液氮冻结含水煤体能量耗散动态变化规律的试验研究

Study on energy dynamic change law in the process of water-contained coal caused by liquid nitrogen freezing

为研究液氮冻结含水煤体过程能量动态变化规律,利用自主研发的液氮冻结煤体全过程声发射试验系统,分析了不同含水率煤体液氮冻结全过程的声发射能量耗散特征及变化规律。结果表明:液氮冻结煤体过程中声发射能量在时间域上,分为陡增期、波动期、平静期,能量一次、二次峰值均与含水率呈正线性关系,含水率5.96%煤体能量一次、二次峰值是干燥煤体的1.66、2.26倍;液氮冻结煤体累计能量与时间关系分为陡增、缓慢增长及稳定等3个阶段,累计能量与含水率呈正线性关系,含水率5.96%煤体是干燥煤体的2.88倍;不同含水率煤体幅值绝大部分集中在40~50 dB之间,占比为94.39%~99.11%,且随着煤体含水率的增加呈线性减小;液氮冻结煤体声发射振铃计数时间序列存在混沌分形特征,陡增阶段、缓慢增长阶段、稳定阶段的关联维数分别与含水率呈正指数、正线性、正线性相关,含水率5.96%煤体陡增阶段关联维数分别是缓慢增长阶段、稳定阶段的2.00、5.78倍;液氮冻结煤体产生裂隙类型以拉伸裂隙为主,其占比随含水率增加呈负指数减小,剪切裂隙占比随含水率增加呈正线性增大。煤体水分增加,增大了液氮冻结过程中水-冰相变产生的冻胀力,能量耗散增加,促使孔裂隙发育,可通过声发射能量反演。

To study the energy dynamic change law of moisture-contained coal in the process of liquid nitrogen freezing,a self-developed acoustic emission(AE)experimental system for the whole process of liquid nitrogen frozen coal was utilized to analyze the characteristics and the change laws of AE energy dissipation in the whole process of liquid nitrogen freezing in coal with different moisture contents.The results shown that AE energy during liquid nitrogen freezing of coal was divided into steep,fluctuating and calm periods in the time domain.The primary and secondary peaks of energy were both positively linearly related to moisture content,and the primary and secondary energy peak of 5.96%moisture content were 1.66 and 2.26 times higher than those of dry coal.The cumulative energy of liquid nitrogen frozen coal,divided into three stages of steep increase,slow growth and stabilization versus time,was positively linearly related to moisture content,which of 5.96%moisture contained coal was 2.88 times higher than that of dry coal.The energy amplitude of different moisture content coals was mostly concentrated in the range of 40-50 dB,accounting for 94.39%-99.11%of the total,and decreased linearly with the increasing moisture content of coal.The time series of acoustic emission ringing counts in liquid nitrogen frozen coals had chaotic fractal characteristics,and the correlation dimensions of the steep increase,slow growth and stable stages were positively exponentially,linearly and linearly correlated with the moisture content,respectively.Furthermore,the correlation dimension in the steep increase stage of 5.96%moisture contained coal was 2.00 and 5.78 times higher than that of the slow growth and stable stage,respectively.The type of coal cracks produced by the liquid nitrogen freezing was mainly tensile,its proportion with the increasing moisture content was a negative exponential decrease,and the proportion of shear cracks positively linearly increased with the increasing moisture content.The increase of moisture in coal strengthened the freezing and expansion force generated by the water-ice phase transition during the liquid nitrogen freezing process,and the increase of energy dissipation contributed to the rapid development of pore-crack and the structural damage and plastic deformation of coal.However,the structural damage was difficult to detect in real time and can be inverted by AE energy.