Study On Rock Fragmentation Performance Of High-pressure Water-jets Cutting Head And Its Dynamics

In order to realize the sustainable development of coal industry, the coal seam difficult to mine, such as that containing gangues and hard inclusions, must be mined nowadays, which was mined rarely and even not mined before. Among all kinds of coal seam difficult to mine, thin coal seam accounts for a large percentage. The coal roof and floor should be cut in the tunnel excavation process in thin coal seam, which results in the poor reliability and efficiency, low utilization of roadway section, and high excavation cost. A new rock cutting method should be studied for decreasing the roadheader volume as well as weight, and improving its rock fragmentation performance, which has extremely vital significance for high-efficiency excavation of half coal-rock roadway in thin seam and coal resources recovery. Taking the high-pressure water-jets cutting head as the object, the rock fragmentation process by mechanical tool and high-pressure water-jet were investigated by theoretical and numerical methods, and the rock fragmentation performance of developed high-pressure water-jets cutting head were studied through experiments, and the dynamics of cutting head in rock fragmentation process were researched by theory of non-linear theory systematically, which provides the theoretical and experimental basis for high-pressure water-jets roadheader application.Based on the fracture mechanics and the solution of Boussinesq problem, the theoretical model of rock fragmentation by mechanical tool is set up. According to elastic wave dynamics, the relationships among rock radial, tangential, shear dynamic stresses and water-jets parameters are obtained under the water-jet impact load, then the mechanical models of crushing and damage zone are developed using the dynamic tensile and shear failure criterion. With the linear superposition of the stress intensity factors at the crack tip, the mode I stress-intensity factor at crack tip is solved, then the mechanical models of cutting force and its reduction percentage of water-jet assisted mechanical tool are developed based on the theoretical model of rock fragmentation by mechanical tool.The numerical model of rock fragmentation by water-jet impact is developed based on the coupled SPH and FEM method, and rock fragmentation process is simulated and investigated under the water-jet impact load. The rock fragmentation is determined by combined action of shear and tensile failure. The crushing zone nearby impact point is mainly caused by the shear failure, while the radial crack initiation and propagation mainly forms due to the tensile failure. However, the spall crack is aroused because of the reflected compressive stress wave from free boundary, and it can improve the rock fragmentation extent. The numerical model of rock fragmentation by mechanical tool is set up using Particle Flow Code, and rock fragmentation process is simulated and investigated. Results show that the rock fragmentation process includes three stages: crack initiation, crushing zone and radial crack formation, major radial crack propagation and rock fragment formation. With the decrease of tool sharpness, the crushing zone, cutting force and the number of radial cracks increases, rock fragments size reduces, which is not beneficial to seal the water in crack and fracture rock by mechanical tool. Therefore, the mechanical tool sharpness should be maintained in the design of rock fragmentation by mechanical tool assisted with water-jet.According to the rock fragmentation condition and experimental requirements of cutting head, the transmission system, parameters measurement and acquisition systems are designed, and the program software for signal acquisition is developed. The rear-mounted device with multi rotating seals for high-pressure water was developed, which enhances the service life of seal device and be replaced expediently. The seal device provides support for reliable implementation of rock fragmentation by assisted water-jet. With the process and characteristic of coal-rock fragmentation in cutting, the mechanical, stress, deformation and fragmentation similar conditions are set up. The artificial coal-rock with different proportion were preparated using cement, gypsum and sand as the raw materials, which provides the reliable crushing object for the experiments of rock fragmentation by water-jets cutting head.On the test bed for rock fragmentation by water-jets cutting head, the rock fragmentation experiments are implemented under different coal-rock strengths, assisted types of mechanical tool with water-jets, water-jet parameters and so on, and torque, pulling force, specific energy of rock fragmentation, drilling depth and so on are used to estimate the rock fragmentation performance of water-jets cutting head. When coal-rock compressive strength is smaller than water-jet pressure, water-jets cutting head I and III can reduce the torque, pulling force and specific energy of cutting head obviously, but the rock fragmentation performance of water-jets cutting head II can not be improved. When the coal-rock compressive strength is higher than water-jet pressure, the rock fragmentation performance of water-jets cutting head III can be enhanced to some extent, but the rock fragmentation performance of water-jets cutting head I can not be improved. The assisted water-jets of cutting head III are arranged out of the mechanical tools, they do not influence the rock fragmentation ability of mechanical tool, and the water-jets cutting head III has good adaptability to the rock with different properties, thus it has better rock fragmentation performance. In addition, the rock fragmentation performance of water-jets cutting head III increase with water-jets pressure. The carbide tip as water-jet nozzle restricts picks self-rotation, which conduces to large wear rate and low service life of conical pick. In addition, the dust level caused by coal-rock drilling can be decreased distinctly under water-jets assistance.With phase-space reconstruction theory, correlation integral and Wolf methods, reconstructed phase space trajectories and Lyapunov exponents of cutting torque are investigated under different conditions. Results show that the system of rock fragmentation possesses chaotic dynamic characteristics. When the rock fragmentation performances are better with assistant water-jets, the maximum Lyapunov exponents change greatly, which indicate that the rock fragmentation mechanism may be changed to some extent. NARX neural network can accurately simulate and effectively predicate the variation characteristics of cutting torque, which provides some basis for controlling the cutting motor to run efficiently.Based on the established coupling dynamic equations between cutting head and coal-rock, the Heaviside functions are introduced to set up the uniform differential equations for dynamic system. The dynamic behaviors of rock fragmentation system are solved using a fourth-order Runge-Kutta method under different system parameters. The results show that the rock fragmentation system may be under periodic motion or chaotic motion. The rock fragmentation efficient is high when the system is under periodic motion, but it is low when the system is with chaotic motion. Therefore, the reasonable parameters should be used to avoid the phenomenon of chaotic dynamic characteristics. The established dynamic model of rock fragmentation system can be used to realize the simulation of coal-rock drilling process by cutting head, which provides a new approach for the design of rock fragmentation and dynamic characteristic of cutting head.