradually decrease to a fixed value;With the increase of cutting angle,the cutting depth of abrasive jet presents an M-shaped change,and the cutting depth is the largest when the angle is 80 °.Through orthogonal analysis and range method,the weight relationship of the influencing factors is explored.It is concluded that the influence of the influencing factors on the cutting depth is as follows: system pressure > nozzle traverse speed > abrasive flow > nozzle cutting angle > jet target distance.The best combination of cutting parameters is system pressure400 MPa,abrasive flow 0.8kg/min,nozzle traverse speed 1mm / s,jet target distance 6mm and jet cutting angle 80 °.(5)Through the trial cutting test in Shaqu No.2 mine,Shanxi Province,the field application is further optimized.Considering the shape of the advancing speed of the working face and the characteristics of the rock properties of the roof,the relationship between the cutting depth and the moving speed of the nozzle is compared and verified with the experimental results.Through the field measurement,it is verified that the tailing deflection angle will increase with the increase of the cutting depth. |
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