Thermal-Mechanical Response Mechanism Of Coal(Rock)Under Microwave-Moisture Coupling

Coal(rock)is fractured or even crushed after absorbing microwave,so microwave technology has been studied and applied in the field of rock(coal)bed dynamic disaster prevention.In the engineering site of coal seam dynamic disaster prevention using microwave fracturing technology,microwave propagation in coal seam coupled with pore water and finally reached the rock roof.In view of the basic process of temperature change,structural evolution and mechanical property deterioration of the cracking induced by microwave on coal(rock)materials,the response characteristics of thermal,structural and mechanical behavior of coal and common coal seam roof rock—sandstone after microwave absorption are studied,especially the influence of water on the microwave effect.The study of the influence of pore water on the thermo-mechanical response of rock materials under microwave could further enrich the control theory of microwave pressure relief and shock reduction of coal seam.The main research results are as follows:(1)The temperature distribution characteristics of sandstone in the direction of microwave propagation were analyzed,and the penetration depth of microwave to rock materials was determined by theoretical analysis.With the development of microwave heating,the thermal effect of microwave-moisture coupling is the main reason for the change of rock temperature distribution in the direction of microwave propagation.According to the distribution characteristics of sandstone structure,it was determined that the generation of intragranular fracture was the main reason leading to microwave damage and even fracturing of sandstone.The water content in sandstone was related to the heating duration and propagation distance of microwave,which could be determined by poly function.(2)The combustion of bituminous coal under microwave-moisture coupling was studied.It was found that the time required for bituminous coal to ignite was shortened with the increase of microwave power and lengthened with the increase of coal permeability.In addition,there was a permeability threshold for coal samples heated by a certain microwave power.Below the threshold,the microwave ignition time of water-bearing coal sample was shorter than that of dry coal sample,while above the threshold,the situation was opposite.Under the condition of a certain amount of microwave energy output,the increase of microwave power could change the loading characteristics of pore steam pressure,so as to produce a mechanical effect similar to constant volume expansion.Short-duration combined with high-power microwave output was more conducive to the formation of fracture network in coals.(3)The dynamic and static mechanical properties of water-containing bituminous coal after different microwave heating times were studied.It was found that the microwave fracturing of water-containing coal samples was time-sensitive.With the increase of microwave heating time,the coal samples changed from shear-tensile failure to tensile failure during static failure,and the fragmentation degree became more uniform during dynamic failure.The peak elastic energy density of coal samples decreased with the increase of microwave heating time,and the coal samples developed from strong dynamic failure to weak dynamic failure.Based on the damage mechanics theory,a mechanical constitutive model considering microwave damage and load damage was established and verified.(4)The influence of microwave heating path on the energy evolution and fracture characteristics of coal and rock composite structures during loading was studied.Under the action of high power combined with short duration microwave fracturing,the energy storage capacity of coal body and rock body decreased obviously and slightly increases,which led to the decrease of the density difference of elastic energy in the coal-rock system.In the process of coal rock structure being loaded,the fracture acoustic signals mainly came from coal body,and the waveform signals were concentrated on the direction of low energy and long time due to the increase of micro-crack density in coal body.Finally,on the basis of the above research,the engineering application scheme of coal seam microwave pressure relief and shock reduction technology was proposed,and the experimental method for determining the reasonable microwave fracturing energy were established.