循环等离子体击穿受载煤体电学响应及孔隙结构演化规律

Electrical response and pore structure evolution affected by cyclical plasma breakdown

煤层增透是提高瓦斯高效抽采效率、降低瓦斯灾害事故的关键,以物理放电为基础的等离子体是煤层增透的有效手段之一。但以往的研究集中在单次击穿煤体孔-裂隙结构的表征,忽略了等离子体对煤体的极化效应,对循环等离子体作用下受载煤体电学性质及孔隙结构的演化特征缺乏深入研究。为此,对多个煤样进行了循环等离子体击穿实验,结合高压衰减棒和罗氏线圈监测了击穿煤体过程中的电压、电流波形,并分析了预击穿周期、等离子体击穿周期、波峰个数、峰值电压、峰值电流及能量转化效率等电学参数的变化规律;采用低场核磁共振方法测试了击穿1次、5次、10次、15次、20次条件下小孔、中孔、大孔及微小裂隙的演化规律;结合几何分形理论对击穿煤体渗流孔分形维数进行了探讨。研究结果表明,预击穿周期仅有首次放电是千微秒级别,之后会“断崖式”下降至几十微秒,而等离子体击穿周期则随着击穿次数的增加呈现出“阶梯式”的增长;能量转化效率维持在28.7%~55.9%,呈现出先快速增加后趋于平稳的趋势,说明等离子体对煤体电学性质的极化效应是有限的;煤体内部中孔、大孔的增长幅度最为明显,部分微小裂隙结构会实现从无到有的突破,表明等离子体对瓦斯的渗流会有显著的改善效果;分形维数在击穿后呈现出降低的趋势,则从孔-裂隙空间维度的角度证实了击穿煤体内原本孤立的孔隙结构会被裂隙沟通;孔隙率与能量转化效率变化趋势的相似性表明等离子体对煤体电学性质产生了影响,煤体电学性质发生改变后又会对等离子体通道的分布产生影响。

Artificial improvement of coal seam permeability is the key to solve the low gas drainage efficiency and prevent gas dynamic disaster.Plasma based on physical discharge is one of the effective means of coal seam antireflection.However,previous studies focus on the characterization of pore-fissure structure of single breakdown,ignore the polarization effect of plasma on coal,and lack of in-depth research on the electrical properties and the evolution characteristics of pore-fissure structure under the action of cyclical plasma of loaded coal.In this paper,the cyclical plasma breakdown ex-periments of coal samples were carried out.The voltage and current waveforms in the process of coal breakdown were monitored thorough combining with high-voltage attenuation rod and the Roche coil,and the variation of electrical para-meters such as pre-breakdown period,plasma breakdown period,number of wave peaks,peak voltage,peak current and energy conversion efficiency were analyzed.The evolution of small pores,medium pores,large pores and micro cracks under the conditions of 1,5,10,15 and 20 times of breakdown was tested by NMR.The fractal dimension of seepage pore was also discussed through geometric fractal theory.The results show that the pre-breakdown period is at the level of thou-sands of microseconds only for the first time,and then it will drop to tens of microseconds,while the plasma breakdown period shows a“ladder”growth with the increase of breakdown times.The energy conversion efficiency is maintained at 28.7%−55.9%,showing a trend of rapid increase at first and then stable,indicating that the polarization effect of plasma on the electrical properties of coal is limited.The growth rate of medium and large pores is the most obvious,and some mi-cro fracture structures will achieve a“0”breakthrough,indicating that plasma will significantly improve the seepage of gas.The fractal dimension shows a decreasing trend after breakdown,which proves that the originally isolated pore struc-ture will be connected by the fracture from the perspective of pore fracture space dimension.The similarity between the change trend of porosity and energy conversion efficiency shows that plasma has an impact on the electrical properties of coal,and the change of coal electrical properties will also have an impact on the distribution of plasma channels.