Experimental study of shock pressure and erosion characteristics of high‐pressure gas–liquid two‐phase jet: Exploration for improving coalbed methane extraction efficiency

Coalbed methane extraction promotes energy exploitation and alleviates the greenhouse effect. This study aimed to analyze the technical potential of a high‐pressure gas–liquid two‐phase jet in increasing coalbed permeability and promoting coalbed methane extraction. A well‐designed experimental setup was used to investigate the erosion characteristics of a high‐pressure gas–liquid two‐phase jet. Through the shock pressure monitoring and erosion tests of the jet, the effects of pump pressure, mixer structure, and nozzle structure on the shock pressure and rock‐breaking performance were studied, and the rock‐breaking mechanism of the high‐pressure gas–liquid two‐phase jet was examined. The results showed that the nonuniformity of mixing and the expansibility of high‐pressure gas continually alters high‐pressure gas–liquid two‐phase jets between single and multiple streams, which makes the jet show obvious pulse effects and considerably improves the rock‐breaking performance. Furthermore, reasonable air pump pressure and water pump pressure settings significantly increase the shock area, whereas the maximum shock pressure only decreases slightly. Although the shock pressure of a high‐pressure gas–liquid two‐phase jet decreases with the increase in air pump pressure, its shock efficiency first increases and then decreases with the increase in air pump pressure. To improve the erosion efficiency of a high‐pressure gas–liquid two‐phase jet, the settings of the air pump pressure and water pump pressure is crucial, but the optimal air pump pressure differs for different water pump pressures. Additionally, the greater the angle between the gas and liquid inlets, the greater the energy loss of the fluid during the mixing. Finally, the rock‐breaking performance showed that the optimal nozzle structure is the single‐cone nozzle.


| INTRODUCTION
Coal dominates the energy consumption of China, constituting 64% of total energy consumption in 2021. Moreover, coal-fired power generation accounted for 60% of the total power generation. 1 Coal production in highgas pressure mines, as well as coal and gas outburst mines, accounts for approximately one-third of the national coal production. Gas disasters severely limit the safe and sustainable development of the coal industry. 2 Methane, the main component of coal mine gases, is not only a clean energy source but also a greenhouse gas. 3 Although coalbed methane extraction effectively controls mine gas disasters, promotes energy exploitation, and alleviates the greenhouse effect, its efficiency decreases continually because of decreased coal seam permeability caused by the increase in coal mining depth. 4,5 Therefore, it is crucial to improve coal seam permeability to ensure safe and efficient coal production in high-gas pressure mines, as well as coal and gas outburst mines. 6 Part of the coal in a coal seam is broken and discharged through the borehole by various means, such as high-pressure water jet slotting and reaming. These methods cause the coal body around the borehole to become creep deformed, reduce the coal in situ stress around the borehole, and promote the crack channel expansion in the coal seams. Water jet technology can be used in soft coal seams to increase coal seam permeability [7][8][9][10][11] , which, being more economical, has been widely used in many mining areas for years.
Abrasive, cavitation, and pulsed jets are used to increase coal seam permeability and improve coalbreaking efficiency. [12][13][14] However, these jet methods have limitations. The abrasive jet makes slag discharge difficult, the cavitation jet impinges on the target area within a small range, and the frequency adjustment of the pulse jet is inconvenient. Therefore, it is crucial to develop a jet flow method with high coal-breaking efficiency, less water consumption, and favorable drilling slag discharge for coal-breaking and permeability enhancement projects. [15][16][17] As early as 1979, Eddingfield and Albrecht 18 concluded that adding air to a pure water jet can effectively improve the target distance of the jet. Momber 19 used an air-water jet nozzle with a diameter of 1.5 mm to conduct erosion tests on concrete with a compressive strength of 39 MPa. The results showed that the addition of air significantly improves the material removal process but has no effect on the cutting depth. The material removal process is sensitive to the amount of air sucked in. Hu et al. 20 established a mathematical model to describe the frequency characteristics of the pulsed air-water jet and performed vibration tests, analyzed the relationship between the vibration acceleration of the pulsed air-water jet and the length of the nozzle cavity, and studied the influence of air concentration in the cavity on the material removal rate. Guha et al. 21 formulated an equation for the volumetric fraction of air entering water, simulated the jet structure and jet velocity distribution of the liquid phase, and captured the process of air entrainment in the jet and the subsequent pressure characteristics. Siamas et al. 22 used Euler's method to describe a two-phase flow system for treating mixed fluid and analyzed the influence of liquid gas density ratio on the flow state. Shang et al. 23 conducted an orthogonal experiment on rock-breaking by underwater gas-liquid two-phase jet and they studied the effects of the water jet system pressure, air flow pressure, transverse velocity, and erosion distance on the rockbreaking performance of the underwater gas-liquid twophase jet. Lin et al. 24 conducted rock-breaking experiments using a gas-liquid two-phase jet. They found that under the same conditions, compared with pure water jet, the erosion depth of the gas-liquid two-phase jet increased by 25% and the rock breaking efficiency increased by 80%.
