Abstract: A series of experiments relating to bubble cloud and erosion mechanism of cavitating water jets were carried out for different nozzles with different pump pressures and ambient pressures. The objective of this work is to find the relation between the erosion and the bubble cloud, determine drilling parameters such as the feed speed or rotating speed for a water jet drilling system, and explore nozzle operating conditions and nozzle designs to reduce the input energy by reducing the pump pressure. Visualization studies of the bubble cloud show that the length and width of the bubble cloud of the convergent-divergent nozzle are larger than those of the convergent-straight nozzle. The erosion tests confirm that it is the bubble collapse that causes the erosion, and the standoff distance plus the drilling depth equals to the bubble cloud length. In drilling brittle hard rock, erosion happens in the first few seconds, and after that, erosion does not increase with time.

Key words: cavitating water jets, bubble cloud, rock-erosion