Analysis on the Shape and Impact Pressure of the High-Pressure Water Jet during the Hydraulic Flushing Cavity Technique

Feng Du,Shouqing Lu,Chengfeng Wang,Wei Wang,Mingjie Li,Dongti Zhang

doi:10.1155/2021/7496540

Abstract

A large proportion of minable coal seams in China belong to low-permeability soft coal seams. Such coal seams suffer serious coal and gas outburst hazards and endure a high incidence of major disasters in coal mines. The adoption of the high-pressure water jet (HPWJ) hydraulic flushing cavity can effectively promote the gas drainage efficiency and volume and eliminate the hidden danger of gas disasters. Nevertheless, the shape and impact pressure of rotating HPWJ are rarely researched. In this study, on the basis of the numerical simulation, the axial and radial stress distributions of HPWJ and the energy-gathering effect of a conical-cylindrical combined nozzle were analyzed. It is concluded that the submerged condition will accelerate the attenuation of jet velocity and reduce the impact strength of the jet. The jet diffusion angle grows with the increases in the nozzle diameter and water pressure, and 24° is the optimal contraction angle. Finally, the influences of factors such as the rotation speed on the shape and impact pressure of HPWJ were explored, and the results show that the rotation speed should be controlled within 90 r/min. The research findings lay the foundation of the study on the mechanism of coal crushing by HPWJ and provide technical support for the research and development of drilling and flushing integrated equipment.

Highlights

China, a country where over 95% of coal mines are recovered by means of underground mining [1,2,3,4], is suffering the most serious coal and gas outburst disasters in the world [5,6,7]
The maximum impact pressure is as high as 188.1 Maximum impact stress (MPa) at the target distance of 10 mm, but it drops by 71.8% to 53.1 MPa at the target distance of 120 mm
The axial and radial stress distributions of the high-pressure water jet (HPWJ) and the energy-gathering effect of a conical-cylindrical combined nozzle were analyzed by simulation

Summary

Introduction

A country where over 95% of coal mines are recovered by means of underground mining [1,2,3,4], is suffering the most serious coal and gas outburst disasters in the world [5,6,7]. Scholars all over the world have conducted extensive research studies on rock crushing by HPWJ and have proposed a variety of theories, including the water hammer effect theory, the stress wave effect theory, the impact effect theory, the water wedge effect theory, the cavitation effect theory, and the pulsed load-induced fatigue damage theory [23,24,25,26] The stress wave effect theory proposed by Singh and Hartman [27] in 1961 is the earliest accessible theory about rock crushing and failure under the action of jets Based on this theory, Farmer and Attewell [28] put forward an empirical formula for the jet cutting depth and the P-wave velocity, i.e., the sound velocity. Wang et al [39, 40] simulated the process of HPWJ-induced rock crushing by means of nonlinear dynamic FEM and rock dynamic damage modeling. The research findings facilitate the study on the mechanism of HPWJ-induced coal crushing and provide technical support for the research and development of drilling and flushing integrated equipment

Impact Characteristics and Shape of HPWJ

Energy-Gathering Effect of the ConicalCylindrical Combined Nozzle

Influence of the Rotation Rate on the Impact Shape and Stress of HPWJ

Findings

Conclusions