Structure optimization of centralbody in dual cavitating jet andthe behavior of coal breaking under its impact
TU Yixiang,FAN Chenxing,QIAN Yi’nan,WANG Xiaosun,KANG Yong,LI Deng
水射流割缝造网技术是增加煤层透气性、提高煤层气开采效率的有效途径。与普通水射流相比，空化射流因空泡溃灭产生的强大冲击具有更高的破岩效率。为进一步增强空化射流破煤岩能力以促进煤层气开发，分别利用中心体和高低压射流产生绕流空化与剪切空化来提升空化效应，提出了新型的非淹没双空化射流。基于多相流Mixture模型和RNG k-ε输运方程，对不同嵌入深度与不同形状中心体下的流场进行了数值模拟，分析了射流流场气含率和轴线速度分布特征，得到了双空化射流中心体结构的最优参数。随后，结合群泡溃灭动力学理论，将最优中心体结构下双空化射流的压力与气含率作为边界条件，利用多物质流固耦合算法与ALE（任意拉格朗日-欧拉）算法对双空化射流冲击煤岩进行了耦合求解。探究了煤岩破碎过程中流体与固体的相互作用特征及煤岩的破碎过程，并对比分析了双空化射流与普通水射流的破岩效果。研究结果表明，在中心体嵌入深度l=0.5 mm时，双空化射流的含气区域最大，在距中心体末端10 mm位置处气含率可达0.9，空化效果最好；中心体AB端均为90°锥角时，双空化射流的集束性与空化效果同时最优。由双空化射流破煤岩的流固耦合结果可知，其对煤岩的冲蚀深度和孔径明显大于普通射流，在100 μs时双空化射流对煤岩的冲蚀深度和孔径分别为普通水射流的2.4倍和1.3倍。随着时间的延长，冲蚀孔径不再明显增加，而冲蚀深度不断增大，且与冲蚀时间呈线性关系。研究结果证明双空化射流具有更强的空化效果，破煤岩能力更强，因而可在一定程度上促进煤层气的开采。
Water jet slotting technology is an effective way to form a crack network in the coal seam and increase the permeability,resulting in the improvement of the efficiency of coalbed methane exploitation. Compared with a conventional water jet,a cavitating jet has higher rock breaking efficiency because of the powerful impact caused by the collapse of numerous bubbles. In order to further enhance the coal rock breaking ability of cavitating jet to promote coalbed methane development,a centralbody and high and low pressure jets were used to produce flowaround cavitation and shear cavitation at the same time. Thus,a new type of non submerged dual cavitating jet which has enhanced cavitating effect was produced. Based on the multiphase flow mixture model and the RNG k-ε transport equation,the flow field under different embedded depths and different shapes of the centralbody was numerically simulated,and the optimal parameters were obtained by analyzing the vapor volume fraction and the velocity distribution at the axis of the jet. Then,based on the theory of group bubble collapse dynamics,the pressure and vapor ratio of the dual cavitating jet with the optimal structure of central body were taken as boundary conditions,and the coupled solution of dual cavitating jet impacting coalrock was carried out by combining the multimaterial fluid structure interaction algorithm and the ALE (Arbitrary Lagrange Euler) algorithm. The interaction characteristics of fluid and solid in the process of crushing coal rock were studied,and the rockbreaking effect of a conventional water jet was used for comparison. The research results show that when the depth of the central body l=0.5 mm,the vaporcontaining area of the dual cavitating jet is the largest,and the cavitation effect is the best with the vapor volume fraction reaching 0.9 at the position 10 mm away from the end of the central body. The dual cavitating jet has the best clustering property and cavitation effect when both the AB ends of the central body have a cone angle of 90°. From the fluidsolid interaction results of the dual cavitating jet breaking coalrock,it was found that the erosion pit depth and diameter of the dual cavitating jet on coal rock are 2.4 and 1.3 times greater than those of a conventional water jet at 100 μs. With the increase of time,the erosion pit diameter no longer increases significantly,but the depth continues increasing linearly as a function of the erosion time. It can be concluded that a dual cavitating jet has a stronger cavitating effect and a greater coal rock breaking ability,and thus it could promote the exploitation of coalbed methane at a certain extent.
coalbed methane;cavitating jet;structural optimization;bubble collapse;fluidstructure coupling