Early research has shown that a gas-liquid two-phase jet has a significant advantage in rock-breaking and material removal, but the research focus has been on the flow field structure, pulse characteristics, and erosion ability of gas-liquid two-phase jet. No research has analyzed the shock force distribution characteristics of a high-pressure gas-liquid two-phase jet, and limited research has been conducted on the influence of mixer and nozzle structure on the shock force and erosion characteristics of a high-pressure gas-liquid two-phase jet. Moreover, the high-efficiency rock-breaking mechanism of the high-pressure gas-liquid two-phase jet requires further analysis.
In this study, a test system of a high-pressure gas-liquid two-phase jet was built for the real-time monitoring of the shock force distribution characteristics of the high-pressure gas-liquid two-phase jet. The shock pressure monitoring and erosion test of the high-pressure gas-liquid two-phase jet were performed. Further, the effects of pump pressure, mixer structure, and nozzle structure on shock pressure and erosion characteristics were studied. Finally, the highefficiency rock-breaking mechanism of the highpressure gas-liquid two-phase jet was examined. The results of this study would contribute to the application of a high-pressure gas-liquid two-phase jet in coalbed methane extraction engineering.

| Experimental apparatus
To study the shock pressure and rock-breaking performance of high-pressure gas-liquid two-phase jet, a test system was built (Figure 1). The rated pressure of the high-pressure air and high-pressure water pumps was 35 and 50 MPa, respectively, and the corresponding rated flow was 1500 and 115 L/min, respectively. The hydraulic cylinders were installed on the jet test bench to provide a confining pressure for the experimental sample and the nominal force of the press was 100 kN. Multiple pressure and flow sensors were used to monitor the flow pressure and gas flow in the pipeline. The jet shock pressure was monitored using a distributed pressure sensor of 45 mm × 45 mm, at 96 measuring points, and the collection frequency was 20 Hz.
Artificial cement mortar samples with good homogeneity, which were made of cement, quartz sand, and lime, were used to avoid the influence of coal bedding and joints instead of coal samples. The dimension of the samples was 100 mm × 100 mm × 100 mm, and their mechanical parameters are shown in Table 1.

| Experimental strategy
The experiments aimed to study the effects of pump pressure, gas-liquid mixer structure, and nozzle structure on the rock-breaking performance of high-pressure gas-liquid two-phase jet, which were divided into six groups, as shown in Table 2. The angle between the gas and water inlets of the gas-liquid mixer and the nozzle channel structure substantially affected the high-pressure F I G U R E 1 Test system of high-pressure gas-liquid two-phase jet.  gas-liquid two-phase jet. Different structures of gas-liquid mixers were designed for the experiments to improve the erosion capacity of the high-pressure gas-liquid two-phase jet, as shown in Figure 2. P g was the air pump pressure, P l was the water pump pressure, and α was the angle between the gas and water inlets of the gas-liquid mixer. A cone-straight nozzle and conical nozzle are typical nozzle structures in the field of coal mining. The cone-straight nozzle structure is mainly composed of a contraction section, a straight pipe section, and an embrasure reaming section, which differ for different work purposes ( Figure 3A). The contraction section is used for gathering fluid and energy; the straight pipe section is used for stabilizing the flow characteristics; and the embrasure reaming section is used for controlling the diffusion angle of the jet outlet. 2,25 To improve the shock force of the jet, the cone-straight nozzle used in coal mining generally lacks the embrasure reaming section. The inner part of the conical nozzle is a conical flow passage ( Figure 3B).
