Damage and Breakage Characteristics of Loaded Coal Impacted by High-Pressure Pulse Water Jet and Its Influence Factors

To elucidate the evolution laws of impact velocity of high-pressure pulse water jet and the breakage characteristics of coal under confining condition, a coupled smoothed particle hydrodynamics-finite element (SPH-FEM) algorithm is adopted. A sinusoidal velocity is applied to the plunger inside the pipeline. The evolution laws of water jet velocity inside and outside the nozzle were obtained, and the temporal damage and breakage characteristics of coal under load and unload conditions impacted by pulse water jet were compared and analyzed. The influence of key parameters such as average velocity, pulse amplitude, and pulse frequency on damage and breakage characteristics of coal was revealed. The results show that the velocity evolution of water jet particles inside and outside the nozzle undergoes four stages: a stationary stage and transient acceleration to a low speed in the pipeline, acceleration inside the convergent section of the nozzle, micro-acceleration inside the straight section of the nozzle, and pulse variation speed following a sinusoidal variation after exiting the nozzle. Under the stress free and two-dimensional stress load conditions, the broken pits of coal specimen exhibit an abnormal development, and undergoes from bowl shape to U-shape, respectively. Two-dimensional stress load has a suppressive effect on the derivation and propagation of internal cracks in coal, reducing the rock-breaking efficiency. Besides, pulse water jet has a higher rock-breaking efficiency on loaded coal specimens than that of continuous water jet. The depth and area of coal fragmentation increase exponentially with the increase of plunger’s average velocity or pulse amplitude, and show a trend of initial increase and subsequent decrease with the increase of pulse frequency, indicating the existence of an optimal pulse frequency for coal fragmentation. The research findings could provide a theoretical guidance for improving the rock-breaking efficiency of high-pressure pulse water jet under confining conditions and optimizing the working parameters.