Mechanical Models And Numerical Methods On Coal And Gas Outburst

As an extremely complex dynamic phenomenon in coal mining,coal and gas outburst shows that a large number of coal with gas is rapidly ejected from the mining face and thrown into the working roadway,resulting in the damage of mines and tools and the casualties.The theoretical research on the outburst mechanism plays an important role in the prevention and control of coal and gas outbursts.Fully understanding the outburst mechanism is the key to put forward effective outburst prevention measures.However,many factors are affecting the occurrence of coal and gas outbursts,and the outbursts have strong randomness and destructiveness,which brings a great challenge to the outburst mechanism study.Domestic and foreign scholars have carried out a large number of experimental and theoretical studies on coal and gas outbursts.The research shows that coal and gas outbursts are the results of in-situ stress,gas pressure,and coal physical and mechanical properties.However,this conclusion only qualitatively points out the causes of coal and gas outbursts.There is no unified understanding of the role and mechanism of these factors in outbursts.Especially,there are still disputes about the dominant position of gas and in-situ stress in outbursts,and the mechanical essence of coal and gas outbursts is still unclear.As a typical mechanical problem,the study of coal and gas outbursts from the perspective of mechanics can not only deeply explain the outburst essence,but also provide theoretical guidance for outburst simulation experiment and outburst prevention.Firstly,based on the porous media seepage theory and the simplification of the coal sample model used in the test,a one-dimensional mechanical model on gas seepage in the coal sample in outbursts is established,the finite volume method is selected to discretize the corresponding governing equations,and the discrete format determined by parameter is discussed.Based on the experimental data and mechanical model,a method for fitting the approximate permeability of coal sample in outbursts is proposed.The rationality of the one-dimensional mechanical model on gas seepage and the effectiveness of the finite volume method are verified by experiment.Furthermore,according to the principle of the one-dimensional mechanical model on gas seepage and considering the desorption effect of adsorbed gas,a two-dimensional mechanical model on gas desorption and seepage in coal samples in coal and gas outbursts is established and the ideal gas state equation is modified.The finite volume method discretization process with non-uniform size elements is derived and the corresponding solution program is compiled.The distribution and variation of gas density,gas pressure,gas pressure gradient,and gas velocity in coal samples in outbursts are calculated in detail.Based on the two-dimensional mechanical model on gas desorption and seepage and the numerical method,the distribution and variation of gas pressure,gas desorption amount,gas pressure gradient,and gas velocity in coal samples in coal and gas outbursts under the conditions of different coal sample porosity and permeability,different gases and different coal sample heterogeneity are calculated,The effects of these factors on the distribution and variation of gas state in the coal sample in outbursts and the role of gas in outbursts and its relationship with the formation of the outburst cavity are analyzed.Then,the role of gas(gas pressure and expansion stress caused by adsorbing gas)is introduced into the classical elasticity equation,and the elasticity equation of the gassy coal sample is deduced.Based on the seepage law of gas in coal samples and the concept of particles’ critical starting velocity in aeolian sand mechanics,the starting criterion of the damaged coal sample is proposed.The elasticity equations of the gassy coal sample are discretized by the finite element method and the corresponding solution program is compiled.The distribution and variation of effective stress in coal samples in outbursts are calculated and the formation process of the outburst cavity is simulated.The characteristic of small openings and large cavities of the outburst cavity is reproduced.Based on the mechanical model and numerical method,the coal and gas outbursts under different conditions of in-situ stress,initial gas pressure,and physical and mechanical properties of coal samples are simulated.The effects of these factors on the outburst intensity and outburst evolution process,and the initiation,evolution,and termination of outbursts are researched.The calculation results show that the gas pressure gradient and gas velocity near the orifice in the coal sample are the largest,the isolines are approximately oval and pear-shaped,and the gas pressure gradient and gas velocity in the coal sample decay rapidly and tend to zero in a short time.The outburst cavity with approximate oval or pear-shaped appears in this area in tests.In addition,there is obvious stress concentration in the front of the outburst cavity and the stress level in this area increases with the outburst cavity evolution.Coal and gas outbursts end with the decrease of gas pressure gradient and gas velocity.The research indicates that the distribution law of gas pressure gradient and gas velocity determines the shape of outburst cavity and its position in coal sample.The stress concentration caused by in-situ stress and gas pressure is the main cause of coal sample failure,in which in-situ stress is dominant,and the influence of vertical in-situ stress on outburst strength is much greater than that of horizontal in-situ stress and gas pressure.The outburst evolution process is characterized by the coal sample is destroyed and started by flowing gas,the continuous expansion of cavity into the coal sample,the increase of effective stress in the coal sample,and the decrease of gas pressure gradient and gas velocity.Coal and gas outbursts end as the gas velocity decreases to less than the critical starting velocity of the crushing coal sample.This paper studies the gas desorption and seepage law in coal samples,the coal sample failure,and the outburst evolution process,defines the main roles of gas and in-situ stress in outbursts,and analyzes the mechanical essence of coal and gas outbursts from the mechanical point of view.The work done in this paper lays a certain theoretical foundation for the further study of the outburst mechanism,and also provides some theoretical guidance for the simulation tests of outbursts and prevention and control of coal and gas outbursts.