Research On Reverse Circulation Mechanism Of Hollow-through DTH Hammer And Bit Dtructure Optimization

Several decades before, resources existing in the area that is easy to reach or in superficial stratum were terribly developed, and they become nearly exhausted. But the resources existing in some outlying areas, plateau, complex condition and deep stratum are waiting for more inputs and development. Investing mining industryand speeding up resource exploration and development turns to be a worldwide (including China) hot economic topic. Advanced drilling equipment and technology should have been researched and developed for a long period.The hollow-through DTH hammer reverse circulation(RC) continuous coring drilling technology has three advantages including high efficiency, reverse circulation and capturing cores continuouslywhile drilling. Its drilling efficiency can be 6-10 times as traditional drilling methods. Reverse circulation will never damage the wall, and it will never be restricted by water, frigid weather, fragmenting stratum, frozen stratum and hard rocks due to its unique drilling mechanism. Research on this drilling technology in our country has already been in the lead of entire world and lots of drilling problems have been solved with it. However, it is found that the RC performanceis not satisfactory enough when drilling in extremely cracked stratum or under heavy air input working condition. There is still a little bit air with small cuttings escape into the annulus under that circumstance. As a result, a small scale of dust appears on the ground and this causes a partial loss of samples. Whether the reverse circulation runs good or not is an essential problem to its promotion. And the key factor which influences RC performance most is the RC bit. The structure types, geometry parameters, nozzle, and etc. are all the important factors of reverse circulation. Therefore, it is absolutely necessary to do more and further research on RC bit structure in order to perfect this RC drilling technology. For the sake of improving slag discharge performance of hollow-through DTH hammer RC drilling system, perfecting and promoting this advanced drilling technology, the author gave three types of innovative designs of high performance RC bit including ejector-based RC bit, annulus-injecting RC bit, and interior-injecting RC bit based on research status at home and abroad. Then, numerical investigation and experiment were carried out to study the relationship between geometry variation and RC performance. It is resulted that the performance of new-designed RC bit works much better than traditional ones, and also the technique support is supplied to design high performance RC bit. More details about research and conclusions as follows:1,Numerical investigation and experiment of ejector-based, annulus-injecting, and interior-injecting RC bitNumerical simulation was carried out to investigate the geometry parameters of ejector-based, annulus-injecting, and interior-injecting RC bit with the method of EFD numerical solution. And mathematic experiment principle belonging to orthogonal design was used to process the data obtained from simulation. The regularity about how those geometry parameter variations influence the RC performance was discussed to find out the most effective parameter combination and the primary and secondary relationship among those parameters. Then, RC experiment rig was designed on the basis of EFD simulation, and it is found that the results from EFD mostly coincide with those from experiment.(1) Main geometry parameters that influence the RC performance of ejector-based and annulus-injecting RC bit include nozzle diameter, interior injector diameter, interior injector height, number of interior injector layers, nozzle position, bottom injector diameter, annulus size, etc. And geometry parameters of nozzle and interior injector have the greatest effect on RC performance among all of those above.Both of the two types of bits have the same regularityhow the RC performance is affected by those parameters. That is, the smaller nozzle diameter is the better RC performance comes; More interior injector layers and larger interior injector diameter bring better RC performance too, while interior injector height acts in an opposite manner; And further distance between nozzle and bottom injector and bigger bottom injector diameter are good for promoting RC performance; However, converge angleθC can be neglected, and, diffuser depth and diffuser angle give little effect on RC performance either.(2) Key geometry parameters of interior-injecting RC bit are interior injector diameter, numbers of interior injector layers and interior injector height, separately. According to its research, smaller interior injector diameter and fewer layers are usually good for RC performance and this regularity is totally contrary to ejector-based and annulus-injecting RC bit, although interior injector height has the same influence to RC performance among those three types of RC bits---the smaller the greater.(3) Results from numerical investigation and experiment show that, interior-injecting RC bit has the most powerful RC capability, then annulus-injecting and ejector-based RC bit are separately ranked second and third, and traditional RC bit comes to the last place. Cuttings and sand grains existing in the annulus can be easilysucked up, and this gives a sufficient proof that these three new-designed bits have extremely high RC performance.2,Numerical investigation and experiment of how the boundary condition affects RC performanceBoundary condition is another critical factor including system air input (QV) and central exhausting tube back pressure (PCEN),and these two boundary condition parameters affect RC performance as well。There are two kinds of key QV. One is called "inflection point" air input, namely, the RC performance will come to the best when QV equals to it. The other is critical air input (QV0)including left critical air input (QV01) and right critical air input(QV02).When QVQV02, air leakiness happens in the annulus again, worsening RC performance greatly. Consequently, system air input must be controlled within an appropriate range for the sake of satisfying RC performance. As to PCEN, increasing it is disadvantageous to reverse circulation, and the chance of air leakiness in annulus rises too. However, poor RC efficiency caused by largePcEN can be avoided by augmenting QV.Main innovative aspects in this dissertation include:(1) Three new types of RC bits were designed based on inheritance and innovation of RC mechanism and RC bit structure researched before by other researchers. Experiment shows that, the three of them can obviously promote RC performance, and interior-injecting bit is proved to be the best of them.(2) Innovative reverse circulation mechanism was put forward that two kinds of key system air input ("inflecting point" air input and critical air input) are extremely essential to RC performance, and both of them change while central exhausting tube back pressure changes. As to some specified issues, finding out key system air input is the guarantee of satisfactory RC performance during drillings.(3) Based on the experience of other experiment rig design before, new-designed experiment rig can test more geometry parameters, and it is also much more replaceable.(4) The EFD simulation was firstly employed to research RC bit geometry parameters and structure optimization systemically. And, orthogonal design and regression design were utilized to study RC bit flow field and conduct experimental design for the first time too.According to the results from simulation and experiment, there is reliability and accuracy using EFD technique to deal with RC bit structure optimization. And, orthogonal design and regression design can save lots of time in numerical investigation and experiment, andmake the experiment much more scientific and efficient.