To study the influence of nozzle structure on the shock pressure and erosion characteristics of a highpressure gas-liquid two-phase jet, five types of commonly used nozzles were selected for high-pressure gas-liquid two-phase jet erosion tests. The nozzles were conestraight nozzle, large-angle cone-straight nozzle, long cone-straight nozzle, single-cone nozzle, and doublecone nozzle. For easy distinction, a conventional nozzle structure was named a cone-straight nozzle. The largeangle cone-straight nozzle, long cone-straight nozzle, and double-cone nozzle are types of the cone-straight nozzle and their names were determined according to their structural characteristics. The angle of contraction section of the large-angle cone-straight nozzle was larger than that of the cone-straight nozzle. Similarly, the straight pipe section of the long cone-straight nozzle was F I G U R E 2 Gas-liquid mixer structure. longer than that of the cone-straight nozzle. The doublecone nozzle was a cone-straight nozzle with an embrasure reaming section. The single-cone nozzle was a conical nozzle ( Figure 4).

| Sample preparation
The test samples were prepared using cement, quartz sand, and lime, and the preparation process was divided into steps of weighing, mixing, shaking, demolding, and curing steps. Mechanical parameter tests were performed after 28 days of curing.

| Experiment of shock pressure monitoring of high-pressure gas-liquid two-phase jet
The shock pressure monitoring experiments were performed under different conditions according to the experimental protocol shown in Table 2. The maximum pressure of the water pump was 7 MPa and the impact time was 12 s to prevent damage to the distributed pressure sensor. To control the shock time accurately, the test sample was covered with an iron plate before the experiment. When the water pump pressure and air pump pressure reached the design values, the iron plate was pulled out instantaneously. After the experiment, the air source and water source were quickly disconnected, and the cover plate was covered.

| Erosion experiment of high-pressure gas-liquid two-phase jet
The distance between the nozzle and the test sample was set to 100 mm. According to the experimental protocol shown in Table 2, the depth, area, and volume of erosion pits were measured using the high-pressure gas-liquid two-phase jet, shocking the specimens for 60 s under different conditions. The erosion pit volume was measured by the sand-filling method. The erosion pit was first filled with sand and the sand was poured into a measuring cylinder to measure the sand volume, which was taken as the erosion pit volume. The effective diameter of the erosion pit was obtained by image processing. The test sample images were imported into image processing software and the perimeter of the erosion pit surface was calculated by tracking points. The erosion pit was equated to a circle and the effective diameter of the erosion pit was back-calculated. Meanwhile, the erosion pit depth was measured directly using vernier calipers.

| Effect of pump pressure on shock pressure and erosion characteristics
Shock pressure is important for examining jet erosion capability and describing rock-breaking performance. The shock pressure and area of the high-pressure gas-liquid two-phase jet under different conditions were monitored using the distributed pressure sensor.  illustrates the relationship between the pump and shock pressures. The figure shows that the magnitude and change range of the jet shock pressure changed considerably because of the gas incorporation. As the air pump pressure increased, the shock pressure decreased. Setting the air pump pressure P g = 0 as a reference, the maximum shock pressure of P g = 5 MPa, P g = 6 MPa, and P g = 7 MPa was reduced by 3%, 26%, and 88%, respectively. The fluctuation range of the shock pressure first increased and then decreased. When the air pump pressure was 5 MPa, the maximum change amplitude of the shock pressure reached 507.12 KPa.
During the experiment, it was found that the structure of the high-pressure gas-liquid two-phase jet changed constantly; the shock area also changed. Figure 6 shows that the jet shock area increased with an increase in the air pump pressure. When the air pump pressure increased to 7 MPa, the maximum shock area of the high-pressure gas-liquid two-phase jet reached 946 mm 2 . Setting the air pump pressure P g = 0 as a reference, the maximum shock area of P g = 5 MPa, P g = 6 MPa, and P g = 7 MPa was increased by 420%, 440%, and 490%, respectively. The high-pressure gas expanded rapidly after being ejected from the nozzle, effectively increasing the jet shock area. The shock area and pressure were inversely proportional.
