Design of teaching experiment for stepped nozzle rotary impact crushing of rocks

As the main energy source in China, coal can be considered the “ballast stone” in energy security. However, as the depth of coal mines increases, the gas pressure and content in the coal seam increase, and the risk of gas-related disaster increases. Current technologies for pressure relief and permeability enhancement (such as dense drilling, hydraulic punching/slotting/fracturing, etc.) face problems such as a large engineering volume and a limited permeability enhancement range. In contrast, hydraulic jet tree drilling can form a large-scale fracture network, which is effective for achieving uniform permeability of coal seams. Under laboratory conditions, it is the basis of the development of this technology to clarify the rock breaking mechanism of the stepped nozzle bit. This study is also of great significance for deepening students’ understanding of the theory of water jet rock breaking and its application in engineering, and improving their analytical skills in engineering and innovation ability.

A four-dimensional water jet comprehensive test system developed in-house was used to carry out the rotary impact rock breaking test using the stepped nozzle bit. This system integrates the rock bearing mechanism, which can accurately simulate the working conditions in the field at the millimeter target distance and realize the functions of fixed-point impact, rotary drilling, and transverse cutting of the drill bit. Standard sandstone samples were selected for drilling and treatment in the experiment. To meet the needs of experimental teaching, the speed is set by the control terminal of the rotary drilling device. When a stable speed is achieved, the high-pressure pump is opened and the pressure is set; when the pressure is stable, the sandstone sample is impacted for 30 s, after which the high-pressure pump is closed. By repeating the above steps, the characteristics of rock failure with variation of the parameters are revealed to assist students in understanding the mechanism of rock breaking using a water jet.

The influence of the rotation speed and pressure on the drilling efficiency is quantitatively analyzed. The rotation speed is optimal for a given pressure, drilling depth, diameter, and volume. The optimal rotation speed for the drilling depth under different pressures is 200 rpm; the optimal rotation speed is 150 rpm at pressures ≤ 12 MPa and 200 rpm at pressures ≥ 12 MPa. The optimal rotation speed for the drilling volume is 150 rpm at a pressure = 8 MPa and 200 rpm at pressures > 8 MPa. The sensitivity of boreholes with different geometries and sizes to the rotational speed is significantly different: the depth is the least sensitive, the volume is the second, and the diameter is the most sensitive.

This study introduces the cascade nozzle rotating impact crushing rock test system for experimental teaching of the course “Water Jet Theory and Practice,” and a teaching platform combining the practice and theory of water jet crushing coal and rock mass is built for application in engineering scenarios. The platform is used to explore the characteristics of self-rotating jet erosion and rock breaking, and to reveal the key influence of the pressure and rotational speed on the drilling efficiency. This platform effectively connects basic theoretical knowledge with complex engineering practice, provides support for scientific research and teaching of water jet rock breaking, creates favorable conditions for exploring the characteristics of jet impact rock breaking, and helps improve students’ professional identity, practical application skills, and innovative consciousness.