| At present,the total global energy consumption continues to rise.Since the largescale use of primary energy has led to the aggravation of climate warming,the development of renewable and clean energy can contribute to mitigating climate warming and improving the energy mix.Hot dry rock(HDR)resources have the advantages of low carbon and environmental protection,wide distribution,stability and efficiency,and huge reserves,and are a clean renewable resource with great development potential.But the deep burial and poor drillability of HDR formations leads to a long drilling time and high drilling cost.To improve the rate of penetration(ROP)of hard rock and reduce the drilling cost of HDR resources,we developed a down-the-hole(DTH)fluidic hammer with high impact energy,and field tests show that this type of DTH hammer can increase the ROP of hard rock formation several times.However,the huge impact energy also poses a serious challenge to the structural safety of the DTH fluidic hammer itself.In many tests and experiments,the fluidic oscillator made of tungsten carbide underwent brittle fracture under the action of the stress wave generated by the impact in the DTH hammer,which seriously hindered the engineering application of the fluidic DTH hammer.To effectively protect cemented carbide fluidic oscillator,combined with the characteristics of the DTH hammer and the high temperature drilling conditions of HDR,this paper proposes to use the layered structure with impedance mismatching as the stress wave protection structure of fluidic oscillator.In this paper,the characteristics of different layered structures were analyzed theoretically,the stress wave attenuation characteristics of layered structures were studied by experimental and numerical simulation methods,and the optimal design of impact resistance characteristics of fluidic oscillator were carried out.The relevant research contents and main conclusions can be encapsulated as follows:(1)Based on one-dimensional stress wave theory,the propagation characteristics of stress wave in layered structure were analyzed.First of all,the difference of stress wave attenuation characteristics between impedance decreasing structure and impedance alternating structure was compared.It is found that the impedance alternating structure is more suitable for the application in drilling engineering.Then the complexity of stress wave propagation in layered structures was analyzed,and the split Hopkinson pressure bar(SHPB)technique was proposed to study the problems.At last,the difference of the waveforms produced by the mass block and the SHPB impact bar was compared to illustrate the rationality of using the SHPB technology for research.The comparative studies indicate that,when impact velocity is same,the wavelength of the stress wave generated by the mass block is smaller than that of the SHPB impact bar,and the stress wave amplitude of the two types of waveforms is the same,so it is reasonable to use SHPB technology to study the stress wave attenuation characteristics of layered structures.(2)The stress wave attenuation performance of steel-poly ether ether ketone(PEEK)layered protective structure(SPLPS)was investigated.Firstly,the accuracy of the numerical simulation model was verified by SHPB experiment,and the stress attenuation performance of SPLPS was analyzed through experiment and numerical simulation.And it is found that the stress amplitude attenuation ratio of 5-layer SPLPS is 77%.Then the effects of the number of layers and the thickness of the interlayer on the attenuation performance of the layered structure were studied respectively.The number of layers has a great influence on the stress attenuation of SPLPS,but when the number of layers is more than 4,the attenuation ability of layered structure does not increase significantly.The stress attenuation effect of SPLPS does not continue to increase with the extension of the interlayer thickness,and the interlayer thickness of 6mm-8 mm has the optimal stress wave attenuation effect.Subsequently,the results of the bench test and HDR well drilling experiment show that SPLPS has good stress wave protection ability.Finally,the shortcomings of the layered structure of homogeneous materials in high-temperature drilling engineering were analyzed.The results illuminate that,for the working conditions where the ratio of transient wave wavelength to the total length of the structure is large,a higher impedance ratio can conducive a better stress attenuation performance of the layered structure.But limited by the insufficient high temperature mechanical properties of low impedance materials,the impedance alternating structure of homogeneous materials cannot obtain sufficient stress wave attenuation performance in high temperature environment.(3)The stress wave attenuation performance of the layered structure of steelcarbon fiber/glass fiber reinforced plastic(CFRP/GFRP)was studied.Firstly,the difference of stress attenuation properties of CFRP specimens in the ply direction and thickness direction was explored,and the monomer properties of CFRP and GFRP materials were compared.The experimental results present that,the stress attenuation performance of CFRP materials in the ply direction is greater than the attenuation capacity in the thickness direction,but the mechanical strength in the ply direction is very low and cannot be used in stress wave protection structures.Then,for the steelCFRP two-layer structure,the influence of CFRP layer thickness on the attenuation capacity was studied.The results show that increasing the thickness of the CFRP layer is beneficial to improve the stress wave energy attenuation ability of the layered structure.Finally,the stress wave attenuation performance of CFRP and GFRP threelayer structures under different arrangements was compared and analyzed.It is found that the CFRP-steel-CFRP three-layer structure has the maximum stress attenuation capacity,and its attenuation ratio on stress amplitude and energy are 12% and 34.7%,respectively.(4)The stress wave attenuation performance of steel layered structure with variable contact area was studied.Firstly,the design concept of layered structure with variable contact area was proposed.The accuracy of the numerical simulation model with variable contact area was verified by SHPB experiment.Then the effects of contact area ratio,contact orientation and number of layers on the stress wave attenuation characteristics of layered structures with variable contact area were analyzed respectively.The analyses reveal that the contact area ratio,the contact orientation and the number of layers can significantly affect the stress wave attenuation performance of the layered structure with variable contact area.A larger contact area ratio always contributes a better stress wave attenuation capacity of the layered structure.When the contact angle between adjacent structures is 90 ° and the number of layers exceeds 4,the layered structure with variable contact area can obtain optimal stress wave attenuation effect.Finally,the optimization measures of variable contact area layered structure in the application of DTH hammer drilling engineering were given,including adding connecting columns and avoiding sharp angles in the layered structure.(5)The structure optimization of the impact resistance characteristics of the fluidic oscillator was carried out.Firstly,the main factors leading to the rupture of cemented carbide fluidic oscillator were discussed,the stress concentration characteristics of fluidic oscillator under the action of stress waves were analyzed,and the influence of increasing rounded corners on the peak stress and stress concentration coefficients at right angles inside the fluidic oscillator was further studied.The results show that the fluidic oscillator has a significant stress concentration phenomenon under the action of stress wave,and increasing the fillet can reduce the peak value of stress wave and stress concentration coefficient at the local right angle.Then,by analyzing the assembly relationship in the DTH hammer,the homogenization effect of the layered structure on the stress wave distribution in the deflected contact state was studied.It is found that the layered structure can effectively avoid the uneven distribution of stress waves inside the fluidic oscillator while attenuating the stress waves. |
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