Ling Cheng, Jing Xu, Shoujin Peng,*, Hilin Yng, Feng Jio, Bin Zhou, Fzhi Yn,c

a State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400030, China

b College of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, China

c College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Keywords:Multiphase flow Coal and gas outburst Dynamic disaster Impact airflow T-shaped bifurcated roadway Coal seam

A B S T R A C T The study of the dynamic disaster mechanism of coal and gas outburst two-phase flow is crucial for improving disaster reduction and rescue ability of coal mine outburst accidents.An outburst test in a T-shaped roadway was conducted using a self-developed large-scale outburst dynamic disaster test system.We investigated the release characteristics of main energy sources in coal seam, and obtained the dynamic characteristics of outburst two-phase flow in a roadway.Additionally, we established a formation model for outburst impact flow and a model for its flow in a bifurcated structure.The results indicate that the outburst process exhibits pulse characteristics, and the rapid destruction process of coal seam and the blocking state of gas flow are the main causes of the pulse phenomenon.The outburst energy is released in stages, and the elastic potential energy is released in the vertical direction before the horizontal direction.In a straight roadway,the impact force oscillates along the roadway.With an increase in the solid–gas ratio, the two-phase flow impact force gradually increases, and the disaster range extends from the middle of the roadway to the coal seam.In the area near the coal seam, the disaster caused by the two-phase flow impact is characterized by intermittent recovery.In a bifurcated roadway, the effect of impact airflow on impact dynamic disaster is much higher than that of two-phase flow,and the impact force tends to weaken with increasing solid-gas ratio.The impact force is asymmetrically distributed;it is higher on the left of the bifurcated roadway.With an increase in the solid-gas ratio, the static pressure rapidly decreases, and the bifurcated structure accelerates the attenuation of static pressure.Moreover,secondary acceleration is observed when the shock wave moves along the T-shaped roadway,indicating that the bifurcated structure increases the shock wave velocity.

1.Introduction

Energy is fundamental for the survival and development of human society.China’s energy resources are characterized by‘‘lack of gas,less oil,and relatively rich coal”.Therefore,coal will be China’s primary energy source in the long term[1].However,the gradual depletion of shallow coal resources has resulted in an increase in the mining depth[2,3].This causes coal and gas outburst(hereinafter referred to as ‘‘outburst”) accidents under mining disturbance, adversely impacting the safe and efficient production in the mine.Outburst is a dynamic mine gas phenomenon caused by the sudden entry of a large amount of coal carrying a large quantity of gas into the mining space; the process is typically accompanied by a rapid and violent release of energy [4].The shock wave, impact airflow, and two-phase coal-gas flow generated in the mining space expose underground workers and production equipment to strong impact dynamic disasters [5–7].Rock cross-cut coal uncovering is one of the main conditions inducing coal and gas outburst, with high risk and intensity [8].When the outburst is induced by rock cross-cut coal uncovering,the outburst fluid mostly migrates along the T-shaped roadway.For example,the Zijiang coal mine mega-outburst accident in Hunan caused 22 deaths and direct economic losses of 11.3 million yuan(Fig.1).The outburst accident occurred during the rock cross-cut coal uncovering.The outburst coal-gas two-phase flow moved from the cross-cut roadway to the main haulage roadway in the early stage.It demonstrates the dynamic disaster-causing phenomenon of outburst two-phase flow in the T-shaped roadway.Hence, exploring the dynamic disaster-causing mechanism of the outburst two-phase flow in the T-shaped bifurcation roadway is of guiding significance to limit the scope of the outburst disastercausing and mitigate the consequences of the accident.Moreover,it can also serve as a reference for auxiliary rescue decision-making after the accident to ensure the safety of rescuers.

Fig.1.Schematic diagram of outburst accident induced by rock cross-cut coal uncovering in the Zijiang coal mine in Hunan Province.

It is known that the outbursts mainly occur under the comprehensive action of in-situ stress, gas pressure, and physical and mechanical properties of coal [9].Statistically, the outburst typically occurs in areas with complex geological structures [10,11].This suggests that the influence of geological structure on these three factors cannot be ignored.However,this makes outburst prevention and control more challenging, resulting in continued occurrence of outburst accidents.When an outburst accident occurs, its strong destructive power and sudden characteristics make on-site monitoring difficult [12].Thus, experimental and numerical simulation analysis has become the most effective means to study outburst disasters [13–15].Initially, researchers explored the basic properties of raw coal,such as mechanics,seepage, adsorption, and desorption based on small samples [16–18].This laid the foundation for the study of the outburst mechanism.Subsequently, some scholars independently attempted to eliminate coal and gas outburst in mines.They focused on the influence of in-situ stress,gas pressure,and physical and mechanical properties of coal on the outburst formation and development process[19–22].In addition, some scholars examined the energy source,energy accumulation principle, and release mechanism of outbursts in terms of outburst energy [23–25].However, the existing research has failed to completely eliminate the occurrence of outburst accidents.

With the gradual deepening of the working face, the geological conditions become more complex, and outbursts become difficult to control [26].Some researchers conducted a preliminary study of the migration characteristics of the outburst fluid and the disaster-causing mechanism.In the early stages of research, the propagation law of outburst gas overpressure was simply monitored experimentally,and numerical simulation was used to assist the analysis [27,28].The development of improved experimental devices has enabled the investigation of the effect of rapid desorption on the migration and flow of outburst airflow and shock waves in the roadway[29–32].However,most experimental or numerical simulation studies often focus on fluid overpressure, which can only reflect the evolution process of outburst gas.Research on the role of pulverized coal in fluid transport remains limited[33,34].Moreover,there is a lack of research on the migration characteristics and the dynamic disaster mechanism underlying the outburst fluid under specific layouts, particularly T-shaped bifurcated roadways.

