| Coal and gas outburst,rock burst are seriously dynamic disasters in coal mine.Once occurred,they would not only directly cause casualties and economic losses,but also induce disasters such as gas explosions.However,with an accurate prediction,those disasters could be avoided after taking effective preventive measures.Field tests show that an abnormal electromagnetic radiation(EMR)usually occurs before the dynamic disasters in coal mine,as well as the short-term,rapid and concentrating micro-fracturing,as result of the abruptly increasing stress.Hence,it would be likely to reduce the risk of these accidents through real-time monitoring EMR,which can give an early warning in time.In order to reveal the EMR mechanism and improve the prediction accuracy of disasters using EMR,great deals of researches have been carried out in theories,experiments and field tests,thus resulting in fruitful results.However,through analyzing those existing researches,it is found that the studies focus on the qualitative analysis of EMR during the static loading or dynamic compression of coal and rock,but it still is unknown what characteristics of EMR when coal and rock are under dynamic tensile.Furthermore,there are still such many researches to be further studied as the mechanism,theoretical models and quantitative indexes of EMR.Therefore,it has a theoretical significance for the predication of dynamitic disasters to study the EMR during fracturing of coal under dynamic tensile,and establishing the coupling model between the EMR and the force in coal mine.Based on the above analysis,this paper focused on the theory analysis,experiment research,numerical simulation and field tests to study the mechanism and characteristics of EMR under dynamic tensile.In the theory study,based on the present researches of EMR,the EMR mechanism of coal under dynamic tensile is put forward,and the source and the movement of charges are elaborated;combined with the gas dynamic and seepage mechanics,the dynamic change of gas pressure difference between the inner and the outer of coal particle is deduced;according to the relationship between dynamic fracture mechanics and static fracture mechanics,the dynamic stress intensity factor of crack under the gas pressure difference is deduced;in accordance with the EMR theory,the fracturing-induced EMR is simplified as an electric dipole,then the EMR field during fracturing is derived;Taking the fracturing area as a link,the relationship between EMR and stress of coal under dynamic tensile is established.In the experiment study,according to the experiment requirements,a non-metallic pressure-bearing cavity and a starting device for pressure relief are designed,and then an experiment system is set up;the EMR signals during the fracturing of coal particles under different pressures of carbon dioxide,nitrogen and methane are tested;the original EMR signal is denoised using the theory of digital signal processing,and then the effective signal is obtained;characteristics of the effective signals including the waveform,frequency,peak and energy are analyzed,at last,the EMR mechanics and the experiment results are verified with each other.In the numerical simulation study,the process of pressure relief is simulated using FLUENT software;according to the seepage model,the gas infiltration process in coal particles is numerically calculated using MATLAB software;the blasting process of coal is simulated using LS-DYNA software.(1)Study on the fracturing mechanism and EMR mechanism of coal under dynamic tensileThe EMR mechanism of loaded coal includes two parts: the charge source and the charge movement.When the coal is loaded under the dynamic tensile,the stress concentration at the crack tip would break the equilibrium state of atoms or molecules and then result in the charge separation.With the crack expanding,the stress concentration and the stress release occur alternately at the crack tip,resulting in crack oscillation due to the elastic waves.At the same time,the charges on the crack walls would move along with the crack oscillation,resulting in EMR.Coal is a material with a double-porosity structure.When the methane,carbon dioxide and nitrogen are adsorbed onto the pore surface,the coal strength would decrease as a result of expanding of coal body.According to the damage theory,the strength damage factor of coal containing gas is deduced based on the effective bearing area and deformation.It shows that the damage factor increases with the increase of porosity and adsorbed deformation of coal.Flow of coal and gas is simplified as a homogeneous flow,and then the pressure declining law of gas with a chock flow is deduced based on the gas dynamics theory.It shows that the gas pressure in pressure-bearing cavity decays exponentially with time.Assuming that the coal particle is spherical,homogenous and isotropic,the seepage model is established based on the seepage mechanics without taking gas desorption,gas diffusion and heat exchange into account.Its analytical solution is obtained using mathematical and physical methods.It shows that the pore pressure of coal particle decays exponentially with time.The dynamic change of gas pressure difference between the inside and the outside of the coal particle is obtained combined the pressure variation between the pressure-bearing cavity and the coal particle.The numerical results show that the gas pressure difference rises to the peak within a very short time,and its peak is approximately equal to the original gas pressure.Assuming that the crack is elliptical,the ?type dynamic stress intensity factor of crack under gas pressure difference is obtained based on the theory of elasticity and fracture mechanics.It indicates that the stress intensity factor is positively related to the gas pressure.According to the dynamic fracturing criterion,the relationship between the fracturing area of coal particle,gas pressure and adsorption deformation is deduced.It indicates that the larger the gas pressure and the adsorption deformation,the larger the fracturing area of coal particle is.Assuming that the generated charges are proportional to the fracturing area,and the crack walls is an oscillated electric dipole,the relationship between EMR and gas pressure,adsorbed deformation is deduced according to the electromagnetic theory.It indicates that the EMR intensity is not only influenced by the gas pressure and the adsorption,but also by the field position and oscillation frequency of crack walls.