Yong Liu

The kinetic energy and the phase-change refrigeration effect generated during the process of high-pressure liquid carbon dioxide (CO2) sprayed into a lower ambient pressure enable the CO2 jet to be widely applied in the cutting and refrigeration fields. The intense phase-change at the nozzle outlet region induces the generation of a downstream Mach disk, which subsequently affects the jet impact performance. This study comprehensively uses high-speed photography, numerical simulation, and impact experiments to analyze the influence mechanism of the Mach disk structure on the impact performance of the CO2 jet. The experimental results show that the position of the Mach disk is positively correlated with the throat diameter of the nozzle under a constant operation condition. The convergent–divergent nozzle can induce the liquid CO2 to undergo a phase-change inside the nozzle in advance, thereby reducing the static pressure at the nozzle outlet and inhibiting the formation of the Mach disk. For the under-expanded state of a CO2 jet, the liquid mass fraction and velocity of the CO2 downstream of the Mach disk rapidly decrease, the pressure increases, and the temperature first increases and then decreases. When the under-expanded CO2 jet impacts the target plate, a Mach disk structure exists in the free flow field, and a plate shock wave structure forms on the target surface. The distance between the two is a key factor affecting the impact performance of the CO2 jet. In the current research, the Mach disk is located 7.4 mm downstream of the nozzle outlet. The impact performance of the under-expanded CO2 jet within a target distance of less than 10 mm increases as the target distance increases. However, when the target distance exceeds 10 mm, the impact performance of the CO2 jet drops sharply. This indicates that the Mach disk in the free flow field and the plate shock wave in the impact flow field jointly determine the impact performance of the CO2 jet. When the positions of the Mach disk and plate shock wave overlap, the influence of the Mach disk can be ignored regarding the impact performance of the jet, and the impact performance of the jet rapidly decreases after the two separate.

Large-diameter reaming without water in coal seam has constantly been an essential demand for gas control in coal mine. To clarify the motion characteristics of FCTS under different reaming parameters, an experiment was carried out to monitor the speed and trajectory of the final cutting tooth (FCT) by a high-speed camera. Analyze the law of kinetic energy dissipation of FCTS in confined space with energy perspective, and study the influence degree of reaming parameters and interaction on the kinetic energy of FCT by response surface method. Discuss the reason for the difference of sensitivity between theoretical analysis and experimental research. Research results indicated that the coal breaking of FCTS relied on the high-speed impact damage to the specimen by the FCT. The intrinsic cause of the variation in the FCT motion trajectory lies in the alteration of the primary driving force. The rotational dynamics model of FCTS based on the rotational motion characteristics in confined space was established. Aiming at the kinetic energy of FCT, the angular velocity, the radius of the confined space, and the mass of FCT as key influencing factors had been grasped. Based on the actual reaming diameter of 0.5m, the optimized reaming parameters included the rotating speed of 458rpm and the FCT mass of 179g. Compared with the standard reaming parameters, the kinetic energy increased by 19.7%, and the rock mass loss rate also increased.

Hard roofs in longwall mining may form large, suspended strata, which induce strong abutment stress redistribution. Therefore, a rational face support design is essential for ground control. This study develops an analytical numerical framework to evaluate support roof interaction under hard roof conditions. A segmented beam foundation model is established for the support roof system, and an equivalent variable foundation modulus is introduced to represent the reduced bearing capacity of yielded coal ahead of the working face. The analytical results are checked against a Particle Flow Code (PFC) and Fast Lagrangian Analysis of Continua (FLAC) coupled model, showing good agreement in the magnitude and location of the peak abutment stress. Parametric analyses are then conducted to examine the effects of support intensity and support distance on roof deflection, rotation, bending moment, shear force, strain energy density, and abutment stress. The results show that increasing support capacity reduces roof deformation and coal wall stress, while redistributing the same capacity over a longer support distance more effectively lowers roof strain energy concentration and inclined fracture development. However, the longer distance, lower density arrangement may transfer stress deeper into the coal seam and increase peak stress fluctuation during face advance. The proposed study provides a practical method for comparing face support schemes under low position hard roof conditions.