In this study,we aimed to address this research gap by focusing on the migration and disaster-causing characteristics of the outburst fluid.To that end, we used a self-developed large-scale outburst dynamic disaster test system to simulate a dynamic disaster in a T-shaped roadway under an outburst induced by rock cross-cut coal uncovering.We examined the key parameters in the process of outburst energy release,namely the evolution processes of gas pressure and in-situ stress.Furthermore,we determined the evolution characteristics of the coal-gas two-phase flow impact force and static pressure in the roadway.We established a formation model for outburst impact flow and a model for the flow in a bifurcated structure,which could adequately explain the phenomena observed in the experiment.The results can serve as a reference and guide coal mines to reasonably arrange effective disaster prevention and resilience facilities and create emergency rescue plans for outburst disasters.

2.Coal and gas outburst experiment

2.1.Experimental system

The self-developed coal and gas outburst dynamic disaster test system used in the experiment is shown in Fig.2[35].The outburst system mainly consists of four subsystems: power, roadway, gas injection, and data acquisition and control.The power subsystem is composed of a specimen chamber, an oil cylinder loading system, and a main bearing support, which can truly reproduce the gas occurrence and stress states of the deep coal seam.The specimen chamber is a 1050 mm × 400 mm × 400 mm steel container for laying coal seams and installing sensors[36].The roadway subsystem is composed of a straight roadway and a bifurcated roadway, which can simulate the dynamic disaster causing the process of coal-gas two-phase flow in a T-shaped roadway during the outburst.The flow direction is shown in Fig.2b.The gas injection subsystem is composed of a flowmeter, a solenoid valve, an intelligent electronic meter, and other components to provide accurate control and monitoring of coal seam gas.The data acquisition and control subsystem include a dynamic signal collector with a sampling rate of 1000 Hz and a DEWE-43A multifunctional data collector with a 20000 Hz,used for data acquisition of impact force and static pressure.

Fig.2.Coal and gas outburst dynamic disaster test system.

2.2.Experimental scheme and steps

The underground roadway layout is complex and diverse.The different roadway structures will cause the weak and strong disturbances produced in the process of airflow migration to have a different distribution in the roadway, which makes the outburst airflow obey different migration laws.It is worth noticing that the essence of outburst coal gas two-phase flow migration in the roadway is that gas drives the movement of pulverized coal through drag force.The movement of pulverized coal flow is closely related to the flow state of the outburst gas flow.Therefore,the roadway layout form is the key subject to clarifying the dynamic disaster characteristics of outburst coal gas two-phase flow.Taking a coal seam gas pressure of 2.0 MPa as the initial condition, this experiment focuses on the evolution characteristics of gas pressure and in-situ stress in the coal seam during an outburst,as well as the flow propagation characteristics of coal gas twophase flow in a T-shaped roadway.

The coal seam is affected by underground mining, and the insitu stress field of the coal rock body, which is in the original rock stress state, changes.The relief stress zone, abutment stress zone,recovered stress zone, and initial stress zone are formed in front of the excavation or mining workings [22].Therefore, to truly restore the stress distribution state of a coal seam in front of the working face,it is divided into four zones,as shown in Fig.3.These zones,beginning from the working face to the deep part of the coal seam,are the relief stress zone(σ14 and σ34),the abutment stress zone(σ13 and σ33),the recovered stress zone(σ12 and σ32), and the initial stress zone (σ11 and σ31).The in-situ stress loading scheme is shown in Table 1 [22,34].

Fig.3.Schematic diagram of coal mass stress distribution.

Fig.4 is a schematic of the roadway structure and sensor layout.In the coal seam,in the center of each of the four stress zones,a gas pressure sensor (P1, P2, P3, and P4) is installed.The flared section connects the coal seam with the T-shaped roadway, which is divided into a straight roadway and a bifurcated roadway.The total length of the straight roadway is 12.8 m,with a total of 12 impact force sensors (F1–F12) and 6 static pressure sensors (S1–S6).The total length of the bifurcated roadway is 7 m.Four impact force sensors (F13–F16) and one static pressure sensor (S7) are placed on the right side.Four impact sensors (F17–F20) and one static pressure sensor (S8) are similarly placed on the left side.In the bifurcated roadway, the sensors on the left and right sides are arranged symmetrically along the central axis of the straight roadway.

Fig.4.Schematic diagram of the roadway structure and sensor placement.

The specific experimental steps are as follows:

(1) Prepare the briquette: crushing, screening, drying, stirring,and forming.The detailed process is described in Ref.[37,38].Table 2 presents the particle size distribution of coal powder when coal briquette is proportioned [34].

(2) Seal the specimen chamber and connect it with the T-shaped roadway through the flared section.

(3) Use the vacuum pump to reduce the internal pressure of the specimen chamber to -0.1 MPa.

(4) Carry out step inflation adsorption, that is, inject gas in batches with pressure increase 0.2 MPa at each step, and allow three hours for gas absorption.The entire adsorption process lasted greater than 40 h.

(5) When the gas pressure reaches the predetermined value and the adsorption is balanced, the in-situ stress is loaded(Table 1).

(6) Debug the data acquisition system,then start the acquisition and trigger the outburst.

(7) Save data and complete pulverized coal collection.

2.3.Experimental results and analysis

2.3.1.Evolution characteristics of gas pressure in coal seam

Table 1 In-situ stress loading scheme.

