Structure optimization of centralbody in dual cavitating jet andthe behavior of coal breaking under its impact

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 centralbody and high and low pressure jets were used to produce flowaround 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 centralbody 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 coalrock was carried out by combining the multimaterial 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 rockbreaking 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 vaporcontaining 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 fluidsolid interaction results of the dual cavitating jet breaking coalrock,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.