Formation Mechanism Of Supercharged Pulsed Water Jet And Evaluation Of Its Hard Rock Fragmentation Performance

Deep underground engineering is related to China’s modernization and sustainable development,and hard rock fragmentation is the key link of deep underground engineering construction.Pulsed water jet（PWJ）shows high-efficiency fragmentation on hard rock and has unique advantages such as dust-free and pollution-free,which is expected to become a new way for hard rock fragmentation in deep underground engineering.Among the existing types of PWJ,percussion,excited and disc-slotted PWJ have the disadvantages of unsustainable emission,obvious“water cushion effect”,uncontrollable frequency and large jet energy loss,respectively,which makes them difficult to be popularized and applied in the field of hard rock fragmentation engineering.Exploring an efficient,low-cost,safe and reliable PWJ generation method suitable for hard rock fragmentation has become an urgent engineering problem to be solved.Therefore,the generation method of SPWJ,pulsation mechanism of jet,dynamic characteristics of jet generation system and rock fragmentation performance of jet are studied by means of theoretical derivation,numerical simulation and indoor test in this paper.The main innovative achievements are as follows:（1）A supercharged pulsed water jet（SPWJ）generation method for driving the piston with low-pressure fluid to intermittently squeeze the water in the chamber was invented.In this method,the low-pressure pump is used as the only power source,the reversing valve core changes the flow direction of the fluid to make the piston move back and forth,and relies on the difference in the effective action area of the front and rear cavities in the piston stroke stage,the formed jet has the dual characteristics of"pressurization"and"pulse".The integrated experimental system of jet pressure acquisition,shape capture and rock fragmentation and key equipment such as jet generation device were developed.The characteristics and advantages of SPWJ was illustrated:The pulse pressure is not limited by the pump pressure,the jet energy can completely act on the target,and the stepless regulation of pulse frequency can be realized by meter-out control without affecting the pulse pressure.（2）The multistage pulsation mechanism of SPWJ was revealed.The theoretical model of jet supercharged state is derived based on the fluid compression law,mass conservation law and Bernoulli’s law,combined with the theory of fluid dynamics,the evolution mechanism of jet pulsation was discussed:The modulation effect of the relief valve causes the fluctuation of the oil inlet flow rate and the oil inlet pressure,which leads to the fluctuation of jet pressure in the initial stage of supercharged and two obvious wave peaks,forming the primary pulsation of the jet;Meanwhile,the shock wave generated by the rapid extrusion of the piston is continuously transmitted to the nozzle outlet and disturbs the jet body,multiple shear vortices are generated around the body of the jet under the viscous shear action of the fluid,forming the secondary pulsation on the basis of the primary pulsation of the jet.（3）The optimal design criterion of SPWJ generation device was established.The influence law of key structural parameters of jet generation device on jet characteristic parameters was obtained through the dynamic simulation model of jet generation system,and the optimization objective and optimization method of jet generation device were clarified:The determination of supercharged ratio and nozzle diameter is based on the principle of ensuring that the peak pulse pressure reaches the threshold pressure for rock fragmentation and the maximum single pulse output power,the determination of piston traveling distance and piston diameter d_p2 makes the jet have a larger proportion of pressure pulsation time and a higher pulse frequency peak,meanwhile,the difference between the length of pressurized chamber and piston traveling distance,and piston diameters d_p3 and d_p2 should be reduced as much as possible.On that foundation,the key structural parameters of the jet generation device are reasonably matched.（4）The rock fragmentation performance of SPWJ was evaluated.The rock fragmentation effect of SPWJ,continuous water jet and dis-slotted PWJ were compared and analyzed through laboratory tests.The results show that the brittle and hard rock can produce large volume spalling by the three kinds of jets when the jet output pressure reaches the threshold pressure for rock fragmentation,but compared with the continuous water jet and dis-slotted PWJ under the same hydraulic parameters,the specific energy consumption of SPWJ is reduced by 60.2%and 54%,respectively.The excellence and reliability of the SPWJ generation method were further quantitative highlighted through the rock fragmentation results.The research results in this work will enrich the generation method of PWJ,have guiding significance for improving the efficiency of PWJ for hard rock fragmentation,and lay a foundation for promoting the application of PWJ in deep underground engineering construction.