Table 2 Coal briquette particle-size distribution.

The evolution process of gas pressure in different stress zones during the outburst is shown in Fig.5.In the relief stress zone,the gas pressure shows a three-stage downward trend of fast,fluctuation,slow;the pressure drop rates in the rapid and slow decline stages are 1.20 and 0.31 MPa/s, respectively.In the fluctuation stage, the gas pressure shows three obvious recovery phenomena(C1–C3), with recovery rates of 13.76%, 42.12%, and 9.46% respectively.It shows that there is a mechanism to inhibit the continuous decline of gas pressure in the outburst process, which reflects the pulse characteristics of outburst [22,37,39].Notably, the recovery rate refers to the ratio of the gas pressure falling to a certain point Aiand then rising to another point Bi,and its rising amplitude(Bi–Ai)to the previous point Ai(i=1,2,3).In the abutment stress zone,the gas pressure shows a three-stage slow, fast, slow downward trend,with decline rates of 0.40,1.59,and 0.37 MPa/s,respectively.It is worth noticing that when the decline rate of gas pressure in the abutment stress zone increases rapidly (D1–D2), the gas pressure in the relief stress zone fluctuates.The analysis shows that,first, when the outburst develops to the depth of the coal seam,the coal seam in the abutment stress zone is damaged rapidly and intermittently, which increases the permeability of the regional coal seam.At the same time,the pressure relief zone is affected by the transmission mechanism from high to low pressure,resulting in pressure rise.Second,when the high-speed fluid reaches the sound velocity at the outburst, it results in an increase in the gas pressure caused by the blockage.It can be seen that the rapid failure process of the coal seam in the abutment stress zone and the blocking state of gas flow are the main factors causing the outburst pulse phenomenon.In addition, when the coal seam in the abutment stress zone is rapidly damaged for the second time (D2),the gas pressure in the recovered stress zone begins to decrease.In the recovered and initial stress zones, due to the distance from the mining space, the coal seam is less affected by the outburst,and the gas pressure shows a slow downward trend.Comparing the four stress zones, the response time of gas pressure is delayed with the depth of the coal seam,and its decline rate decreases with the depth of the coal seam.

Fig.5.Evolution of the coal seam gas pressure.

2.3.2.Evolution characteristics of in-situ stress in coal seam

As a measure of the elastic potential of coal and rock mass, insitu stress is one of the sources of outburst energy.Fig.6 shows the evolution of in-situ stress in the process of outburst.As can be seen from Fig.6a,in the vertical stress direction,the in-situ stress in the relief, abutment, and recovered stress zones shows a downward trend, with a decrease of 0.93, 3.28, and 2.10 MPa, respectively;the release of elastic potential energy in the vertical stress direction occurs mainly in the abutment and recovered stress zones.In the horizontal stress direction, as shown in Fig.6b, the stress in all four zones, exhibits a downward trend, amounting to 0.60,2.40,0.21,and 0.49 MPa,respectively.The release of elastic potential energy in the direction of horizontal stress in the process of outburst occurs mainly in the abutment stress zone.Comparing the declining trend of in-situ stress in different regions,the declining percentage rates of in-situ stress in the relief and abutment stress zones in the vertical stress direction are 93%and 82%respectively, and 100% in the horizontal stress direction, which shows that the elastic potential energy in the relief and abutment stress zones can almost be completely released.At the same time, in the recovered stress zone, the percentage rates of in-situ stress decrease in the vertical and horizontal directions are 70% and 11.67% respectively, indicating that the release of elastic potential energy starts from the vertical stress direction during the outburst process.In addition, Fig.6b and c show that the initial stress zone exhibits a downward trend in the horizontal stress direction.However, the vertical stress direction shows an upward trend, indicating that the stress gradually transfers to the vertical stress direction of the deep coal seam during the outburst process.It is worth noticing that in the relief, abutment, and recovered stress zones, there is a phased recovery in the decline process of the insitu stress, indicating that the release of outburst energy is completed in stages [22].

2.3.3.Evolution characteristics of impact force

During an outburst in a coal mine,the strong impact pressure of the shock wave, impact airflow, and coal-gas two-phase flow on objects is the main factor of dynamic disaster.In this paper, the impact pressure generated by the impact airflow and coal-gas two-phase flow movement during coal and gas outburst is collectively referred to as the impact force.The discussion on the evolution of impact force has important reference value for the location of refuge facilities and the establishment of an underground base for rescue investigation.Table 3 shows the injury effects of different impact forces on the human body [40].A force higher than 19.6 kPa can affect the escape ability and cause local myocardial rupture, middle ear and lung contusion, and even cause death under the action of harmful gas after the outburst.When the impact force is higher than 49 kPa,it can directly cause great harm and death.