To further analyze the characteristics of the shock force of the high-pressure gas-liquid two-phase jet, the shock force on each position of the target was studied. Figures 7-10 illustrate the distribution state of the shock pressure. The water jet shock area was concentrated and the shock pressure was large. The shock pressure and F I G U R E 5 Relationship between shock pressure and pump pressure.
F I G U R E 6 Relationship between shock area and pump pressure.
shock area of the high-pressure gas-liquid two-phase jet changed rapidly, and the shock force and shock area were negatively correlated. The two main reasons for the above phenomena are as follows: first, as the air pump pressure increased, the amount of air entering the mixer increased, the water volume decreased, the jet shock energy decreased, and the shock pressure decayed. Second, gas expansion caused the jet structure to be more divergent, and the nonuniformity of the gas-liquid mixing caused the jet structure to change constantly; therefore, the jet shock force and shock area fluctuated considerably. Under the influence of high-pressure gas, a single jet was converted into multiple jets, and the higher the air pump pressure was, the more obvious the jet conversion was. When the air pump pressure was 5 MPa, the jet constantly alternated between single strand and multistrand. When the air pump pressure was 7 MPa, the high-pressure gas-liquid two-phase jet comprised multiple jets. Figure 11 shows that the shock pressure and shock area of the jet continued to decrease as the water pump pressure decreased. At the same water pump pressure, the shock force decreased as the air pump pressure increased and the shock area increased as the air pump pressure increased. When the water pump pressure P l = 7 MPa, the air pump pressure P g changed from 0 to 5 MPa, the maximum shock area of the jet increased from 194 to 823 mm 2 , and the maximum shock pressure decreased from 550 to 526 KPa. Thus, the maximum shock area of the jet increased by 324.23% and the maximum shock pressure decreased by only 4.36%. These results show that by properly configuring the air and water pump pressures, the jet can slightly reduce the maximum shock pressure but considerably increase the shock area.
To study the coal breaking effect of a high-pressure gas-liquid two-phase jet under different pump pressure conditions, an erosion test of high-pressure gas-liquid two-phase jet was performed. Figures 12 and 13 show that the rock-breaking performance of the jet increased with increasing pump pressure. Mixing a certain amount of high-pressure air with the high-pressure water considerably improved the rock-breaking performance of the jet. When the water pump pressure was P g = 16 MPa, the air pump pressure increased from 0 to 10 MPa and the erosion pit depth, effective diameter, and volume increased by 13.32%, 24.07%, and 73.91%, respectively. When the air pump pressure P g = 0 MPa and the water pump pressure P l = 8, 12, and 16 MPa, respectively, the erosion pit depth was 20.38, 26.54, and 32.89 mm, respectively. When the water pump pressure P l increased from 8 to 12 MPa, the erosion pit depth increased by 30.23%. When the water pump pressure P l increased from 12 to 16 MPa, the erosion pit depth increased by 23.93%.
When the water pump pressure was 8 MPa, the effective diameter of the erosion pit increased with the increase in air pump pressure; when the water pump pressure was 12 MPa, the effective diameter of the erosion pit first increased significantly and then decreased slightly with the increase in air pump pressure. When the water pump pressure was 16 MPa, the effective diameter of the erosion pit first increased, then decreased, and then increased with the increase in air pump pressure. These results show that differences exist in the variation law of the effective diameter of the erosion pit under the three conditions, which is mainly related to the nonuniformity of the test samples. However, the overall law is that the effective diameter of the erosion pit increases with the increase in air pump pressure.
The depth, effective diameter, and volume of erosion pits are usually considered as direct indicators in evaluating the erosion efficiency. As the shock time was the same for each experiment and the shape of the erosion pit was irregular, the volume of the erosion pit was used as the main evaluation index of the erosion efficiency. As shown in Figure 12, the erosion efficiency of the high-pressure gas-liquid two-phase jet first increased and then decreased with increasing air pump pressure. Combined with the experimental results at F I G U R E 12 Rock-breaking performance of jet under different pump pressures. (A) P g (0 MPa) + P l (8 MPa); (B) P g (5 MPa) + P l (8 MPa); (C) P g (5.5 MPa) + P l (8 MPa); (D) P g (6.5 MPa) + P l (8 MPa). (B) P g (0 MPa) + P l (12 MPa); (C) P g (7 MPa) + P l (12 MPa); (D) P g (7. 5 MPa) + P l (12 MPa); (E) P g (8 MPa) + P l (12 MPa). (F) P g (0 MPa) + P l (16 MPa); (G) P g (10 MPa) + P l (16 MPa); (H) P g (10.5 MPa) + P l (16 MPa); (I) P g (11 MPa) + P l (16 MPa). different pump shock pressures, it was found that although the pure water jet produced a more considerable shock pressure, it produced a significant water cushion effect during rock shock, reducing the rockbreaking efficiency of the pure water jet. The mixture of high-pressure water and high-pressure gas formed a complex jet structure with a considerable variation in jet shock pressure, which inhibited the water cushion effect. The addition of high-pressure gas increased the shock area of the jet, further improving the rock-breaking efficiency.
