低阶长焰煤细观结构定量分析与表征单元确定

Quantitative analysis of meso-structured and representative elementary volume determination of low rank long flame coal

表征单元体(REV)作为量化多孔介质材料尺度效应的一种基本方法,是构建煤体宏-细观跨尺度结构模型的有效手段。为提高煤体细观结构REV结果的准确性,基于煤样细观尺度下孔裂隙结构特征研究,提出了煤样细观结构定量表征方法,以煤样试件不同位置与不同破裂方向的423个尺度下的孔裂隙二值图像为依据,分别从孔裂隙分布密度、几何形状、方位定向3个角度分析了煤样细观结构的几何特性。在考虑煤体非均质性与各向异性的采样方法基础上,分析了REV尺寸的空间效应与方向效应,最后,利用变异系数法确定了合理的煤样细观REV尺寸。采用图像处理、统计学与变异系数判别相结合的方法对低阶长焰煤细观结构的定量表征与REV确定方法展开相关研究。研究结果表明:①针对灰度不均、边界模糊、对比度低的煤样SEM图像提出了一种孔裂隙结构准确识别与定量分析的方法;②确定了反映煤样各向异性的细观结构定量表征指标,包含孔隙率、整体形状因子、分形维数与定向分布系数;③与平行层理相比,垂直层理方向微裂隙更为发育,孔隙率为平行层理的2倍,大孔径孔隙贡献率为85%,具有较强的空间充填能力,孔裂隙整体形态规则性较差,但边缘形态光滑性较好,定向分布更趋于简单;④基于变异系数可接受值10%确定了煤体细观结构表征单元尺寸为200μm×200μm。

As a basic method to quantify the scale effect of porous media materials,representative elementary volume(REV)is an effective means to construct the macro-meso structure of coal.In order to improve the accuracy of REV results of coal meso-structure,a quantitative characterization method of coal meso-structure was proposed based on the study of pore-fracture structure characteristics at mesoscale of coal samples.Based on the binary images of pore-fracture at 423 scales at different positions and different fracture directions of coal samples,the geometric characteristics of coal meso-structure were analyzed from three aspects:pore-fracture distribution density,geometric shape and orientation.Based on the sampling method considering the heterogeneity and anisotropy of coal,the spatial effect and directional effect of REV size are analyzed.Finally,the reasonable microscopic REV size of coal sample is determined by the coefficient of variation method.The results show that:①A method for accurate identification and quantitative analysis of pore fracture structure is proposed for SEM images of coal samples with uneven gray level,blurred boundary and low contrast.②A quantitative characterization system of meso-structure reflecting the anisotropy of coal samples was established,including porosity,overall shape factor,fractal dimension and directional distribution coefficient.③Compared with the parallel bedding,the micro-fractures in the vertical bedding direction are more developed,the porosity is 2 times that of the parallel bedding,the contribution rate of large aperture pores is 85%,and it has strong spatial filling ability.The overall morphology of pores and fractures is poor,but the edge morphology is smooth and the directional distribution tends to be simpler.④Based on the acceptable value of the coefficient of variation of 10%,the size of the coal meso-structure characterization unit is determined to be 200μm×200μm.