Fig.7 shows the evolution process of the impact force in the straight roadway.The essence of the coal-gas two-phase flow in the roadway is that the gas drives the pulverized coal through drag force.The two-phase fluid presents the flow state of dilute phase and dense phase flows from front to back[29].Therefore,the evolution of the impact force during an outburst process can be divided into two stages according to different fluid media:gas flow and sparse two-phase flow stage (gray area in Fig.7), and dense two-phase flow stage.In the gas flow and sparse two-phase flow stage, the impact force shows a rapid rise, rapid decline trend.As the distance from the working face increases,there is a gentle period after the decline.Sensors F1, F7, F8, and F11, show that the impact force of the gas flow and sparse two-phase flow stage there can cause trauma or death, showing obvious dynamic disastercausing characteristics.The maximum impact forces at this stage are 45.15, 79.10, 54.75, and 29.56 kPa, respectively.The disaster intensity shows a trend of first increasing and then decreasing.In the dense two-phase flow stage, the impact force shows an evolutionary rise and slow trend.Among them, at F1, because of its proximity to the coal seam, the impact force is most violently affected by the two-phase flow, and the fluctuation frequency and disaster intensity reach their maximum.Except for F10 and F12, the impact force at other locations can cause injury or death in the dense two-phase flow stage.Their maximum values from front to back are 168.88, 55.89, 56.69, 42.90, 47.63, 63.36, 70.55,58.63, 34.71, and 58.45 kPa, respectively.The disaster intensity increases in the middle of the straight roadway.It is worth noticing that in the middle and front end of the straight roadway,there are obvious fluctuations in the process of impact force decline, which coincides with the pulse characteristics of gas pressure decline.The phenomenon reflects the fact that the outburst development process actually occurs intermittently and multiple times [39].

Table 3 Injury degree of impact force on human body.

Fig.8 shows the evolution process of impact force in the bifurcated roadway.When the outburst fluid enters the bifurcated roadway,there is no area where the impact force directly causes heavy injuries or death.According to whether there is human trauma or not,the impact force in the bifurcated roadway is divided into the evolution with disaster-causing characteristics(F13, F15,and F18)and the evolution without disaster-causing characteristics (F14,F16, F17, F19, and F20).When the impact force has disastercausing characteristics,its evolution shows that it rapidly increases to the disaster-causing range during the early stage of the outburst,then decreases,and then rises again in the middle stage of the outburst.When the impact force has no disaster-causing characteristics, its evolution shows a first stable and then slowly rising trend in the safety range.The impact force evolution in the bifurcated roadway is more affected in the early stage of gas flow and sparse two-phase flow than in the middle and late stages of dense twophase flow.In the gas flow and sparse two-phase flow stages, the maximum impact forces of F13, F15, and F18 are 38.13, 37.12,and 41.77 kPa, respectively.Among them, F13 will rapidly drop to the safe range in this stage, whereas F15 and F18 will remain stable in the disaster-causing range.And the disaster-causing degree is significantly higher than that in other locations, indicating that there is a disaster concentration area in the bifurcates roadway.It is worth noticing that, as shown in Fig.8, taking the center of the bifurcated roadway as the reference point, F13 and F17,F14 and F18,F15 and F19,F16 and F20 all show different evolution characteristics and disaster-causing characteristics.That is,in the bifurcated roadway with a symmetrical structure, there are great differences in the evolution of impact force between the left and right sides.

2.3.4.Distribution characteristics of impact force along roadway

Fig.9 shows the distribution characteristics of impact force along the roadway at different times.At t=20 ms,the impact force 1 m away from the coal seam jumps to 30.07 kPa.The impact force at 5 m is negative, indicating that the static pressure of airflow is negative.The outburst shock wave moves to 10 m and does not reach the bifurcation.At t=40 ms, the evolution of the first 7 m in the straight roadway does not differ significantly from that at 20 ms.The impact force at 8 m rises rapidly to 25.11 kPa.The evolution of the impact force on the left and right sides of a bifurcated roadways begins to show differences.At t=60 ms,the impact force at 8 m continues to rise to 49.03 kPa.The impact force at the middle distance of 7 and 10 m in the straight roadway and 14 m in the bifurcated left roadway increases to 24.08, 9.42, and 22.4 kPa,respectively, which exhibits disaster-causing characteristics for the first time in the bifurcated left roadway.The impact force in the roadway presents a serrated distribution.When t=80 ms, the maximum impact force in the roadway moves from 8 to 7 m,with a maximum value of 61.98 kPa.Both are high-intensity disastercausing impact forces.The impact force of the bifurcated right roadway begins to increase.When t=100 ms, the impact force at 13 and 14.4 m in the bifurcated right roadway rises to 28.26 and 23.06 kPa, respectively.At this time, the disaster area appears in being the middle of the straight roadway and the left and right sides of the bifurcated roadway; the most serious disaster area appears in the middle of the straight roadway.When t=200 ms,the impact force at 7 m begins to decline; the disaster intensity to the left and right sides of the bifurcation gradually increases,with the disaster intensity on the left side being greater than that on the right side.When t=400 ms, the impact force from the first 6 m of the coal seam gradually increases, and the disastercausing characteristics are shown in the front and middle of the straight roadway, with the middle still being the strongest.At t=800 ms, the disaster area in the roadway continues to expand(mainly along the straight roadway), and the impact force at 11 m suddenly increases to 56.53 kPa, becoming a secondary serious disaster area.When t=1000 ms, the impact force at 1 m exceeds 100 kPa,reflecting high intensity disaster-causing characteristics, and the disaster area extends to the front end of the straight roadway.After t=1200 ms,the impact force in the roadway begins to decline as a whole.Until t=3000 ms, the impact force shows a strong–weak alternating distribution along the roadway,and the disaster intensity on the left side of the bifurcated roadway is still higher than that on the right side.

Fig.9.Distribution characteristics of impact force along roadway.

