Research On The Starting Behavior Of Sedimentary Particles In Rough Fractures Under The Action Of Water Flow

Gas drainage is the fundamental measure to control coal mine gas disasters.In the practice of gas drainage engineering,the gas flow rate of boreholes（ground drilling and underground drilling）will decrease slowly or suddenly,which greatly shortens the efficient gas drainage cycle of boreholes.The research shows that the deposition of coal and rock particles carried in the extraction airflow in the fracture is the main reason for this engineering problem.After the gas drainage flow in the borehole drops sharply,the hydraulic scouring of the blocking area in the mining-induced fracture field can significantly reduce the amount of particle deposition in the fracture,thereby prolonging the service cycle of the borehole and improving the gas drainage effect.However,the incipient law of sedimentary particles in rough fissures under the action of water flow is still unclear,which leads to the fact that the field hydraulic scouring work is extremely dependent on previous engineering experience.Therefore,this thesis uses experimental research and numerical simulation to reconstruct the rough fracture surface and analyze the starting law of deposited particles in rough fractures.The research results can provide some theoretical guidance for the hydraulic scouring process of plugging fracture field.The main research results obtained in this thesis are as follows:（1）The three-dimensional coordinate data of mining fracture surface were obtained,the rough fracture surface was reconstructed,and the solid model of rough fracture surface was prepared.The digital reconstruction of the standard contour line was carried out,and the relative coordinate data of the standard contour line was obtained.The mathematical correlation between the roughness coefficient of the joint surface（JRC）and the root mean square of the first derivative（Z₂）was fitted.Based on the mathematical correlation,the roughness coefficient of the rough fracture surface was quantified.（2）An experimental platform for the starting of deposited particles in rough fractures was built,and the critical starting velocity of single particles in rough fractures and its influencing factors were studied.The start-up behavior,start-up efficiency and the influencing factors of start-up efficiency of particles in granular beds in rough fractures were investigated.The results show that the critical incipient velocity of single particle is positively correlated with fracture roughness and particle size.It is revealed that the start-up efficiency of group particles in granular bed increases with the increase of flow velocity.Furthermore,according to the growth rate of start-up efficiency of group particles,the start-up behavior of group particles in granular bed is divided into three parts:slow rising section,sharp rising section and gentle stable section.The influence of fracture roughness and particle gradation on the starting efficiency of group particles in granular bed was investigated.（3）A numerical calculation model of particle incipient liquid-solid two-phase flow in rough fractures was established.The fluctuation law of water pressure and flow velocity in rough fractures was explored,and the variation law of water pressure and flow velocity at the same position was revealed.In addition,the relationship between the local pressure loss and the roughness coefficient in the rough fracture is analyzed.The influence of fracture roughness on flow streamline is explored.The results show that the rough fracture can cause the bending of the flow streamline,and the closer to the rough wall surface,the greater the degree of streamline bending.With the increase of fracture roughness,the degree of streamline bending increases gradually.Furthermore,based on the established numerical calculation model of liquid-solid two-phase flow,the dynamic characteristics of particles in the rough fracture during the starting process are studied,and the effects of fracture roughness and particle size on the coupling torque and particle trajectory are analyzed.