To improve the erosion efficiency of a highpressure gas-liquid two-phase jet, the settings of the air pump pressure and water pump pressure are crucial, but the optimal air pump pressure differs for different water pump pressures. In this study, it can be concluded that the air pump pressure should be set to 5.5, 7.5, and 10 MPa when the water pump pressure is set to 8, 12, and 16 MPa, respectively. More tests should be conducted for other pressure levels to determine the values of the pressure combination of air pump and water pump.
During the experiment, flow sensors were used to record the amount of water and gas used in the system under different pump pressures. Figure 14 shows the variation of water and air flow rates with pump pressure during the experiment: the water flow rate decreased considerably with increasing the air pump pressure. When the water pump pressure was P l = 16 MPa, the air pump pressure increased from 0 to 10 MPa and the water flow rate decreased by 22.73%. The high-pressure gas-liquid two-phase jet effectively reduced the water consumption of the project.

| Effect of gas-liquid mixer structure on shock pressure and rockbreaking performance
Experiments on the shock pressure monitoring of highpressure gas-liquid two-phase jet by using gas-liquid mixers with different structures. As shown in Figures 15  and 16, as the angle between the gas and water inlets of the gas-liquid mixer decreased, the fluctuation range of the jet shock pressure and shock area increased continuously. This phenomenon shows that the smaller the angle between the gas and water inlets, the more unstable the two-phase flow is, causing the jet structure to change drastically quickly. When the included angle α was 0°, the amplitude of the jet shock pressure reached 507.12 KPa and the frequency reached 10 Hz. When the angle α was 90°, the jet shock force was considerably reduced because of the large amount of energy consumed in the gas-liquid mixing process. Figure 17 shows that the maximum shock pressure and shock area of the jet increased continuously as the angle between the gas and liquid inlets decreased. When the angle between the gas and liquid inlets was reduced from 90°to 0°, the maximum shock pressure increased by 52.77% and the maximum shock area increased by 18.93%. The average shock pressure and shock area are critical parameters for describing the jet energy. The average shock pressure and shock area increased slightly as the gas-liquid inlet angle decreased, showing that the smaller the gas-liquid inlet angle, the smaller the energy loss during gas-liquid mixing.
To analyze the influence of the gas-liquid mixer structure on the erosion efficiency of a high-pressure gas-liquid two-phase jet, erosion experiments of a highpressure gas-liquid two-phase jet under different mixer structures were performed. Figures 18 and 19 show that the average effective diameters of the erosion pits were 31.89, 27.80, 28.74, and 27.73 mm at the same pump pressure conditions when the inclusion angles of the gas and liquid inlets of the mixer were 90°, 60°, 30°, and 0°, respectively. The experimental results show that the average effective diameter of the erosion pits changes slightly and has no specific relationship with the angle between the gas and liquid inlet of the mixer. The depth and volume of the erosion pit increased with a decrease in the angle between the gas and liquid inlets. This phenomenon occurred because the depth and volume of the erosion pit were mainly affected by the shock force and shock energy. The lower the energy loss of the liquid during mixing, the greater the depth and volume of the erosion pit.