Summarizing the process of outburst,the disaster area in the Tshaped roadway first appears near the coal seam and in the middle of the straight roadway and soon after that the middle part of the straight roadway develops into a serious disaster area.The disaster scope of the roadway gradually expands,mainly in the direction of the coal seam.There is a disaster-causing area in the bifurcated roadway, and the disaster-causing intensity of the left roadway is higher than that of the right roadway.In the process of outburst,the impact force of the roadway mostly presents a serrated distribution.The analysis suggests that a series of complex expansioncompression wave systems are formed after the outburst impact airflow enters the roadway.Compression wave propagation has superposition, which can be superimposed to form a strong compression wave.The propagation of the expansion wave is not superimposed, and its energy gradually decays [34,37].Therefore,it leads to a strong-weak alternating distribution characteristic of the impact force in the roadway.There are areas of low impact disaster-causing strength in the middle and rear end of the straight roadway and some areas of the bifurcated roadway,which are the best places for underground personnel to avoid impact dynamic injury.

2.3.5.Evolution characteristics of wall static pressure

The process of outburst dynamic disaster can be regarded as the violent movement of coal driven by gas.Gas plays a key role in the movement of the coal-gas two-phase flow.The change in static pressure represents the state of gas movement.Therefore, it is essential to monitor the static pressure in the roadway in the process of outburst.The movement characteristics of outburst fluid can be reflected from the evolution of static pressure, which has a certain reference value for the research on the migration characteristics of outburst fluid and its dynamic disaster-causing characteristics.Fig.10a shows the evolution process of static pressure in the roadway during the outburst process.It can be seen that the static pressure shows a downward trend of cyclic fluctuation.To facilitate further analysis,as in Section 2.3.3,based on the fluid medium,the evolution of static pressure is divided into the airflow stage(stage 1), transition stage (stage 2), and dense two-phase flow stage (stage 3) in chronological order.Among them, the transition stage refers to the process of fluid medium from gas flow to sparse two-phase flow and finally to dense two-phase flow.

Fig.10.Evolution of the static pressure in the roadway.

From Fig.10b it follows that in the stage of impact airflow, the starting time of static pressure reaction is positively correlated with the distance from the coal seam.The peak static pressures shown by S1–S8 are 2.74, 2.79, 2.80, 2.61, 2.54, 2.08, 1.66, and 1.63 kPa, respectively.The peak static pressure increases first and then decreases along the roadway.It is worth noticing that the static pressure evolution process on the left and right sides of the bifurcation (S7, S8) is similar and shows symmetry.After S3, the attenuation rates of the S4–S7 static pressure peak are 6.8%, 2.7%,18.1%, and 20.2%, respectively.The bifurcated structure can accelerate the attenuation process of static pressure in the airflow stage.In Fig.10c we can see that in the transition stage,with an increase in the solid-gas ratio of coal gas two-phase flow,the static pressure in the roadway continues to decrease.The solid-gas ratio is a crucial factor affecting the evolution of static pressure.In Fig.10d we observe that in the dense two-phase flow stage,the static pressure is reduced by more than 10 times compared with the airflow stage.The disaster caused by gas static pressure is negligible.

2.3.6.Evolution characteristics of air shock wave

According to the theory of gas dynamics, after the outburst starts,the pulverized coal carried by the gas is quickly sprayed into the roadway space, compressing the air [41].A series of moving compression waves are generated in the compressed airflow.When the compression waves are superimposed, a strong compression wave (air shock wave) is formed.Research shows that the shock wave is often located before the compressed airflow[27,29].Therefore, the propagation velocity of the shock wave is characterized by the starting time of static pressure reaction and the spacing between measuring points,as shown in Fig.11a.Notably,the sampling rate of experimental static pressure is 20000 Hz,which provides rapid response and accuracy.Fig.11b shows the distribution characteristics of air shock wave migration velocity along the roadway.The shock wave velocities from front to back are 354.0,444.4,412.4,317.5,384.6,and 400 m/s respectively,first increasing, then decreasing, and then increasing.During the outburst process, the shock wave velocity reaches its maximum between S2 and S3.It is worth noticing that the shock wave velocity increases at the end of the straight and bifurcated roadways,and there is a secondary acceleration in the T-shaped bifurcation.

3.Theoretical analysis of the formation of outburst impact airflow

3.1.Formation model of outburst impact airflow

In the process of outburst, gas is the participating medium and the main energy source.Therefore, the formation and migration mechanism of impact airflow and shock wave produced in the process of an outburst should be explored because of its vital role in the disaster-causing dynamic characteristics.It has practical guiding significance for reasonably arranging the layout of outburstproof air doors, outburst-prevention barrier, refuge chambers,and other facilities, as well as the design of post-disaster emergency rescue measures.

The formation model of coal and gas outburst impact airflow is established.As shown in Fig.12, the mining space is divided into the roadway undisturbed zone, shock wave, air impact airflow zone, and gas impact airflow zone.Among them, both shock wave and impact airflow have strong destructiveness in the roadway.The occurrence of the outburst is a process of energy accumulation,storage, and sudden release.Under the disturbance of mining, the stress of the coal seam is redistributed, resulting in the formation of a high-energy accumulation area near the mining space.In the process of outburst initiation and development, the high amount of energy accumulated in the coal seam is suddenly released into the direction of the mining space.A release channel connecting the high-energy storage area of the coal seam and mining space is formed instantly.Under the action of the gas pressure gradient,the high-pressure gas flow formed in the coal seam carries a large number of broken coal that sprays into the mining space through the channel.As shown in Fig.12, an outburst hole with small mouth and large cavity is gradually formed [9].The highpressure gas flow is accelerated to sound speed under the influence of hole shape and expands rapidly in the mining space to form a gas airflow with impact damage characteristics.The undisturbed air in the mining space is compressed by the gas impact airflow to form the air impact airflow, and there is a shock wave in front of the air impact airflow.In addition, according to the theory of gas dynamics, the process of gas impact airflow entering the mining space can be regarded as the flow from a small section pipe into a large section pipe, and the flow is in unsteady turbulent state[41].When the pipe section suddenly expands, the gas impact airflow occupies the whole roadway section after moving for a certain distance.Before that, many vortices are generated in the wall of the mining space near the coal seam.

