Energy criterion of coal breaking by abrasive jets based on stress wave effect
CHEN Changjiang,ZHU Ying,LI Zhiping,ZHANG Huidong,LIU Yong
磨料射流是一种操作简单、切割效率高的绿色加工技术。在矿山开采、油气开发等领域，被广泛应用于破碎煤岩。磨料射流破煤是射流能量传递、转化和释放的过程。射流能量以循环、冲击载荷形式直接加载于射流-煤岩接触区域，并以球面应力波形式向煤岩内部传递。当应力波能量高于煤体破坏阈值，煤体发生破坏。然而，目前磨料射流破煤过程中的应力波传播规律及有效破煤范围不明确。因此，基于应力波效应分析了局部微裂纹时起裂时的煤岩能量密度，结合磨料射流特性，建立了描述磨料射流破煤能量准则；采用能量等效方法，基于SHPB（Split Hopkinson Pressure Bar，分离式霍普金森杆）实验，研究了相同入射能量下煤样能耗规律及破坏特征，计算了能量准则中的煤岩参数；在此基础上，采用DIC（Digital Image Correlation，数字图像相关法）实验获取了磨料射流冲击下应力波传播规律，并采用能量准则计算了磨料射流破煤的理论范围；通过对比实际和理论破煤深度，验证了磨料射流破煤能量准则。结果表明：磨料射流冲击过程中，煤体发生破坏的能量阈值决定于应力波波速、煤岩破碎能耗密度、碎块平均粒径及磨料射流冲击频率。随入射能量增加，煤样应变率增大，能耗密度增加，但入射能量有效利用率降低；煤体吸收的能量，主要是被用于微裂纹扩展，且破碎后的碎块平均直径与煤样应变率呈一次函数关系。球面应力波传播过程中具有明显时空特征，且沿传播方向能量发生衰减；又由于煤体原生孔、裂隙发育，应力波会出现聚集、叠加，导致局部能量增加。当应力波能量高于煤体裂纹起裂时的能量阈值时，煤体出现破坏。
Abrasive jet is a green processing technology with easy operation and high cutting efficiency. It is widely used in mining,oil and gas development and other fields to break coal rock. Abrasive jet coal breaking is a process of jet energy transfer,transformation and release. The jet energy is directly loaded on the jetcoal rock contact area in the form of cyclic and impact load. At the same time,it is transferred to the interior of the coal rock in the form of spherical stress waves. When the stress wave energy is higher than the coal damage threshold,coal damage occurs. However,the current stress wave propagation law and effective coal breaking range in the process of coal breaking by abrasive jet are not clear. Therefore,in this paper,the energy density of coal rock at crack initiation during local microcracking was analyzed based on the stress wave effect,and the energy criterion describing coal breaking by abrasive jet was established by combining with the characteristics of abrasive jet. The energy equivalence method was used to study the energy consumption law and damage characteristics of coal samples under the same incident energy based on SHPB (Split Hopkinson Pressure Bar) experiment,and the coal rock parameters in the energy criterion were calculated. On this basis,the stress wave propagation law under the impact of abrasive jet was obtained by DIC (Digital Image Correlation) experiment,and the theoretical range of coal breaking by abrasive jet was calculated by using the energy criterion. The energy criterion of coal breaking by abrasive jet was verified by comparing the actual and theoretical coal breaking depth. The results show that the energy threshold for coal destruction during abrasive jet impact is determined by the stress wave velocity,the energy density of coal rock breaking,the average particle size of broken pieces and the abrasive jet impact frequency. With the increase of incident energy,the strain rate of the coal sample increases and the energy consumption density increases,but the effective utilization of incident energy decreases. The energy absorbed by the coal body is mainly used for microcrack expansion,and the average diameter of broken pieces after crushing is a primary function of the strain rate of coal sample. The spherical stress wave propagation process has obvious spatial and temporal characteristics,and the energy attenuation occurs along the propagation direction. However,due to the development of primary pores and fissures in the coal,the stress wave is aggregated and superimposed,resulting in an increase of local energy. If the stress wave energy is higher than the energy threshold of the coal body when the cracks start to crack,the coal body appears to be damaged.
abrasive jets;energy criterion;SHPB experiments;DIC experiments;stress wave effect