Experimental study of flow field structure of interrupted pulsed water jet and breakage of hard rock

• The tip structure is a tile-like structure after contacting the wall.
• The internal damage of rock was produced mainly by the action of stress waves.
• Jet diameter, pulsation frequency and jet power are key factor on erosion rate.
• The concept of composite pulsating stress was proposed.

Interrupted pulsed water jet has great potential for application in hard rock breakage as it can be easily generated and controlled. The sharp nature of the head and tail of the jet significantly influences on water hammer pressure of the jet and efficiency of hard rock breakage. In this paper, the evolution of a single pulse was captured using high-speed photography and analyzed by numerical simulation. It was found that the velocity of the jet head was slower than the jet velocity at the nozzle exit, but the former still had high turbulent kinetic energy. A U-shaped structure was formed after the jet head made contact with the target, and it in turn affects the peak and duration of the water hammer pressure, as well as its distribution on target. The effect of stress wave propagation on rock breakage during erosion of granite by interrupted pulsed water jet was investigated by ultrasonic testing. Results showed that there were two modes of rock breakage, namely macroscopic fracture of rock blocks, and fixed-depth erosion hole formation. These were caused by the combined effects of the impact stress wave and the pulsating water wedge pressure. The effects of jet diameter, pulsation frequency and jet power on erosion rate and specific erosion energy were analyzed.