Fig.12.Coal and gas outburst impact airflow formation model.

3.2.Analysis of gas flow state in outburst hole

Fig.12 shows that the outburst holes in the coal seam show the morphological characteristics of a small mouth and large cavity.In the process of ejection of high-pressure gas into the mining space,the influence of the variation of the cross-sectional area is prominent.To investigate the flow state of gas in the outburst hole, it is assumed that the gas has no heat and work exchange with the surroundings during the outburst process, the friction between the gas and the outburst hole wall is ignored, and the gas flow is a one-dimensional constant.According to the theory of gas dynamics, the differential relationship between the flow parameters in the outburst hole and the change in the cross-sectional area can be obtained [41]:

where p is the outburst gas pressure; k the adiabatic index of the outburst gas; Ma the Mach number of the outburst gas; A the sectional area;ρ the outburst gas density;V the outburst gas velocity;and T the temperature of the outburst gas.

After the outburst starts, under the action of the gas pressure gradient, the gas begins to migrate along the hole to the mining space, and the cross-sectional area gradually decreases.From Experiment in Section 2.3.1,the gas pressure in the hole is decreasing during the outburst process.Combined with the above differential equation [Eq.(1)], the outburst gas flow in the hole presents a subsonic state (Ma<1).When the cross-sectional area of subsonic outburst airflow (Ma<1) decreases, it will inevitably lead to the decrease in gas pressure, density, and temperature, as well as the increase in gas velocity and Mach number.The outburst airflow accelerates in the outburst hole.For the convenience of discussion, we denote P0as the gas pressure in the outburst hole in the coal seam, Peas the section pressure at the outlet of the outburst hole, Pbas the environmental pressure of the mining space,and βcras the critical pressure ratio.The critical pressure ratio can be calculated using formula 6, and when k=1.3, βcr=0.546.When the pressure of the outlet airflow is equal to the ambient pressure, the gas expands completely in the outburst hole.At this time,the gas flow is in a critical flow state,with a critical pressure ratio βcr=Pb/P0.When βcr>Pb/P0, the gas flow is in the supercritical flow state, also known as the incomplete expansion state.At this time, the airflow velocity of the outlet section has reached the sound velocity,and the outlet section becomes the critical section.According to the evolution of P1 in Section 2.3.1, Pb/P0in the outburst hole is always less than 0.546 during the outburst process.In the outburst process,the airflow in the outburst hole is in the state of incomplete expansion, and the gas is sprayed into the mining space at the speed of sound to form a gas impact airflow.The process is accompanied by flow choking.This explains the rebound phenomenon in the process of coal seam gas pressure decline.

3.3.Formation of air impact airflow and shock wave

After the outburst starts, when the gas impact airflow spreads into the mining space at the speed of sound,the gas impact airflow speed continues to increase because of the sudden section increase.This process can be modeled by a piston at the end of the coal seam, which accelerates to the right to compress the undisturbed air in the mining space,as shown in Fig.13a.To facilitate the explanation of the problem,the movement of the piston from a stationary state to a certain speed V is divided into many small speed increments △V.When the piston speed increases from 0 to △V,as shown in Fig.13b,the air on the right side of the piston is compressed, and its pressure, density, and temperature slightly increase.Therefore, an air compression wave is generated that moves to the right.Because the velocity increment △V of the piston is very small, the compression wave is weak, and its propagation speed is the sound velocity, c1, in undisturbed air.The pressure change in the figure indicates the location of this weak compression wave, which moves to the right with the piston at the speed△V to form an air impact airflow.When the piston speed increases from △V to 2△V,as shown in Fig.13c,a second weak compression wave is generated in the mining space.The second weak compression wave propagates at the local sound velocity relative to the air impact airflow.The air impact airflow is compressed and the temperature increases,increasing the local sound velocity to c2.Therefore, the propagation velocity of the second weak compression wave relative to the mining space is c2+△V, which is faster than that of the first compression wave.Subsequently, every time the piston slightly accelerates, a weak shock wave will be generated in the air impact airflow,with a slightly higher speed than the previous weak compression wave, as shown in Fig.13d and 13e,where c4+3△V>c3+2△V>c2+△V>c1.Over time,the distance between waves gradually decreases, as shown in Fig.13f.This process continues until the latter weak compression wave superimposes with the former one to form a strong compression wave,namely a shock wave.In the process of shock wave formation, the air impact airflow is continuously compressed by the piston, and its internal pressure, density, and temperature also increase whereas the airflow moves to the right at increasing speed.

4.Discussion

4.1.Migration characteristics of outburst impact airflow in a bifurcated structure

To highlight more the migration characteristics of the impact airflow at the bifurcated structure.We obtained the distribution of peak impact force and peak static pressure at the bifurcation structure from the experiment, as shown in Fig.14.The peak impact force tends to rise after the bifurcation structure, and the peak impact force on the left side is slightly larger than that on the right side.The peak static pressure is decreasing after the bifurcation structure, and the peak static pressure on the right side is slightly larger than that on the left side.The peak impact force and the peak static pressure show a negative correlation distribution.That is,the greater the peak impact force rises,the greater the peak static pressure falls.

Fig.14.Distribution of peak impact force and peak static pressure at the bifurcated structure.

