Effect of Nozzle Structure on Rock Drilling Performances of Abrasive Waterjet
Authors: Li Huan , Huang Zhongwei , Li Jingbin , Cheng Kang , Hu Jingru , Li Wenbin 
Published: September 2023

ABSTRACT
Jet nozzle plays an important role in an abrasive waterjet (AWJ) machining system and determines the rock-breaking performance. Here, we conducted AWJ rock-breaking experiments with straight cone nozzle (SCN), fan-shaped nozzle (FSN), swirling impeller nozzle (SIN) and straight-swirling mixed nozzle (SSMN). Granite and sandstone were selected as the target rock materials. Jet-drilled hole morphology, diameter, depth and volume were performed to evaluate the rock-breaking performances of the nozzles. And, the application scenarios of different AWJ nozzles were illustrated and recommended. Results showed that the SCN and FSN created spindle-shaped holes, and the jet-drilled holes’ cross sections of them were circular and ellipse respectively. They are more suitable for deep hole drilling, such as hydraulic perforation, rock excavation in underground space constructing. The SIN and SSMN formed large circular holes and there was a conical bulge at the hole bottom, it should be noted that the conical bulge of the SSMN drilled hole was shorter than that of SIN. Thus, they are recommended for radial branches drilling in exploiting hot dry rock resources, natural gas hydrate and coalbed methane. The research results are expected to provide a theoretical basis for nozzle selecting of different application scenarios.

INTRODUCTION
Abrasive water jet (AWJ) is one of the advanced modern material machining methods, which is widely used in manufacturing and mining industry for its excellent cutting/drilling ability. In recent years, the applications of AWJ technology in geotechnical engineering (such as unconventional oil and gas exploitation, coal mining, tunnel excavation, geothermal exploitation, natural gas hydrate exploitation and so on) are actively explored and tried by many scholars and engineers. Yang et al. (Yang et al., 2019) proposed an innovative coalbed methane stimulation approach referred to as radial abrasive liquid nitrogen (LN2) jet drilling (ALN-RJD) that uses abrasive LN2 jet to drill multiple laterals radiating out from a main borehole. The experiment and simulation results showed that the abrasive LN2 jet has superior coal-breakage performance and the ALN-RJD can significantly increase the gas production and achieve higher Net Present Value. Liu et al. (Liu et al., 2019) conducted numerical simulation on rock breaking by abrasive water jet-pick. The results indicated that the addition of abrasive water jet reduces the sensitivity of rock strength to confining pressure and brings a better rock breaking performance. Hong et al. (Hong et al., 2021) carried out laboratory experiments on coal-breakage performances by abrasive nitrogen-gas jet. The results illustrated the coal-breakage mechanisms and influence of parameters. Besides, the dimensionless standoff distance and nozzle diameter are the key factors for improving coal-breakage efficiency. Tae-Min Oh et al. (Oh & Cho, 2014, 2016; Oh et al., 2019) systematically studied the granite cutting performance of abrasive water jet and developed a rock cutting depth model based on kinetic energy. Their research results showed that cutting depth efficiency tends to increase with an increase in the water pressure and traverse speed and with a decrease in the standoff distance and uniaxial compressive strength. Besides, the optimum abrasive/water ratio ranges from 0.08 to 0.2 under the conditions given in their study.