| Effect of nozzle structure on shock pressure and rock-breaking performance
The nozzle is an important part of the jet system. To analyze the influence of nozzle structure on the shock force of high-pressure gas-liquid two-phase jet, shock pressure monitoring experiments were performed. Figure 20 shows that conical nozzles with different structures had higher shock pressures, whereas doublecone nozzles had consistently lower shock pressures. Different structures of conical nozzles had different frequencies of shock pressure variation. The conical nozzle had the highest shock pressure amplitude. Figure 21 shows that the shock area of the singlecone nozzle changed periodically, and the frequency of the larger shock area was the largest. Figure 22 shows that the nozzle structure had a considerable influence on the jet shock pressure and shock area. When the nozzle was changed from a double-cone to a single-cone structure, the jet maximum shock pressure increased by 33.83%, and the jet maximum shock area increased by 238.18%. The average shock pressure and average shock area of the single-cone nozzle were larger than those of other structures, showing that the two-phase flow through the single-cone nozzle had the least capacity loss. To analyze the influence of nozzle structure on the erosion efficiency of a high-pressure gas-liquid two-phase jet, erosion experiments were performed using different nozzle structures of a high-pressure gas-liquid two-phase jet. As shown in Figure 23, the large-angle cone-straight nozzle had considerable advantage in the effective diameter of the erosion pit. The effective diameters of erosion pits produced by the cone-straight nozzle, largeangle cone-straight nozzle, long cone-straight nozzle, single-cone nozzle, and double-cone nozzle were 28, 32, 26.78, 29.97, and 27.55 mm, respectively. The effective diameter of the erosion pit produced by the large angle cone-straight nozzle was 12.50%, 16.31%, 6.34%, and 13.91% greater than that produced by the cone-straight nozzle, long cone-straight nozzle, single-cone nozzle, and double-cone nozzle. The volume and depth of erosion pit produced by the cone-straight nozzle, large-angle conestraight nozzle, long cone-straight nozzle, single-cone nozzle, and double-cone nozzle were 4.2 cm 3 and 17.12 mm, 4.9 cm 3 and 23.03 mm, 4.1 cm 3 and 15.81 mm, 5.8 cm 3 and 27.98 mm, and 3.2 cm 3 and 17 mm, respectively. The volume and depth of the erosion pit produced F I G U R E 19 Relationship between characteristic parameters of rock-breaking performance and gas-liquid mixer structure.
F I G U R E 20 Relationship between shock pressure and nozzle structure. by the single-cone nozzle were 27.59% and 38.81%, 15.52% and 17.69%, 29.31% and 43.50%, and 44.82% and 39.24%, respectively, higher than those produced by the cone-straight nozzle, large-angle cone-straight nozzle, long cone-straight nozzle, and double-cone nozzle. Thus, the single-cone nozzle had considerable advantage in the volume and depth of the erosion pit. Although the effective diameter of the erosion pit produced by the large-angle cone-straight nozzle was the largest, the depth and volume of the erosion pit produced by the nozzle were smaller than those of the single-cone nozzle. Therefore, the single-cone nozzle has better rock-breaking efficiency.
4 | DISCUSSION 4.1 | The high-efficiency rock-breaking mechanism of the high-pressure gas-liquid two-phase jet Artificial cement mortar samples were used to replace coal samples in the experiments. Thus, the coalbreaking efficiency of the high-pressure gas-liquid two-phase jet was determined by studying its efficiency on the artificial cement mortar sample erosion. Based on the above analysis, the high-pressure gas-liquid F I G U R E 21 Relationship between shock area and nozzle structure. two-phase jet has a higher coal-breaking efficiency than that of the water jet. The main reasons for this phenomenon are as follows: 1. The high-pressure gas-liquid two-phase jet can generate pulsating shock pressure. The uneven mixture of the high-pressure water and gas creates a significant pulsation effect in the shock pressure of the high-pressure gas-liquid two-phase jet. Under changing shock pressure, the rock fatigues, the water cushion effect is reduced,and the rock is more susceptible to internal tensile or shear damage. 2. The structure of the high-pressure gas-liquid twophase jet changes constantly. The flow regime of the high-pressure gas-liquid two-phase jet changes continuously from a single jet to multiple jets within a short time. The flow regime has a considerable impact on rock damage development. When the jet is in a single-jet state, the jet shock force is relatively large and the jet produces a relatively deep pilot erosion pit. When the jets are in a multijet state, they form smaller erosion pits around the pilot erosion pit. Under the action of high-pressure gas-liquid two-phase jetting, the fracture develops from smaller erosion pits to the pilot erosion pit. As the crack penetrates the pilot erosion pit, the rock mass is eroded in blocks, considerably increasing the efficiency of the jet breaker in breaking the coal ( Figure 24A,B).