As shown in Fig.15, when the outburst impact airflow flows through the T-shaped structure, it can be regarded as flowing through two convex folding surfaces (E1O1F1, E2O2F2) and one blunt head obstacle (G1G2) at the same time.When the highspeed airflow flows through the convex surface, a fan-shaped expansion beam forms at the inner corner; when flowing through a blunt obstacle,an arched curved shock wave(AB)is often generated in front of the object[41].Therefore,when the impact airflow flows through the T-shaped structure, a fan-shaped expansion beam will be generated at the inner walls O1and O2of the bifurcation, and a curved shock wave will be generated at one end near the wall G1G2.The expansion beam and shock wave form a complex wave system at the bifurcation, which changes the flow state of the impact airflow.After the impact airflow enters the bifurcated structure, its velocity begins to increase and gradually deviates to both sides under the influence of the expansion beam.The impact airflow enters the left and right sides of the bifurcation at a higher speed,and the static pressure of the roadways on both sides of the bifurcation decreases gradually, i.e.>V0,

Fig.15.Flow model of impact airflow at a bifurcated structure.

In addition, part of the impact flow forms a compression zone near the G1G2wall of the bifurcated structure through the shock wave AB, and the static pressure in this region increases rapidly.around the bifurcation, the impact airflow velocity increases, and the disaster caused by dynamic pressure significantly increases.In the compression zone,the increase in the gas static pressure significantly worsens the disaster caused by positive pressure.In conclusion, owing to the particularity of the bifurcated structure, the disaster-causing factors are not the same in different locations.Clarifying the distribution of disaster-causing factors and reasonably arranging disaster reduction facilities such as outburst-proof air doors, prevent-outburst barriers, and refuge chambers according to their inherent characteristics have key significance for the weakening of outburst dynamic disasters.

4.2.Discussion on asymmetry of dynamic disaster caused by bifurcated left and right roadway

From the above experiments, it is concluded that the dynamic disaster of the coal-gas two-phase flow exhibits an asymmetric phenomenon in a bifurcated roadway.It is worth noticing that the essence of outburst coal-gas two-phase flow migration is that gas drives the movement of pulverized coal.The movement of pulverized coal flow is closely related to the flow state of outburst gas flow.Therefore, the asymmetry of disaster-causing characteristics in the bifurcated roadways is explored from the perspective of airflow.

For one reason,according to the jet principle in fluid mechanics,when the high-speed fluid flows in a limited space,the influence of turbulence results in an asymmetric fluid flow [42].When the impact airflow is ejected from the outburst hole to the mining space,the impact airflow will be influenced by the nature of turbulence, and the gas on both sides of the space will be sucked.The side with the large volume of suction will have a large complementary flow velocity.From Bernoulli’s principle, it is known that the greater the fluid flow velocity,the lower the pressure.The pressure difference between the left and right sides of the impact airflow transport process will occur,and then deflection will occur,resulting in a wall attachment effect.Eventually, the wall attachment effect causes the impact airflow to move away from the center axis of the roadway before it reaches the bifurcation structure.Therefore, the dynamic disaster in the bifurcated roadway is asymmetric.Another reason, according to the analysis in Section 4.1,when the high-speed airflow flows through the bifurcated structure, it generates a series of complex disturbance waves, and the flow state of the airflow changes.The impact airflow is influenced by the complex disturbance wave,which intensifies the turbulence effect,thus making a difference in the fluid flowing to both sides of it.

A numerical simulation of outburst airflow in a T-shaped roadway and its boundary conditions are shown in Fig.16.The numerical simulation method is detailed in reference[39].The geometric model is based on the experimental device.The total length of the straight roadway is 12.2 m and the total length of the bifurcated roadway is 7 m.The gas pressure (P1) in the relief stress zone obtained from the experiment is used as the inlet pressure to control the change in gas pressure in the outburst hole to reproduce the movement of outburst airflow in the roadway during the experiment.The outlet of the model is in the bifurcated roadway,the conditions on the left and right sides are the same,the pressure setting is adopted,and the flow condition is a subsonic-supersonic mixed flow.The initial velocity, pressure, and temperature in the roadway model are 0 m/s, 1 atm, and 293.15 K, respectively; the wall boundary condition is non-slip flow.

Fig.16.Geometric model and boundary conditions of numerical simulation.

