Study On The Inducement And Distribution Of Damage In Coal Impacted By High Pressure Water Jets

With the advancement of coal resources to deep mining,the complication of geological environment poses a challenge to traditional mining techniques.At the same time,the need for economic development to optimize the capacity of the coal industry also makes the urgent for more safe,scientific,and efficient coal mining methods.Under such circumstances,high pressure water jet is favored by scholars and engineers for rock breaking because of its advantages such as high cutting efficiency,low risk,energy saving,environmental protection,and ease of handling.In the coal mining process,high pressure water jet is applied to drill holes in the coal,thereby assisting gas pre-pumping and coal and gas co-extraction,and has developed into a mature technology.Based on the experimental and numerical methods,this paper investigates the propagation of stress wave,the inducement mechanism of microscopic damage and the distribution of damage field during the process of coal impacted by high pressure water jets,aiming to reveal the coal permeability increase mechanism under high pressure water jets and lay the theoretical foundation for engineering applications.The main results are as follows.(1)The evolvement of internal structure of coal under the impact of high pressure water jet was studied experimentally,and the digital image processing technology was used to realize the three-dimensional visualization of the damage field.At the same time,the distribution of damage field and the evolvement of the damage-inducing ability of high pressure water jets were quantified based on damage mechanics and autonomous programming,which provides the evidence for the mechanism revealing of microscopic damage induced by high-pressure water jets in coal.(2)Based on ALE-FEM algorithm,a numerical model for high-pressure water jet impact on coal rock is established.At the same time,a material surrogate method was used to establish the model of abrasive jet impinging on the coal,achieving the simulation of multiphase flow breaking rock.In additon,the scientificity of the numerical model is verified by comparing the numerical simulation results with the experimental results under the same impact conditions.(3)Based on the dynamic damage model of coal impacted under high pressure water jets,paper simulated the temporal evolution of damage field.The distribution charateristics of the damage field in the horizontal and vertical directions are discussed and compared with the experimental results.In addition,based on the comparison of multiple sets of simulation results,the effects of jet velocity and rock properties on the evolution and spatial distribution of damage fields were quantified.(4)Paper simulated of the stress wave effect of high pressure water jet breaking coal.Based on the effective stress history along the selected path in coal model,the waveform of the stress wave at different time steps was extracted.Based on the evolution of stress wave and the strain characteristics of coal,the mechanism of the transition from non-elastic wave to elastic wave during the propagation of stress wave is discussed.At the same time,the law of spatial and temporal decline of stress wave is quantitatively discussed according to the change of peak effective stress.Based on the comparison of multiple sets of numerical simulation results,the effects of jet velocity and rock properties on stress wave propagation and attenuation are quantified.(5)Based on the stress and damage characteristics of coal impacted under high pressure water jets,paper proposed the failure mechanism of coal breaking under jets impact.The damage and failure of coal was divided into three typical stages: elastic strain stage,damage accumulation stage and plastic failure stage.At the same time,combining with the strain law of coal with different coordinates,the development of punching hole is discussed from three perspectives: the formation of punching,the horizontal development of punching,and the vertical development of punching.(6)The numerical and experimental methods are adopted to compare the differences in the mechanism of coal breaking under abrasive jets and pure water jets from the perspective of the initial fracture of coal,the deepening of punching and the widening of punching.Based on the damage fields obtained from simulations and experiments,the evolvement of the damage-inducing capability of abrasive jets and pure water jets with erosion depth was quantitatively discussed,which provides the guidance for the various application of jets in different engineering environments.