| Although countries continue to increase the development of renewable resources such as solar energy,water energy,wind energy,biomass energy,geothermal energy,etc,due to technological constraints,renewable resources can not be used in living and industry on a large scale,so coal will remain an important energy pillar for living and industry for a long time in the future.Most of China’s coal resources are deep in the ground,and gas gushing can occur during coal mining,causing safety hazards.Gas drainage can reduce the content of gas in coal seams and reduce crustal stress,which is an important way to effectively prevent gas gushing and achieve safe production in coal mines.Relevant studies have shown that water jet penetration enhancement and pressure relief is a simple,safe,reliable and effective method to deal with coal seam gas.However,due to the complex and diverse construction conditions,working parameters and coal body parameters in the working process,there are many deficiencies in practical application of water jets in breaking coal to increase permeability,such as the influence of key parameters on water jet performance is not clear enough,the evaluation index for the effect of water jet on coal breaking and punching is relatively single,and the fragmentation mechanism and crack development law of different properties of coal are not consistent,etc.Therefore,the jet performance,coal breaking mechanism,crack development law and coal breaking characteristics of high-pressure water jet for coal breaking and punching nozzle have been deeply studied in this paper,in order to improve its coal breaking and punching penetration enhancement effect.Firstly,based on the computational fluid dynamics method,a numerical model of the nozzle jet is established based on the structure of the improved high-pressure water jet coal breaking and punching nozzle.The numerical simulation method is determined and the jet performance of the improved nozzle is verified,the influence law of the key structural parameters of the nozzle on the jet performance is investigated,and on this basis,two indexes for evaluating the jet performance,namely,the maximum velocity of the jet and the effective extension diameter are proposed.Secondly,the numerical model of water jet impacting coal is establishedbased on the method of display dynamics,and the parameters and boundary conditions of numerical simulation are set according to the properties of coal and the actual operation conditions.The whole process of water jet impacting and breaking coal is reproduced,the mechanism of coal breaking and punching by water jet is explained from the microscopic point of view,and the damage evolution process of coal breaking area and crack propagation area is analyzed deeply,The influence of different water jet velocity and effective expansion distance on coal breaking and punching is studied.Thirdly,taking the maximum water jet velocity and effective expansion distance as the optimization objectives of water jet performance,based on orthogonal test matrix analysis,BP neural network algorithm and genetic algorithm,a multi-objective and multiparameter collaborative optimization method is proposed.And the optimization research is carried out on the coal breaking and punching nozzle,and the influence weights of key structural parameters on the comprehensive optimization objectives of water jet are obtained,the optimal parameter combination and the best comprehensive water jet performance in the global range are obtained.Finally,using the self-developed high-pressure water jet coal-breaking and punching device,the coal breaking and punching by water jet experiments are carried out on three types of nozzles.It is proved that under the same operating parameters,the water jet performance of nozzle after the collaborative optimization is the best,with a hole depth of over 13 cm.It is found that the effective damage time of the water jet on the coal is about200 s.On this basis,the breaking and crack development of non-hole/hole coal under pressure are studied by means of field strain and acoustic emission combined monitoring.The results show that the hole will change the structure of the coal,generate a large number of cracks under the pressure and increase the development rate of cracks.The larger the hole diameter,the more obvious the effect,which indirectly proves that the coal will produce more cracks after water jet punching under the action of confining pressure,and improve the fluidity of gas and enhance the penetration effect of water jet punching.The research results of this paper improve the theory of coal breaking and punching by water jet,provide ideas for the design and optimization of coal breaking and punching nozzle,provide important experimental data and reference for the research of coal breaking and punching by water jet,and have certain guiding significance and academic value for improving the penetration effect of water jet and ensuring the safe and efficient production of low permeability coal seams. |
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