The dynamic disaster-causing characteristics of outburst impact airflow are mainly reflected in the changes of static pressure and dynamic pressure.The evolution of static and dynamic pressure of impact airflow in a T-shaped roadway during the outburst is shown in Fig.17.Taking the front and back of the bifurcated structure as the research object, the dynamic disaster process of outburst impact airflow is described.At t=30 ms, the impact airflow moves to the end of the straight roadway, and the static pressure of the airflow is higher than 50 kPa, showing the characteristics of a high-intensity positive pressure disaster.At this time, most of the dynamic pressure is lower than 30 kPa, that is, the disaster intensity of dynamic pressure is lower than that of static pressure.At t=40 ms, the impact airflows into the bifurcated roadway, and the static pressure and dynamic pressure of the left and right roadways are symmetric.At this time, the static pressure is still high,and the disaster intensity caused by dynamic pressure increases significantly at the bifurcated structure, exceeding 50 kPa.At t=60 ms, the static pressure decreases to nearly atmospheric pressure except at the bifurcated structure.At the same time, the straight roadway shows a downward trend from the tail to the front.It is worth noticing that a semicircular high-strength positive pressure disaster area appears at the bifurcated structure.Regarding the dynamic pressure, the bifurcated structure presents highintensity dynamic pressure-induced damage symmetrically on both sides.Generally speaking, at this time, the straight roadway mainly presents the characteristics of static pressure disaster,whereas the bifurcated roadway mainly presents the characteristics of dynamic pressure disaster.At t=100 ms, the static pressure in the bifurcated roadway changes from positive to negative, and its minimum value is less than -50 kPa.The static pressure in the straight roadway also decreases gradually and changes from positive to negative.At the same time,the area and strength of the disaster caused by the semicircular positive pressure at the bifurcated structure tends to decrease.It is worth noticing that the dynamic pressure disaster intensity and action area in the straight and bifurcated roadway have decreased significantly.At this time,the dynamic and static pressures of the impact airflow in the straight roadway have initially shown asymmetry.On the whole,roadway disaster is mainly caused by dynamic pressure.At t=150 ms, the static pressure at the end of the straight roadway,the bifurcation, and the bifurcated roadway exhibits an increasing trend, whereas the dynamic pressure in the roadway still shows the characteristics of high-intensity disaster.At the same time,both static and dynamic pressures show obvious asymmetric distribution characteristics in a bifurcated roadway.At t=350 ms,the disaster caused by negative pressure appears in the straight roadway, and the semicircular positive pressure disaster area at the bifurcated structure gradually changes to an irregular shape.At the same time, the static pressure in the bifurcated roadway fluctuates, showing the formation of disaster concentration areas caused by positive pressure, and the asymmetry is obvious.From the dynamic pressure distribution, it is observed that the impact airflow in the straight roadway reflects an obvious wall attachment effect.In the bifurcated roadway,only the left side shows the characteristics of dynamic pressure disaster.

Fig.17.Evolution of static and dynamic pressure of outburst impact airflow.

5.Conclusions

Using the self-developed dynamic disaster test system of coal and gas outburst, we experimentally study the dynamic disaster characteristics of outburst coal gas two-phase flow in T-branch roadway.Based on gas dynamics,the formation model of outburst impact flow and its flow model in a bifurcated structure are established to explain the phenomena observed in the experiment.

(1) An evident rebound phenomenon exists in the process of coal seam gas pressure decline,which indicates that the outburst process has pulse characteristics.The rapid failure process of the coal seam in the abutment stress zone and the blocking state of gas flow are the main factors causing the outburst pulse phenomenon.The decline rate of gas pressure decreases with the deepening of the coal seam.The elastic potential energy in the relief and abutment stress zones can be completely released.The vertical elastic potential energy is released before the horizontal direction.

(2) In the straight roadway, the impact force in the stage of gas flow and sparse two-phase flow,shows an evolutionary process of rapid rise-rapid decline.The impact force in the dense two-phase flow stage shows an evolutionary process of stable rise-slow decline.In the middle and front end of the straight roadway, there is an obvious fluctuation phenomenon in the decline of impact force, indicating that the disaster caused by two-phase flow has the characteristics of intermittent recovery.In the bifurcated roadway, the impact force evolution is much more affected by the stage of gas flow and sparse two-phase flow than that of dense two-phase flow, and there is a disaster concentration area.

(3) The impact force in the T-shaped roadway presents the characteristics of serrated distribution,and the disaster area first appears near the coal seam and in the middle of the straight roadway,which rapidly becomes a serious disaster area.The disaster scope of the roadway gradually expands, mainly in the direction of the coal seam.In the bifurcated roadway,the disaster-causing intensity is higher on the left roadway than that on the right one.

(4) The static pressure shows a fluctuating downward trend.In the stage of impinging airflow, the peak static pressure first increases and then decreases along the roadway.The bifurcated structure can accelerate the attenuation process of static pressure,and the left and right sides of the bifurcation are symmetrically distributed.In the transition stage, the static pressure decreases with an increase in the solid-gas ratio of coal gas two-phase flow,indicating that the solid-gas ratio is a crucial factor affecting the evolution of static pressure.In the dense two-phase flow stage, the static pressure drops sharply compared with the gas flow stage.The shock wave shows the distribution characteristics of increasing,decreasing,increasing.There is a phenomenon of secondary acceleration when the shock wave moves into the T-shaped roadway.The bifurcated structure promotes an increase in shock wave velocity.

(5) Based on gas dynamics, the formation model of outburst impact flow and its flow model in a bifurcated structure are established.The outburst airflow accelerates in the outburst hole and reaches the sound velocity at the outlet section, which is accompanied by flow choking.Under the interference of disturbance waves in a bifurcated structure,asymmetric fluids show more obvious differences in a bifurcated roadway.In the early stage of the outburst, the disaster-causing factors in the bifurcated structure are mainly caused by positive pressure, and the static and dynamic pressures at the left and right sides of the bifurcation are symmetrically distributed.Over time, the disastercausing factors gradually change from positive pressure to negative pressure and dynamic pressure.The static pressure and dynamic pressure on the left and right sides become asymmetric.

The outburst impact effect is a research topic in the field of multiphase flow engineering, and its flow process is quite complex.This study mainly analyzes the results of the physical simulation experiment and further verifies the experimental phenomenon through theoretical and numerical simulation.In addition, here we focus on the gas-phase medium in the process of outburst in both theory and numerical simulation.The flow characteristics of the coal-gas two-phase medium in the actual outburst process affect each other.In the future, the multiphase flow model can be introduced to guide the experiment and field research more accurately.Research on the hazard characteristics of outbursts involves the intersection and integration of multiple disciplines,and research in this area is still in its infancy.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos.51874055, 52074047, and 52064016).