The supercritical carbon dioxide (SC-CO ₂ ) jet has great potential for application in oil and gas drilling and exploitation engineering as its low threshold pressure and high efficiency. However, currently, the high energy consumption of SC-CO ₂ jet rock breaking limits its application. It is an effective way to reduce energy consumption and further improve rock breaking efficiency by taking advantage of SC-CO ₂ jet’s high impact stress, dissolution and thermal cracking. To clarify the thermal cracking effect of SC-CO ₂ jet on coal and rock mass, the phase transition temperature and its effect on ambient temperature during SC-CO ₂ jet injection were investigated experimentally in this paper. Under the premise of isolating impact stress, dissolution and extraction, experiments were conducted to examine the coal cracks induced by thermal stress. Additionally, experiments were conducted to induce coal crack using low-temperature nitrogen. Scanning electron microscopy and industrial CT scanning were used to analyze the surface and internal cracks of the samples. The results show that the environment temperature is decreased while the temperature of SC-CO ₂ jet is decreased. And when the initial temperature of the jet is increased, the environment temperature will be further reduced. In addition, the thermal stress applied to coal at low temperature can promote the development and expansion of cracks on the coal surface and inside. In contrast, the thermal stress produced by low temperature nitrogen can only affect the surface cracks of coal. Furthermore, increasing the initial temperature of the jet can improve the development and expansion of the crack.

This study addresses the limitations of conventional hydraulic fracturing techniques, such as high water consumption and the potential to trigger water-lock effects. It systematically investigates the thermo-mechanical degradation behavior and fracture evolution mechanisms of low-permeability coal under the influence of cyclic heating and liquid nitrogen (LN2) cooling, a promising waterless stimulation technology. Using a combination of methods, including Brazilian splitting tests, three-point bending tests, acoustic emission (AE) monitoring, and three-dimensional (3D) surface profilometry, the progressive damage characteristics of coal's mechanical integrity over 15 cycles were revealed. Key findings include: cyclic thermal shock causes an 81.98% decrease in coal's P-wave velocity, a 63.31% reduction in tensile strength, and an 85.15% attenuation in fracture toughness, indicating significant cumulative damage due to repeated thermal shock. AE analysis confirms that microcrack nucleation and propagation intensify during the cycles, with the coal's failure mode transitioning from brittle to ductile. Fractal dimension and roughness index-based fracture surface characterization shows a significant increase in fracture network complexity with the number of cycles.

To investigate the control factors of mine disasters induced by high ground pressure, this paper takes a longwall face of Pingdingshan No. 10 coal mine as the engineering background. By employing the analytical model and numerical simulation methods, this study quantified the movement characteristics of the hard roof strata and the evolution of mining‐induced stress. The control mechanisms of various geological factors on the distribution of the abutment pressure were evaluated. The results indicate that (1) the PFC‐FLAC coupled numerical model can effectively simulate and predict the characteristics of roof collapse and the distribution characteristics of abutment pressure. (2) The first weighting distance, the peak of the abutment pressure, and the influence range of the abutment pressure can be measured by the proposed model, whereas the periodic weighting barely can be observed. (3) The tensile strength and thickness of the hard roof directly control the peak value of the abutment pressure and the first weighting distance. As the strength and thickness of the hard roof increase, both the peak value and influence distance of the abutment pressure significantly increase. When the tensile strength of the hard roof reaches a high value, the development of fractures in the hard roof shows a clear periodicity pattern. The vertical stress significantly affects the movement and fracturing of the overburden, while the horizontal stress has a limited impact on the movement and fracturing of the overburden. The proposed methods provide technical guidance and theoretical support for controlling ground pressure in deep‐buried longwall faces.