| Application of high-pressure gas-liquid two-phase jet in coalbed methane extraction engineering
Erosion characteristic is a vital factor in determining whether high-pressure jet can be applied to coalbed methane extraction engineering. 26,27 According to the above experimental studies, high-pressure gas-liquid two-phase jet not only has good erosion characteristics but also has good water-saving performance. According to the analysis of the actual engineering situation on site, using a high-pressure gas-liquid two-phase jet to facilitate coalbed methane extraction has the following three potential advantages compared with using water jet: (1) Coal-breaking efficiency improvement and system working pressure reduction. High-pressure water jet erosion of coal can cause the coal body around the borehole to become creep deformed, reduce the coal in situ stress around the borehole, promote the crack channel expansion in the coalbed, and increase the permeability of the coalbed. 10,28-30 A reasonable increase in the broken volume of coal can effectively improve the effect of permeability increasing, so the jet erosion efficiency can reflect the efficiency of coalbed permeability increasing to a certain extent. Experiments showed that when a certain amount of gas was injected into the water jet without changing the water pump pressure, the jet erosion efficiency increased by 73.91%. Therefore, high-pressure gas-liquid two-phase jet has great technical potential in increasing coalbed permeability and promoting coalbed methane extraction.
High-pressure equipment in coal mines typically pose serious danger; therefore, reducing the working pressure of equipment is crucial to ensuring safe production in mines. (2) Water consumption reduction and drainage pressure relief of mine roadway. Compared with the water jet water consumption, that of the high-pressure gas-liquid two-phase jet can be reduced by more than 20%. The reduction in water consumption not only prevents water wastage but also relieves the drainage pressure of the mine roadway and avoids large-scale water accumulation in the mine roadway. (3) Borehole drainage facilitation. After hydraulic punching, a large quantity of water accumulates in the drilled hole, severely affecting the desorption of coal seam gas ( Figure 25). During coal-breaking and permeability enhancement, the high-pressure gas-liquid two-phase jet easily transports water from the borehole, thereby reducing water accumulation in the borehole and facilitating coal seam gas desorption.
Compared with the water jet system, the highpressure gas-liquid two-phase jet system has certain complexities. Factors such as pump pressure and jet component structure considerably influence the rockbreaking effect of the high-pressure gas-liquid two-phase jet system. There are many details that still need to be determined in the development of high-pressure gas-liquid two-phase jet equipment to increase coal seam permeability. In this study, we investigated the influence of pump pressure, mixer structure, and nozzle structure on the rock-breaking performance of highpressure gas-liquid two-phase jet. The results can provide an important theoretical basis for designing high-pressure gas-liquid two-phase jet equipment to increase coalbed permeability and coalbed methane extraction efficiency.

| CONCLUSIONS
In this study, a high-pressure gas-liquid two-phase jet test system was set up and jet shock pressure monitoring and erosion tests were performed at different pump pressures, mixer structures, and nozzle structures. The rock-breaking mechanism of the high-pressure gas-liquid two-phase jet was analyzed and the following conclusions were obtained: (1) The nonuniformity of mixing and the expansibility of high-pressure gas causes the continuous alternation of high-pressure gas-liquid two-phase jets between single and multiple streams, which makes the jet show obvious pulse effects and significantly improves the rock-breaking performance. Reasonable air pump pressure and water pump pressure settings can significantly increase the shock area while slightly decreasing the maximum shock pressure. (2) Although the shock pressure of a high-pressure gas-liquid two-phase jet decreases with the increase in air pump pressure, its shock efficiency first increases and then decreases with the increase in air pump pressure. To improve the erosion efficiency of a high-pressure gas-liquid two-phase jet, the setting of the air pump pressure and water pump pressure is crucial, but the optimal air pump pressure differs for different water pump pressures. (3) The greater the angle between the gas and liquid inlets, the greater the energy loss of the fluid during the mixing. When the angle between the gas and liquid inlets changes from 90°to 0°, the erosion pit volume increases by 106.67%. The average effective diameter of the erosion pit has no specific relationship with the angle between the gas and liquid inlet of the mixer. The choice of the single-cone nozzle can improve the rock-breaking performance of the high-pressure gas-liquid two-phase jet.