| The top coal caving mining technology is an effective method for mining thick coal seams,but compared to the full height mining at once with large mining heights,it has the problem of low top coal recovery rate caused by unreasonable coal caving technology.The flow characteristics of top coal caving are the basis for determining a reasonable coal caving technology.Therefore,it is necessary to study the flow pattern of top coal caving to better guide the coal caving process and maximize the recovery rate of top coal in the working face.When studying the flow law of top coal caving,most scholars use the ideal spherical particles in the discrete element simulation software PFC to simulate the coal gangue dispersion,which is inconsistent with the actual situation and cannot truly reflect the flow state of the coal gangue behind the scaffolding.Therefore,this thesis is based on irregular particles and uses PFC for numerical simulation to study the flow law of top coal caving.The main research results of the thesis are as follows:(1)Establish a top coal compression model,analyze the development law of top coal cracks and the characteristics of broken block size changes.The research results show that top coal breakage first occurs at the tail of the support top beam,and then the cracks expand towards the direction of advancing towards the working face;The fragmentation of the top coal at the tail of the support top beam is smaller and the degree of fragmentation is greater;The coal expands towards the goaf side until it is completely separated from the original coal.(2)In PFC software,irregular shaped blocks are simulated by generating a rigid cluster Clump.The fineness of their appearance is not only affected by the generation parameters,but also affects the computational efficiency.By comparing the characterization accuracy and numerical simulation calculation efficiency under different generation parameters,the block generation parameters under the condition of balancing fineness and computational efficiency are determined.(3)Three factors that affect the shape of particles were analyzed: the number of particle vertices,the distance between particle vertices and the origin,and the angle of particle vertices.In order to reduce the randomness and diversity of the shape of coal gangue dispersion as much as possible,a Gaussian distribution function was used to randomly generate the above three influencing factors,and ultimately 36 particles of different shapes were randomly generated as the basis for the shape of the model particles.(4)The numerical model is established with top coal of different block sizes.The recovery of top coal in working face increases first and then decreases with the increase of the block size of top coal.The recovery rate reaches its maximum when the top coal block size value is 270 mm.The smaller the block size of the top coal,the more favorable it is for the release of the top coal on the side of the goaf,but the gangue in the goaf is also more likely to advance into the coal discharge port and cause the coal discharge to stop;The larger the block size of the top coal,the more favorable it is for the release of the top coal on the support side.However,the number of arches basically follows the trend of increasing with the increase of the block size of the top coal.Excessive block size of the top coal can seriously affect the coal release operation.To improve the advancing efficiency of the working face,measures should be taken to reduce the block size of the top coal.(5)Establishing a numerical model with different inclination angles of the shield beam,the top coal recovery rate gradually increases with the increase of the inclination angle of the shield beam,reaching a maximum value around 45 °,and then decreases with the increase of the inclination angle.When the inclination angle of the shield beam is too large,it is easy to cause arching due to the limited horizontal space for coal caving,which is not conducive to the release of top coal.The thesis has 94 figures,5 tables,and 71 references. |
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