| Coal industry is the important basic industry related to country’s economic lifeline andenergy security. In recent years, although our country has been to adjust energy structure andreduce the proportion of coal in energy consumption. The proportion of coal in primary energystructure remains above60%and in the future for a long time, the status of coal as the mainenergy will not change due to the main energy status of coal in our country. Due to theunderlying weakness of China’s coal mine safety, the complicated production conditions of coalenergy and the low overall technological level of coal production, a variety of disasters andaccidents occurs, which results in huge losses of state property and people’s lives. Thus the coalindustry has become the most serious casualty industry in China.According to relevant statistics survey, the annual number of deaths and death rate permillion tons in coal disaster is gradually declining though the coal production is increasing yearby year in China. With the basic formation of laws and regulations on coal mine safety, thegradual improvement of economic policy and the further consolidation on safety management ofbasic work in recent years, the situation of safe production has sustainably and stably improved.However, there is a big gap in safety production situation with a comparison to advancedcoal-producing countries in the world. Analyzing the reasons, in addition to the complicatedgeological conditions of coal mining and the difficulties in production, the huge gap ofmechanization degree between advanced coal-producing countries and our country, the rescuetechniques and equipment is far behind other advanced coal-producing countries.According to the low penetration rate, high frequency of drilling accidents in fast rescuedrilling in fractured and complicated formations, the reverse circulation Down-the-Hole (DTH)air hammer drilling technology has been proposed to be applied to rapid mine emergency rescuedrilling in this paper. The new type of reverse circulation drilling bit with powerful suction ofannular jet has been designed on the basis of Coanda wall-attachment effect, which is applicableto drilling in complicated formations. The suction properties and reverse circulation formation characteristics of reverse circulation drilling bit with annular jet has been studied by means ofnumerical simulation and laboratory experiments. Meanwhile, the comparative analysis onexisted reverse circulation drilling bit with inner suction nozzle has been accomplished. Thecontents and conclusions of this paper are as follows:(1) It has been concluded that the drilling technology with positive circulation DTH airhammer should be more popular according to the summary on successful cases of mine rescue inrecent years. However, on the practice of drilling technology with positive circulation DTH airhammer, the air is difficult to effectively form a cycle while drilling in fractured and complicatedstratum and the discharge of debris or core is difficult. Moreover, the consumed amount of air islarge, the penetration rate is low, the service life of drill bit is short, and the frequency ofunderground drilling accidents is high. In view of the characteristics and advantages of reversecirculation drilling technology with DTH air hammer, it can be applied to the mine rescuedrilling, and effectively solve the problems while drilling with positive circulation drillingtechnology in some working conditions, which could make a contribution to the improvement ofdrilling technology and equipment level in rapid emergency and rescue drilling.(2) The successful application of reverse circulation drilling technology with DTH airhammer to production and engineering construction can brought huge economic benefits fordrilling projects, and the reverse circulation drill bit, as the key part of reverse drillingtechnology, will play an important role. Field test results have shown that the failure of flushnozzle in the bottom of reverse circulation drill bit lead to the poor formation of reversecirculation, easy return of compressed air in orifices, overdue discharge of debris in bottom ofdrill hole and low penetration rate. The location of suction nozzles of drilling bit is changed andthe flush nozzles are blocked while the actual effect of reverse circulation is still imperfect inexperiments field. Therefore, the structural parameters of reverse circulation drilling bit still needfurther improvement and optimization.(3) Due to the flow direction of compressed air can be changed by wall-attachment effect,and a large negative pressure zone can be formed inside the nozzles, it can be found that the airamplifier and the annular jet structure of two-stage annular foam breaker can have a strongsuction effect. Thus, a new type structure of drill bit with cyclic ejection nozzles has beenproposed in the innovative design of reverse circulation DTH drill bit, and the conventional structure with suction nozzles has been updated to new structure with annular ejection nozzles,which can make a contribution to the improvement of reverse circulation formation.(4) On the basis of a reverse circulation drill bit in diameter of180mm, the internal flowfield of reverse circulation drill bit with internal ejection nozzles and the new drill bit withannular ejection nozzles has been numerically simulated by computational fluid dynamics (CFD)software FLUENT. The simulation results have shown that reverse circulation drill bit withannular ejection nozzles is better than the drill bit with internal ejection nozzles regardless of theexistence of the flush nozzles in the bottom of drill bit. The drill bit with annular ejection nozzlescan have a greater negative pressure inside the drill bit, and a strong suction function can also beachieved. Furthermore, the suction coefficient of reverse circulation drill bit with cyclic ejectionnozzles is1.3times as that of drill bit with internal ejection nozzles.(5) The designed imitated experiment apparatuses and the features of experimentapparatuses with reverse circulation drill bit has been summarized and analyzed according todifferent research content in multi-process percussive and rotary drilling team. Meanwhile, animitated tester JJ-180-00has been innovatively designed by the combination of projectrequirements and the former design experience of imitated tester with reverse circulation drillingtechnology. In addition, the whole test system and each part of the imitated tester, includingexperiment apparatus and measuring instruments, have been introduced in detail.(6) Using graphical programming language LabVIEW platform set up a multi-channel dataacquisition recording system. The pressure and suction flow of reverse circulation drilling bit canbe dynamic monitoring.(7) The reverse circulation drill bit with annular ejection nozzles has been tested and studiedby stepwise optimization on the basis of imitated tester of JJ-180-00. Test results have shownthat, as to the reverse circulation drill bit in diameter of180mm, the formation of reversecirculation effect is better while the annular gap is1mm, than that of the gap are0.5mm、1.5mmand2mm. The lower annular nozzles from bottom, and the smaller spray angle, the better effectof the suction property and the formation of reverse circulation effect will be.(8) the reverse circulation effect has been experimented which the flow area is differentthrough the way of symmetrical block the inner nozzles. Results show that the symmetryreduction in number of nozzle (reduce the flow area) can improve the suction performance of drill bits, but flow area is too small easy to cause pressure-out in the air inlet.(9) Contrast to the pressure in bottom hole and the suction volume of compressed air inannulus space while the reverse circulation drill bit is internal ejection nozzles and annularejection nozzles, it can be concluded that the actual effect of reverse circulation drill bit withannular ejection nozzles is better than the drill bit with internal ejection nozzles whether or notthe flush nozzles are existed in the bottom of drill bit. |
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