(2)Study on the characteristics of fracturing coal under dynamic tensileThe nonmetal pressure-bearing cavity and the starting device for the pressure relief are developed.This experiment system can meet the sealing of gas pressure of 0-2 MPa and rapid relief.The experiment system is used to test EMR signal during ejection of coal within 6-10 mm and gas with pressure of 0.60,0.75,0.90,1.05,1.20,1.35,1.50,1.65 and 1.80 for methane,carbon dioxide and nitrogen.The results show that the coal samples are broken after gas pressure release.The distribution of mass of coal particles with original size and fracturing sizes after pressure relief is analyzed.Results show that the larger the particle size,the greater the mass is,and both of them have a good linear relationship.The distribution of areas of the coal particles with original size and fracturing size after pressure relief is analyzed.Results show that the larger the particle size,the smaller the surface area of the corresponding coal particles,and the relationship between them meet an exponential function.The influence of gas pressure on mass of coal particles with the original size and fracturing sizes is analyzed.Results show that the mass of coal with original size is negatively correlated with the pressure,and both have a good linear relationship.The mass of the coal particles with fracturing size is analyzed,and both have a good linear relationship.The influence of gas pressure on the area of coal particles with fracturing size is analyzed.Results show that the larger the gas pressure,the larger the surface area and total fracturing area.The mass and surface area of coal particles with fracturing size under N2?CH4 and CO2 are compared.Results show that the relationship between the mass of the coal particles under the three types of gases is: the methane group>the nitrogen group>the carbon dioxide group,surface area relationship is the nitrogen group> the methane>the carbon dioxide group,and fracturing area is the methane group> the nitrogen group>the carbon dioxide group.However,these relationships are not significant.The fracture theory model and experimental results under dynamic tensile are compared.It shows that the relationship between the fracturing areas and pressures in experimental results is in good agreement with the fracture theory model.However,the relationship between the fracturing areas and adsorption capacity in experimental results is not in good agreement with the fracture theory model.(3)Study on characteristic of EMR signals of coal under dynamic tensileThe nonmetal pressure-bearing cavity and the starting device for the pressure relief are developed.This experiment system can meet the sealing of gas pressure of 0-2 MPa and rapid relief.The experiment system is used to test EMR signal during ejection of coal within 6-10 mm and gas with pressure of 0.60,0.75,0.90,1.05,1.20,1.35,1.50,1.65 and 1.80 for methane,carbon dioxide and nitrogen.The results show that EMR signals appeared abnormal during the fracturing of coal particles.Through comparing the difference of the time-domain and frequency-domain characteristics between the effective EMR signal and the noise signal,it is found that the amplitude of the effective EMR signal increases significantly in the time domain,while the frequency of the noise signal is mainly 50 Hz and its multiple.According to the characteristics of the frequency domain and the time domain,FIR filter and IIR filter are used to reduce the noise.Results show that this method can effectively reduce the noise,but remain the effective EMR signal.On the whole,the waveforms of effective EMR signals appear three types,namely harmonics,unimodal pulses and bimodal pulses,while the effective signal is composed of several to tens of pulses with different amplitudes in the local.The number of pulses of effective EMR signals is counted.It is found that it increases with the increase of gas pressure.The number of pulses received by two antennas shows a proportional relationship.The frequencoes of the effective EMR signal are 0-200 Hz,which belongs to the ultra-low frequency and very low frequency signals.Results show that it has no significant relationship with the gas properties and the gas pressure.The frequencies of EMR signals received by the two antennas are approximately equal.The peak value of effective EMR signal ranges between 0.05 and 1.32 mv,which is not significantly affected by the gas properties,but has a positive correlation with gas pressure.The peak values of EMR signals received by two antennas are different,but in a positive relationship.The sum of squares of the effective EMR signal is used as the energy.Results show that energy has no obvious relationship with the gas properties,but increases with the increase of gas pressure and fracturing area.The energy of EMR signals received by two antennas are in a positive relationship.The characteristics of effective EMR signal are reasonably explained according to the EMR mechanism under dynamic tensile.And the force-electric coupling model is verified by the experiment results.(4)Numerical simulationFLUENT software was used to simulate the dynamic changes of gas pressure during pressure relief of pressure-bearing cavity under initial pressures of 0.60,0.90,1.20,150 and 1.80 MPa.Results show that gas pressure has decreased to the minimum after about 170 ?s,and the closer the location is from the pressure relief opening,the faster the gas pressure decays.According to the seepage model,the finite difference equation is established using the finite difference method,and the dynamitic changes of pore pressure during gas seepage under the initial pressures of 0.60,0.90,1.20,150,and 1.80 MPa is programmed using MATLAB software.Results show that the pore pressure decreases from the outside to the inside as the time increases,and the greater the initial pressure,the faster the pore pressure decays.Combining the numerical simulation results of gas pressure in cavity and the pore pressure in coal particles,the dynamic changes of gas pressure difference between the inside and outside of coal particles during the pressure relief process is obtained.Results show that the gas pressure difference increases quickly first,and then decays slowly as time increases,and the peaks of pressure difference are close to the initial pressure.The closer to the center of coal particles,the slower the gas pressure decays.According to the peridynamics theory,the dynamic fracturing process of crack under pressure difference is simulated using Fortran language.The results show that the crack propagates along the x-axis and y-axis as loading time increases.The relationship between the number of damaged units and the gas pressure under different size crack is simulated.The results show that the greater the gas pressure,the greater the number of damaged units.The simulation results and experimental results were compared and analyzed,which showed that the numerical simulation results were consistent with the results of experiments. |
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