Experimental Investigation on Mechanical and Acoustic Emission Characteristics of Gassy Coal under Different Stress Paths.

Coal mining leads to stress loading-unloading variation in front of the working face, which influences the occurrence of disasters. In order to study the influence mechanism of stress loading-unloading to the coal failure, a series of experiments of gas-bearing coal deformation and failure under triaxial stress were conducted and acoustic emission (AE) was monitored. In this study, the effect of gas pressure on the mechanical behavior of gas-bearing coal in conventional triaxial stress (CTS) experiments and fixed axial stress and unloading confining stress (FASUCS) experiments was analyzed, and the damage evolution rules of gas-bearing coal in the CTS experiments and FASUCS experiments were determined using AE. The results show that with the increasing of gas pressure, the peak strength and peak strain of gas-bearing coal in the CTS experiments and FASUCS experiments gradually decrease, and the peak of AE ring-down counts lags behind the peak strength. Compared with the CTS experiments, the strength of gas-bearing coal in the FASUCS experiments is lower and the precursor information appears later. The trends in calculated stress and damage coefficient D are consistent with the stress path during unloading, and both begin to rise sharply after the sample enters the plastic stage. Therefore, AE ring-down counts, damage coefficient D, and calculated stress can be used as precursor information for failure of coal and rock, which has great significance for the further study of coal-rock material and for early hazard warning.

Study on the dust removal and temperature reduction coupling performances of magnetized water spray.

Dust pollution and heat damage hazards are important problems affecting underground safety production. This paper is aimed at exploring the optimal magnetization conditions of magnetized water for dust removal and temperature reduction and improving the utilization rate of water. First, the surface tension, viscosity, and specific heat capacity of water under different magnetization conditions were measured experimentally. Then, the influence law of ejection pressure on spray atomization and the changes of dust removal performance before and after magnetization of water were analyzed. Based on this, the temperature reduction effect of magnetized water under different wind speeds was analyzed, and the magnetization conditions with the best coupling performance of magnetized water were obtained. Finally, a spray system was designed to control the magnetization conditions strictly. The results demonstrate that the dust removal performance is better when the magnetic field intensity is 150 mT and the magnetization time is 80 s. Under this condition, the specific heat capacity also reaches the maximum. These research results can provide a theoretical basis for the selection of dust pollution and heat damage control measures in mines.

Influence of Confining Pressure on Nonlinear Failure Characteristics of Coal Subjected to Triaxial Compression.

The stress of a coal seam increases with an increase in the mining depth, which makes the failure mechanism of a coal mass more complex. To reveal the deformation and failure law of deep coal, a series of triaxial experiments was carried out via laboratory experiments and numerical simulation experiments to analyze the influence of the confining stress on the nonlinear failure characteristics of coal. Based on the crack-propagation model, the values for the inelastic flexibility S1 and the damage variable D were calculated. The results showed that the value of S1 decreased with an increase in the confining stress, which indicated that the increase in the confining pressure could inhibit the crack propagation and that the inhibitory effect was more obvious when the confining pressure increased in a small range of 4 to 12 MPa. The damage variable decreased with an increase in the confining pressure at the yield point; moreover, with an increase in the initial confining pressure, the damage rate gradually decreased. The coal body changed from the compression state to the expansion state when moving from the yield point to the peak point, and the compression value of the yield point and the dilation value of the peak point increased with the increase in the confining pressure. After the coal body entered the yield stage, the change in the confining pressure had a more significant effect on the damage to the coal body.